RE: RS-232 Tx / Rx monitoring LEDs?
-Original Message- From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Brent Hilpert Sent: Sunday, August 23, 2015 3:07 AM To: General Discussion: On-Topic and Off-Topic Posts Subject: Re: RS-232 Tx / Rx monitoring LEDs? On 2015-Aug-22, at 11:55 PM, drlegendre . wrote: On Sun, Aug 23, 2015 at 1:17 AM, Chuck Guzis ccl...@sydex.com wrote: On 08/22/2015 10:23 PM, dwight wrote: I would think the reverse voltage sum of the diodes is enough. Different diodes also can handle different voltages. Since the sum of the forward voltages is enough to handle AC, I'd suspect the reverse voltages each would handle is quite small as well. The problem is when the current limiting is done with a resistor that in the forward direction drops a lot of voltage. The diode has to handle the voltage until breakdown when reversed. If the resistor was handling 1 Watts, with the right break down, the LED could be taking .5 Watts. This is more than most are designed for. ...and that's just the nub of it. The success of this depends largely on the consistent characteristics of every LED in the string. Since LEDs tend to fail short if submitted to overvoltage, I've often wondered if a spike in the AC supply would precipitate a cascade failure in the string. I've looked hard and there are no rectifier diodes in the string--just the LEDs themselves. Probably saves about 5 cents or so of manufacturing cost. I've also seen LED night lights from China that employ nothing more than a safety capacitor (usually about 104) in series with a resistor connected to two back-to-back LEDs, all across the AC line. I've wondered what the lifetime of such a setup is. --Chuck. I've also seen C-R series voltage dropping circuits, here there. Correct me if I'm wrong, but doesn't the series cap dissipate power just as it would, were it a series resistor? I mean, if the LED is passing 20mA, the cap is also doing 20mA - and at whatever the Vdrop is. Right? If not, why? I doubt if any brief explanation here is going to the topic justice. Look up power factor or reactive power. FWIW: The impedance (capacitive reactance, Z=Xc=1/(2*pi*f*C) of the C does produce the desired voltage drop but the C also shifts the phase of the current relative to that of the V. To apply the power equation P=VI properly, you can't just multiply the RMS values together, you multiply the instantaneous values of the V I sine waves together through a cycle. You get a third sine wave, that for power. If V I are in phase, the power sine wave will all be in the positive region and is real power consumption. When they are out of phase, some portion of the power sine wave will be negative: a portion of the energy the C sucked down the line is being returned during each cycle. Yes, it does reduce energy consumption relative to a purely R solution. On a large scale, the power company doesn't like it because it unnecessarily adds to the currents circulating in the system, but then, this is from C which shifts the current in one direction, so it's doing some compensation for the inductive wall warts you have plugged in around the house, which as L shift the current in the other direction. --- Brent, that is an excellent explanation in just a few sentences. One quibble however. The power company does indeed like components that shift the current in the capacitive direction. Taken as a whole for the power grid, the power source sees the load as inductive because of all the industrial motors it powers, including the ones that exist in almost every home (washer, dryer, heating/air conditioning, mixer, disposal, etc.) Capacitor banks are frequently installed in large industrial operations to shift the inductive load more toward the capacitive power factor. This is because the power company, as you have implied, charges more for power that is current shifted away from zero %. I have even seen large motors installed in industrial situations that run continuously without load, because such motors appear as a capacitive load, and indeed are called 'rotary capacitors'. I recall one time when the CFO ordered such a motor turned off because it is wasting power. It took a little plain and fancy instruction by the engineer to let him know that it was actually saving the company money.
Re: RS-232 Tx / Rx monitoring LEDs?
On 2015-Aug-23, at 9:06 AM, Chuck Guzis wrote: On 08/23/2015 07:10 AM, dwight wrote: I've used the capacitor method to provide most of the drop in the past. I don't usually max out the LEDs at 20ma. I find there is little difference between 10 and 20ma. Yes, the 10 ( or 20ma ) is current flow through the capacitor. It is necessary to have some resistor in series as well to suppress line spikes. Another similar dirty trick back in the day was to run a 6SL7 dual-triode form the line using a 1.0 uF nonpolar capacitor in series with the line to provide a supply for the 500 ma 6.3 v heater and then use one of the triode sections as a half-wave rectifier. You thus had the other triode section for whatever stupid purpose. Of course, this was horrible abuse of the tube, particularly in the area of heater-cathode voltage ratings. It probably wouldn't work as well in 220VAC countries, but it worked well enough in the 120VAC ones. Those ubiquitous motion detectors for outdoor lights use capacitive dropping to supply the low voltage for the ICs and electronics. Cap and small R in series with the AC input to a bridge rectifier. Works out well in that the bridge rectifier permits current flow in both directions, which is necessary to get current flow through the cap (the cap has to charge and discharge).
