Hi The gotcha with "standard" RS-232 is that the range from -3 to +3 volts is defined as "undefined". In other words, a negative should be below -3V and a positive should be above +3 volts. Indeed most (but not all) chips count zero as a negative. A few have hysteresis and others have some odd thresholds.
Bob On Jul 26, 2013, at 12:14 PM, Angus <[email protected]> wrote: > Hi, > > #5 is one of the reasons I preferred not to take the signal negative. I > suppose it depends on the situation, but for the spitting data into a pc I > used to find that a 0 to 5v or so signal powered from a control line almost > always worked. The most common problem was either a faulty com port or an > optocoupler circuit that did not have enough gain to pull the line close to > 0v. ( that was a while ago though - not so much 'legacy' as 'vintage'! ) > > Angus. > > > From: "Bob Camp" > To: "Discussion of precise time and frequency measurement" > Sent: July 26, 2013 1:37 PM > Subject: Re: [time-nuts] RS 232 > > Hi > > In order to get voltages from the other pins on the D connector: > > 1) You have to program them to be in the "correct" state (either high or low) > 2) They have to be present on the "driver" side (usually, but not always true) > 3) They need to be > 3 V (diode drops etc…) > 4) They need to supply enough current (as in no isolating resistors, usually > true) > 5) You need to have two of them to spare that meet these criteria (one > positive and one negative) > > Its a little like rolling dice, sometimes it will work, sometimes it won't > work. > > Bob > > On Jul 26, 2013, at 7:01 AM, [email protected] wrote: > >> Since there is repeated request for the circuit se attachment. My question >> is what can be done to get a positive voltage from some of the other unused >> pins of the DB 9 connector. What is needed is any where from 3 to 15 volt. >> Diodes work with 1 mA but I set Rx for 2 mA. Any thing else look at H11 >> data sheet. >> Bert Kehren >> >> >> In a message dated 7/26/2013 4:06:37 A.M. Eastern Daylight Time, >> [email protected] writes: >> >> FWIW, it was clear as mud for me too. >> >> Bert began with, "Since joining time nuts over four years ago I have >> not used a single MAX >> 232 chip. Two reasons MAX do not give me isolation and do generate noise >> in critical applications." >> >> From that I took that he was doing RS232 using opto isolators. That >> implies + and - voltages to me. Where do they come from and where are >> they relative to the isolation boundary? If the goal is really >> isolation, how do these supplies get isolated? If the noise is cured by >> isolation, these details seem important as the supplies need isolation >> too. Maybe the switcher in the MAX232 is causing the noise. Then how do >> we get a negative supply from, say, +5V without the noise? Then, maybe >> he was saying RS232 sucks and this other way (not RS232 compatible) >> works better. >> >> The word picture of Bert's solution, which provided more details, left >> me less much less than clear too. Maybe I'm just not up on circuit >> shorthand terminology enough to follow what sort of current limiter is >> limiting what current to what, and what is being blocked by a diode from >> which negative level. Not really sure if I even got the big picture of >> what he is describing. Is it an isolated equivalent of a MAX232 >> interface or something else that wouldn't talk to an RS232 device? >> >> So, more clarification, or possibly that picture (~= 1k words) might >> help. Or maybe I'm just obtuse and everyone else is getting it. (Seems >> Marki may also be in the confused camp.) >> >> >> >> On 7/25/2013 3:34 PM, Mark C. Stephens wrote: >>> Although your description, >>> " I prefer the use of two H11 opto couplers which >>> work perfect. On the receiving end the diode along with a current >> limiter >>> and blocking diode for the negative level works perfect. On the output >> side >>> a power source is needed." Is a perfect circuit description, I'd be >> more confident with a schematic :) >>> >>> >>> --marki >>> >>> >>> -----Original Message----- >>> From: [email protected] [mailto:[email protected]] On >> Behalf Of [email protected] >>> Sent: Friday, 26 July 2013 5:32 AM >>> To: [email protected] >>> Subject: Re: [time-nuts] RS 232 >>> >>> I do not understand your question, I am referring to low noise >> applications like counters for dual mixers or other AV measurements, but >> also Shera >> and even Tbolt where external noise should be kept to a minimum. When you >> chase 1 E-14, isolation is key and I always like to err on the cautious >> side and as I stated we use blue tooth or USB but in the case of USB there >> are always H11 in the circuit. Some still like to use RS 232 and the >> subject came up and I have on my boards H11's like on the counter Corby >> uses >> but he ended up using an external power source and I like to eliminate >> that >> requirement. David had the right answer using the power that the RS 232 >> mouse uses out of a DB 9, started looking but I do not have one any more >> and I can not find any data. >>> Bert >>> >>> >>> In a message dated 7/25/2013 2:48:43 P.M. Eastern