RE: RS-232 Tx / Rx monitoring LEDs?
Those ubiquitous motion detectors for outdoor lights use capacitive dropping to supply the low voltage for the ICs and electronics. Cap and small R in series with the AC input to a bridge rectifier. Works out well in that the bridge rectifier permits current flow in both directions, which is necessary to get current flow through the cap (the cap has to charge and discharge). Over here that sort of circuit was common for the power supply for the control electronics in washing machines and the like. It may still be, I've not worked on anything that recent. -tony
Re: RS-232 Tx / Rx monitoring LEDs?
On 2015-Aug-23, at 9:54 AM, Steve Moulding wrote: -Original Message- From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Brent Hilpert Sent: Sunday, August 23, 2015 3:07 AM To: General Discussion: On-Topic and Off-Topic Posts Subject: Re: RS-232 Tx / Rx monitoring LEDs? On 2015-Aug-22, at 11:55 PM, drlegendre . wrote: On Sun, Aug 23, 2015 at 1:17 AM, Chuck Guzis ccl...@sydex.com wrote: On 08/22/2015 10:23 PM, dwight wrote: I would think the reverse voltage sum of the diodes is enough. Different diodes also can handle different voltages. Since the sum of the forward voltages is enough to handle AC, I'd suspect the reverse voltages each would handle is quite small as well. The problem is when the current limiting is done with a resistor that in the forward direction drops a lot of voltage. The diode has to handle the voltage until breakdown when reversed. If the resistor was handling 1 Watts, with the right break down, the LED could be taking .5 Watts. This is more than most are designed for. ...and that's just the nub of it. The success of this depends largely on the consistent characteristics of every LED in the string. Since LEDs tend to fail short if submitted to overvoltage, I've often wondered if a spike in the AC supply would precipitate a cascade failure in the string. I've looked hard and there are no rectifier diodes in the string--just the LEDs themselves. Probably saves about 5 cents or so of manufacturing cost. I've also seen LED night lights from China that employ nothing more than a safety capacitor (usually about 104) in series with a resistor connected to two back-to-back LEDs, all across the AC line. I've wondered what the lifetime of such a setup is. --Chuck. I've also seen C-R series voltage dropping circuits, here there. Correct me if I'm wrong, but doesn't the series cap dissipate power just as it would, were it a series resistor? I mean, if the LED is passing 20mA, the cap is also doing 20mA - and at whatever the Vdrop is. Right? If not, why? I doubt if any brief explanation here is going to the topic justice. Look up power factor or reactive power. FWIW: The impedance (capacitive reactance, Z=Xc=1/(2*pi*f*C) of the C does produce the desired voltage drop but the C also shifts the phase of the current relative to that of the V. To apply the power equation P=VI properly, you can't just multiply the RMS values together, you multiply the instantaneous values of the V I sine waves together through a cycle. You get a third sine wave, that for power. If V I are in phase, the power sine wave will all be in the positive region and is real power consumption. When they are out of phase, some portion of the power sine wave will be negative: a portion of the energy the C sucked down the line is being returned during each cycle. Yes, it does reduce energy consumption relative to a purely R solution. On a large scale, the power company doesn't like it because it unnecessarily adds to the currents circulating in the system, but then, this is from C which shifts the current in one direction, so it's doing some compensation for the inductive wall warts you have plugged in around the house, which as L shift the current in the other direction. --- Brent, that is an excellent explanation in just a few sentences. One quibble however. The power company does indeed like components that shift the current in the capacitive direction. Taken as a whole for the power grid, the power source sees the load as inductive because of all the industrial motors it powers, including the ones that exist in almost every home (washer, dryer, heating/air conditioning, mixer, disposal, etc.) Capacitor banks are frequently installed in large industrial operations to shift the inductive load more toward the capacitive power factor. This is because the power company, as you have implied, charges more for power that is current shifted away from zero %. I have even seen large motors installed in industrial situations that run continuously without load, because such motors appear as a capacitive load, and indeed are called 'rotary capacitors'. I recall one time when the CFO ordered such a motor turned off because it is wasting power. It took a little plain and fancy instruction by the engineer to let him know that it was actually saving the company money. Sure, L shift predominates in the system, I was just speaking in general terms at that point. The power companies may be playing it both ways though: I was once told a story of a small mill or manufacturing co. and an electro-plating company situated next door to each other. But the mill was inductive (motors) and the electro-plating was effectively capacitive (not sure how that worked), so they pretty much cancelled each other out and weren't contributing to line losses for the power company. The power
Re: RS-232 Tx / Rx monitoring LEDs?
Heh, all you had to say was power factor and I've have understood. I suspected that was the case, but it seemed too easy.. Guess I'm too used to thinking in simple ohmic terms, with Watt Kirchhoff always looming large. Something told me that, in the end, there was no way around dealing with the E^2/R heat - anything else seemed like a thermodynamic cheat. On Sun, Aug 23, 2015 at 11:54 AM, Steve Moulding fti1...@xmission.com wrote: -Original Message- From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Brent Hilpert Sent: Sunday, August 23, 2015 3:07 AM To: General Discussion: On-Topic and Off-Topic Posts Subject: Re: RS-232 Tx / Rx monitoring LEDs? On 2015-Aug-22, at 11:55 PM, drlegendre . wrote: On Sun, Aug 23, 2015 at 1:17 AM, Chuck Guzis ccl...@sydex.com wrote: On 08/22/2015 10:23 PM, dwight wrote: I would think the reverse voltage sum of the diodes is enough. Different diodes also can handle different voltages. Since the sum of the forward voltages is enough to handle AC, I'd suspect the reverse voltages each would handle is quite small as well. The problem is when the current limiting is done with a resistor that in the forward direction drops a lot of voltage. The diode has to handle the voltage until breakdown when reversed. If the resistor was handling 1 Watts, with the right break down, the LED could be taking .5 Watts. This is more than most are designed for. ...and that's just the nub of it. The success of this depends largely on the consistent characteristics of every LED in the string. Since LEDs tend to fail short if submitted to overvoltage, I've often wondered if a spike in the AC supply would precipitate a cascade failure in the string. I've looked hard and there are no rectifier diodes in the string--just the LEDs themselves. Probably saves about 5 cents or so of manufacturing cost. I've also seen LED night lights from China that employ nothing more than a safety capacitor (usually about 104) in series with a resistor connected to two back-to-back LEDs, all across the AC line. I've wondered what the lifetime of such a setup is. --Chuck. I've also seen C-R series voltage dropping circuits, here there. Correct me if I'm wrong, but doesn't the series cap dissipate power just as it would, were it a series resistor? I mean, if the LED is passing 20mA, the cap is also doing 20mA - and at whatever the Vdrop is. Right? If not, why? I doubt if any brief explanation here is going to the topic justice. Look up power factor or reactive power. FWIW: The impedance (capacitive reactance, Z=Xc=1/(2*pi*f*C) of the C does produce the desired voltage drop but the C also shifts the phase of the current relative to that of the V. To apply the power equation P=VI properly, you can't just multiply the RMS values together, you multiply the instantaneous values of the V I sine waves together through a cycle. You get a third sine wave, that for power. If V I are in phase, the power sine wave will all be in the positive region and is real power consumption. When they are out of phase, some portion of the power sine wave will be negative: a portion of the energy the C sucked down the line is being returned during each cycle. Yes, it does reduce energy consumption relative to a purely R solution. On a large scale, the power company doesn't like it because it unnecessarily adds to the currents circulating in the system, but then, this is from C which shifts the current in one direction, so it's doing some compensation for the inductive wall warts you have plugged in around the house, which as L shift the current in the other direction. --- Brent, that is an excellent explanation in just a few sentences. One quibble however. The power company does indeed like components that shift the current in the capacitive direction. Taken as a whole for the power grid, the power source sees the load as inductive because of all the industrial motors it powers, including the ones that exist in almost every home (washer, dryer, heating/air conditioning, mixer, disposal, etc.) Capacitor banks are frequently installed in large industrial operations to shift the inductive load more toward the capacitive power factor. This is because the power company, as you have implied, charges more for power that is current shifted away from zero %. I have even seen large motors installed in industrial situations that run continuously without load, because such motors appear as a capacitive load, and indeed are called 'rotary capacitors'. I recall one time when the CFO ordered such a motor turned off because it is wasting power. It took a little plain and fancy instruction by the engineer to let him know that it was actually saving the company money.
Re: RS-232 Tx / Rx monitoring LEDs?
On 08/22/2015 11:55 PM, drlegendre . wrote: I've also seen C-R series voltage dropping circuits, here there. Correct me if I'm wrong, but doesn't the series cap dissipate power just as it would, were it a series resistor? I mean, if the LED is passing 20mA, the cap is also doing 20mA - and at whatever the Vdrop is. Right? If not, why? That's correct--but there's the matter of the voltage. Eventually, you'll see that the brightness of the LEDs will drop fairly rapidly before the thing is useless. A common dodge used by Chinese manufacturers. Saves a few components, doesn't it. --Chuck
Re: RS-232 Tx / Rx monitoring LEDs?
On 08/23/2015 07:10 AM, dwight wrote: I've used the capacitor method to provide most of the drop in the past. I don't usually max out the LEDs at 20ma. I find there is little difference between 10 and 20ma. Yes, the 10 ( or 20ma ) is current flow through the capacitor. It is necessary to have some resistor in series as well to suppress line spikes. Another similar dirty trick back in the day was to run a 6SL7 dual-triode form the line using a 1.0 uF nonpolar capacitor in series with the line to provide a supply for the 500 ma 6.3 v heater and then use one of the triode sections as a half-wave rectifier. You thus had the other triode section for whatever stupid purpose. Of course, this was horrible abuse of the tube, particularly in the area of heater-cathode voltage ratings. It probably wouldn't work as well in 220VAC countries, but it worked well enough in the 120VAC ones. --Chuck
RE: RS-232 Tx / Rx monitoring LEDs?
I've also seen C-R series voltage dropping circuits, here there. Correct me if I'm wrong, but doesn't the series cap dissipate power just as it would, were it a series resistor? I mean, if the LED is passing 20mA, the cap is also doing 20mA - and at whatever the Vdrop is. Right? If not, why? Yes, the capacitor passes 20mA. But unlike a resistor the voltage drop across a capacitor is not in phase with the current. For a pure capacitor, it is out of phase by 90 degrees. For a pure inductor, BTW, it is 90 degrees out in the oposite direction. Now, if you consider the power at any instant, it is, indeed the product of the voltage and current at that instant. For a resistor, this is alwas positive, so if you add up all the instantaneous powers over a full cycle (mathematically, this is integration, of cource), the total power consumed is positive. But for a capacitor, sometimes the instantaneous power is negative, in effect the capacitor is supplying energy back to the circuit. And if you integrate that over a full cycle, you end up with zero. Intuitively : With a resistor, voltage and current are either both positive or both negative, so the product is positive. With a capacitor, due to that phase shift, there are 4 regions to consider : V +ve., I +ve; V +ve, I -ve; V -ve, I -ve; V -ve, I+ve. The first and third of those give a +ve power, the second and fourth a -ve power and they exactly cancel out. It turns out that the power in an AC circuit can be calcualted as the product of the RMS voltage, RMS current and the cosine of the phase angle between them. The RMS values are the ones normally quotes (115V mains had an RMS value of 115V). The cosine(phase) term is known as the 'power factor', and is probably the thing to look up in a book on AC electric circuits. -tony
Re: RS-232 Tx / Rx monitoring LEDs?
On 2015-Aug-22, at 11:55 PM, drlegendre . wrote: On Sun, Aug 23, 2015 at 1:17 AM, Chuck Guzis ccl...@sydex.com wrote: On 08/22/2015 10:23 PM, dwight wrote: I would think the reverse voltage sum of the diodes is enough. Different diodes also can handle different voltages. Since the sum of the forward voltages is enough to handle AC, I'd suspect the reverse voltages each would handle is quite small as well. The problem is when the current limiting is done with a resistor that in the forward direction drops a lot of voltage. The diode has to handle the voltage until breakdown when reversed. If the resistor was handling 1 Watts, with the right break down, the LED could be taking .5 Watts. This is more than most are designed for. ...and that's just the nub of it. The success of this depends largely on the consistent characteristics of every LED in the string. Since LEDs tend to fail short if submitted to overvoltage, I've often wondered if a spike in the AC supply would precipitate a cascade failure in the string. I've looked hard and there are no rectifier diodes in the string--just the LEDs themselves. Probably saves about 5 cents or so of manufacturing cost. I've also seen LED night lights from China that employ nothing more than a safety capacitor (usually about 104) in series with a resistor connected to two back-to-back LEDs, all across the AC line. I've wondered what the lifetime of such a setup is. --Chuck. I've also seen C-R series voltage dropping circuits, here there. Correct me if I'm wrong, but doesn't the series cap dissipate power just as it would, were it a series resistor? I mean, if the LED is passing 20mA, the cap is also doing 20mA - and at whatever the Vdrop is. Right? If not, why? I doubt if any brief explanation here is going to the topic justice. Look up power factor or reactive power. FWIW: The impedance (capacitive reactance, Z=Xc=1/(2*pi*f*C) of the C does produce the desired voltage drop but the C also shifts the phase of the current relative to that of the V. To apply the power equation P=VI properly, you can't just multiply the RMS values together, you multiply the instantaneous values of the V I sine waves together through a cycle. You get a third sine wave, that for power. If V I are in phase, the power sine wave will all be in the positive region and is real power consumption. When they are out of phase, some portion of the power sine wave will be negative: a portion of the energy the C sucked down the line is being returned during each cycle. Yes, it does reduce energy consumption relative to a purely R solution. On a large scale, the power company doesn't like it because it unnecessarily adds to the currents circulating in the system, but then, this is from C which shifts the current in one direction, so it's doing some compensation for the inductive wall warts you have plugged in around the house, which as L shift the current in the other direction.
Re: RS-232 Tx / Rx monitoring LEDs?
On 08/22/2015 10:23 PM, dwight wrote: I would think the reverse voltage sum of the diodes is enough. Different diodes also can handle different voltages. Since the sum of the forward voltages is enough to handle AC, I'd suspect the reverse voltages each would handle is quite small as well. The problem is when the current limiting is done with a resistor that in the forward direction drops a lot of voltage. The diode has to handle the voltage until breakdown when reversed. If the resistor was handling 1 Watts, with the right break down, the LED could be taking .5 Watts. This is more than most are designed for. ...and that's just the nub of it. The success of this depends largely on the consistent characteristics of every LED in the string. Since LEDs tend to fail short if submitted to overvoltage, I've often wondered if a spike in the AC supply would precipitate a cascade failure in the string. I've looked hard and there are no rectifier diodes in the string--just the LEDs themselves. Probably saves about 5 cents or so of manufacturing cost. I've also seen LED night lights from China that employ nothing more than a safety capacitor (usually about 104) in series with a resistor connected to two back-to-back LEDs, all across the AC line. I've wondered what the lifetime of such a setup is. --Chuck.
RE: RS-232 Tx / Rx monitoring LEDs?
From: ccl...@sydex.com On 08/21/2015 08:36 AM, dwight wrote: I was going to add something but it has already been said several times. I will add that if using a LED on an AC like signal of high voltage, one should use a diode. I recommend using a shunt diode rather than a series diode when high voltages are being dropped by the resistor. It reduces the need for a high voltage diode but makes the resistor hotter. Some red LEDs glow orange when not protected from 12VAC. You can ask how I know. Dwight Ever take a close look at a string of Christmas-tree LEDs? Most are composed of a string of LEDs hooked directly across the AC line--no rectifier diode to be found. Some seek to reduce the 60Hz flicker by employing two strings to illuminate on both half-cycles, reducing the flicker somewhat. Since my eyes react to the flicker (it's like ants crawling over the string), I found that simply employing a full-wave bridge rectifier can reduce the appearance of flicker tremendously. --Chuck I would think the reverse voltage sum of the diodes is enough. Different diodes also can handle different voltages. Since the sum of the forward voltages is enough to handle AC, I'd suspect the reverse voltages each would handle is quite small as well. The problem is when the current limiting is done with a resistor that in the forward direction drops a lot of voltage. The diode has to handle the voltage until breakdown when reversed. If the resistor was handling 1 Watts, with the right break down, the LED could be taking .5 Watts. This is more than most are designed for. Dwight
RS-232 Tx / Rx monitoring LEDs?
I was going to add something but it has already been said several times. I will add that if using a LED on an AC like signal of high voltage, one should use a diode. I recommend using a shunt diode rather than a series diode when high voltages are being dropped by the resistor. It reduces the need for a high voltage diode but makes the resistor hotter. Some red LEDs glow orange when not protected from 12VAC. You can ask how I know. Dwight
Re: RS-232 Tx / Rx monitoring LEDs?
On Fri, Aug 21, 2015 at 8:36 AM, dwight dkel...@hotmail.com wrote: I was going to add something but it has already been said several times. I will add that if using a LED on an AC like signal of high voltage, one should use a diode. I recommend using a shunt diode rather than a series diode when high voltages are being dropped by the resistor. It reduces the need for a high voltage diode but makes the resistor hotter. Some red LEDs glow orange when not protected from 12VAC. You can ask how I know. Dwight You can also turn on all of the pretty blues lights in a UV-erasable PROM by putting it in the PROM burner the wrong way around. -- Charles
RE: RS-232 Tx / Rx monitoring LEDs?
for a high voltage diode but makes the resistor hotter. Some red LEDs glow orange when not protected from 12VAC. I discovered (over 35 years ago) that green LEDs glow orange if massively overcurrented (you know what I mean). No they don't last long like that. It doesn't appear to be a thermal effect though. -tony
Re: RS-232 Tx / Rx monitoring LEDs?
On 08/21/2015 08:36 AM, dwight wrote: I was going to add something but it has already been said several times. I will add that if using a LED on an AC like signal of high voltage, one should use a diode. I recommend using a shunt diode rather than a series diode when high voltages are being dropped by the resistor. It reduces the need for a high voltage diode but makes the resistor hotter. Some red LEDs glow orange when not protected from 12VAC. You can ask how I know. Dwight Ever take a close look at a string of Christmas-tree LEDs? Most are composed of a string of LEDs hooked directly across the AC line--no rectifier diode to be found. Some seek to reduce the 60Hz flicker by employing two strings to illuminate on both half-cycles, reducing the flicker somewhat. Since my eyes react to the flicker (it's like ants crawling over the string), I found that simply employing a full-wave bridge rectifier can reduce the appearance of flicker tremendously. --Chuck
Re: RS-232 Tx / Rx monitoring LEDs?
Totem pole outputs have comparable drive strength in both directions, that$ That's true for CMOS outputs. TTL outputs pull down much more strongly than$ Ok, but when you refer to drive strength I assumed you were talking about $ Maybe _rated_ current, but, even there, I don't think so (my TTL doc hasn't been unpacked yet, or I'd go check, but I'm fairly sure they are generally specced to sink more current to GND than source from Vcc). Certainly, look at the equivalent circuit for a TTL totem-pole output: there's a resistor in series with the top half, to Vcc, but to GND there's nothing but the E-C of a transistor. Or just take a chip you can afford to sacrifice :-), get it to drive an output low, connect it to Vcc, and observe that it fries the output; ground a driven-high output and notice that it does nothing much. But make sure it's real TTL first; as others have pointed out, this is not, in general, true of TTL-compatible CMOS circuits - the TTL compatability refers to input thresholds and Vcc and the like. This does have the arguable advantage that, if you accidentally wire two outputs together, it doesn't usually fry anything if they fight. I'm inclined to doubt that's the reason, though; it seems more likely to me that the resistor was put there to give warm fuzzies about ensuring nothing fries if the output transistors happen to both turn on for a split nanosecond as signals propagate through the circuit. But even that is pure speculation on my part. /~\ The ASCII Mouse \ / Ribbon Campaign X Against HTMLmo...@rodents-montreal.org / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B
Re: RS-232 Tx / Rx monitoring LEDs?
A consideration with RS232 is that the signals swing to either side of 0, so$ It's been a while since I read the spec, but I think it's -3 to -20 volts one way and +3 to +20 the other, with the -3 to +3 range deliberately left ambiguous. I think there are slew rate limits, too, but I don't recall what they are even approximately. Of course, serial lines are normally run over unshielded wires with a single shared signal ground, so, for noise tolerance, transmitters usually drive the outputs substantially beyond 3V either side of ground. (There are also short-circuit tolerance specs which mean you don't want your output impedance to be too low, either. I think it's something like, any pins or ccombinations of pins may be shorted to one another and/or any voltage or voltages from -20 to +20 indefinitely without damage.) /~\ The ASCII Mouse \ / Ribbon Campaign X Against HTMLmo...@rodents-montreal.org / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B
Re: RS-232 Tx / Rx monitoring LEDs?
On Thu, 8/20/15, Mouse mo...@rodents-montreal.org wrote: Maybe _rated_ current, but, even there, I don't think so (my TTL doc hasn't been unpacked yet, or I'd go check, but I'm fairly sure they are generally specced to sink more current to GND than source from Vcc). It so happens I have a TTL handbook to hand at the moment. From the 1985 TI TTL data book: Ioh Iol 7400 -0.4 16 74H00 -0.5 20 74LS00 -0.4 8 74S00 -1 20 all in mA. So as rated, TTL devices can sink a solid order of magnitude more current than they can source. BLS
Re: RS-232 Tx / Rx monitoring LEDs?
On Aug 20, 2015, at 09:54 , Paul Koning paulkon...@comcast.net wrote: Ok, but when you refer to drive strength I assumed you were talking about current, not voltage. By that measure totem pole outputs are pretty much symmetrical. Again, CMOS totem pole outputs are pretty much symmetrical, but TTL totem pole outputs aren't even close. Looking at a Fairchild 74LS04 datasheet, I see a 20x difference in recommended drive-high current (Ioh = 0.4mA) vs. recommended drive-low current (Iol = 8mA). And the voltages are correspondingly different, too: at rated drive current of 0.4mA, Voh = 2.7V (min) to 3.4V (typ) with a 5V supply, while Vol is 0.35V (typ) to 0.5V (max) at 8mA drive current. So that's 1.6V drop from Vcc while sourcing a mere 0.4mA, vs. 0.5V rise from GND while sinking 8mA. Totem pole outputs just mean that the output driver actively drives both up and down, with two stacked drive transistors. It does not imply that the drive strengths are even close to being equal, particularly when we're talking about TTL logic in a vintage computer. Incidentally, TTL inputs also present asymmetric loads for high vs. low inputs at about the same ratio (18x input current ratio in the 74LS04 example when driven at the input thresholds), and have asymmetrical input threshold voltages. So unloaded TTL output voltages aren't relevant if we assume that the output is driving a TTL input of the same logic family. Even if a TTL output appears to drive all the way up to Vcc with no load, it won't once it's driving a typical load. So you might think of those TTL totem pole drivers as being symmetrical when they're strictly driving TTL inputs of the same family, since the TTL inputs and TTL outputs are designed to work together. But they're very strongly asymmetrical when driving things other than TTL inputs, such as the LEDs in question here. -- Mark J. Blair, NF6X n...@nf6x.net http://www.nf6x.net/
Re: RS-232 Tx / Rx monitoring LEDs?
On Aug 19, 2015, at 11:40 PM, dwight dkel...@hotmail.com wrote: I think the main issue is that TTL is usually a weak pullup and a hard pulldown. RS 232 levels are equal up and down. That depends. Open collector outputs, yes, but those are the less common case. Totem pole outputs have comparable drive strength in both directions, that's precisely their purpose (to provide symmetric rise/fall times when driving capacitive loads). Putting it on the RS232 lines could even improve signal quality because the signal is usually poorly terminated, causing reflections. A consideration with RS232 is that the signals swing to either side of 0, so if you use an LED referenced to 0, it either needs a series diode or a sufficiently high reverse voltage rating. And the driver voltage range spec for RS232 is quite loose; it might be just a couple of volts but it can be as high as 15 volts or so. paul
Re: RS-232 Tx / Rx monitoring LEDs?
On Thu, Aug 20, 2015 at 10:54 AM, Paul Koning paulkon...@comcast.net wrote: Ok, but when you refer to drive strength I assumed you were talking about current, not voltage. By that measure totem pole outputs are pretty much symmetrical. Not for true (bipolar) TTL. See the specs for the 7400, 74LS00, 74S00: http://www.ti.com/lit/ds/symlink/sn74ls00.pdf Ioh(max) Iol(max) 7400 -0.4 16 74LS00-0.4 8 74S00 -120 CMOS for comparison: http://www.ti.com/lit/ds/symlink/sn74hct00.pdf http://www.ti.com/lit/ds/symlink/sn74act00.pdf Ioh(max) Iol(max) 74HCT00 -44 74ACT00 -24 24
Re: RS-232 Tx / Rx monitoring LEDs?
On Aug 20, 2015, at 3:40 PM, Eric Smith space...@gmail.com wrote: On Thu, Aug 20, 2015 at 10:54 AM, Paul Koning paulkon...@comcast.net wrote: Ok, but when you refer to drive strength I assumed you were talking about current, not voltage. By that measure totem pole outputs are pretty much symmetrical. Not for true (bipolar) TTL. See the specs for the 7400, 74LS00, 74S00: http://www.ti.com/lit/ds/symlink/sn74ls00.pdf Ioh(max) Iol(max) 7400 -0.4 16 74LS00-0.4 8 74S00 -120 CMOS for comparison: http://www.ti.com/lit/ds/symlink/sn74hct00.pdf http://www.ti.com/lit/ds/symlink/sn74act00.pdf Ioh(max) Iol(max) 74HCT00 -44 74ACT00 -24 24 Ok, clearly my memory was faulty. Thanks for the corrections. paul
Re: RS-232 Tx / Rx monitoring LEDs?
On Aug 20, 2015, at 11:58 AM, Mark J. Blair n...@nf6x.net wrote: On Aug 20, 2015, at 06:27, Paul Koning paulkon...@comcast.net wrote: Totem pole outputs have comparable drive strength in both directions, that's precisely their purpose (to provide symmetric rise/fall times when driving capacitive loads). That's true for CMOS outputs. TTL outputs pull down much more strongly than they pull up, which is why older designers are still in the habit of driving LEDs with active low outputs even though active high outputs work just fine with modern CMOS logic. Look at nearly any TTL datasheet, and note that VOL is much closer to ground than VOH is to VCC. Ok, but when you refer to drive strength I assumed you were talking about current, not voltage. By that measure totem pole outputs are pretty much symmetrical. palu
RE: RS-232 Tx / Rx monitoring LEDs?
A consideration with RS232 is that the signals swing to either side of 0, so if you use an LED referenced to 0, it either needs a series diode or a sufficiently high reverse voltage rating. A diode in inverse parallel with the LED Is more normal when you want to run an LED off effectively an AC supply. In the case of RS232 signals it is common to use a red and a green LED in inverse parallel, either separate LEDs or one of the 2-wire bicolour LEDs which are precisely a pair of LEDs in inverse parallel. Most of the little RS232 terters are just that, I think with 3k series resistors for the LED pairs. -tony
Re: RS-232 Tx / Rx monitoring LEDs?
On Aug 20, 2015, at 06:27, Paul Koning paulkon...@comcast.net wrote: Totem pole outputs have comparable drive strength in both directions, that's precisely their purpose (to provide symmetric rise/fall times when driving capacitive loads). That's true for CMOS outputs. TTL outputs pull down much more strongly than they pull up, which is why older designers are still in the habit of driving LEDs with active low outputs even though active high outputs work just fine with modern CMOS logic. Look at nearly any TTL datasheet, and note that VOL is much closer to ground than VOH is to VCC. -- Mark J. Blair, NF6X n...@nf6x.net http://www.nf6x.net/
RE: RS-232 Tx / Rx monitoring LEDs?
I think the main issue is that TTL is usually a weak pullup and a hard pulldown. RS 232 levels are equal up and down. Putting it on the RS232 lines could even improve signal quality because the signal is usually poorly terminated, causing reflections. The recommended lengths and Baud rates are intended to account for this. Usually the line is lower impedance than the input termination, requiring the source to absorb the reflected signal. This is why 422/485 came to be. The impedance of the loads better match that of the termination. Dwight Subject: Re: RS-232 Tx / Rx monitoring LEDs? From: n...@nf6x.net Date: Tue, 18 Aug 2015 21:27:19 -0700 To: cctalk@classiccmp.org Those little RS232 testers with LEDs built into a double DB25 connector box are usually just made of LEDs and resistors connected to each signal line. They can load signals enough to cause problems at high speeds, with weak drivers, or with long cables, but usually they don't cause problems. If you're concerned, you could always include jumpers or switches to disconnect the LEDs when they're not needed. Of course, buffering the TTL signals eliminate any such problems. But on the other hand, using two LEDs connected with opposite polarities on each RS232 level signal lets your discriminate between driven positive, driven negative, and open. That can come in handy when debugging things where the other end may or may not be driving properly, or may be mis-wired. For an example, feel free to take a look at this little modular jack RS232 tester that I made: https://github.com/NF6X/YostTester/blob/master/YostTester.pdf The red/green LED pairs show whether each line is high, low or open. Resistor values may vary depending on the LEDs that you choose. -- Mark J. Blair, NF6X n...@nf6x.net http://www.nf6x.net/
RS-232 Tx / Rx monitoring LEDs?
As part of my C-64 RS-232 to M15 60mA CL project, I've put together a simple MAX232 based interface to connect the TTL levels on the C-64 to the standard RS-232 +/- levels. Is there any reason that I can or cannot install LEDs - on either side of the MAX232 converter - to give some indication of line status? Specifically, a pair of LEDs, one each for Tx and Rx lines, to blink / etc. as line status changes and data moves. I don't see any obvious issue - but I can conceive of a situation where it might screw up the line in some way I haven't considered. Comments?
Re: RS-232 Tx / Rx monitoring LEDs?
On 8/18/2015 10:40 PM, drlegendre . wrote: As part of my C-64 RS-232 to M15 60mA CL project, I've put together a simple MAX232 based interface to connect the TTL levels on the C-64 to the standard RS-232 +/- levels. Is there any reason that I can or cannot install LEDs - on either side of the MAX232 converter - to give some indication of line status? Specifically, a pair of LEDs, one each for Tx and Rx lines, to blink / etc. as line status changes and data moves. I don't see any obvious issue - but I can conceive of a situation where it might screw up the line in some way I haven't considered. Comments? Probably best to feed the TTL RX and TX lines to a 74'04 and then hook the output to a 330 ohm and then the cathode of a LED, with anode going to 5V. But, probably good enough is to just put the diode cathode on the TX and RX lines, 330 ohm to 5V, and have fun. Jim -- Jim Brain br...@jbrain.com www.jbrain.com
Re: RS-232 Tx / Rx monitoring LEDs?
Those little RS232 testers with LEDs built into a double DB25 connector box are usually just made of LEDs and resistors connected to each signal line. They can load signals enough to cause problems at high speeds, with weak drivers, or with long cables, but usually they don't cause problems. If you're concerned, you could always include jumpers or switches to disconnect the LEDs when they're not needed. Of course, buffering the TTL signals eliminate any such problems. But on the other hand, using two LEDs connected with opposite polarities on each RS232 level signal lets your discriminate between driven positive, driven negative, and open. That can come in handy when debugging things where the other end may or may not be driving properly, or may be mis-wired. For an example, feel free to take a look at this little modular jack RS232 tester that I made: https://github.com/NF6X/YostTester/blob/master/YostTester.pdf The red/green LED pairs show whether each line is high, low or open. Resistor values may vary depending on the LEDs that you choose. -- Mark J. Blair, NF6X n...@nf6x.net http://www.nf6x.net/
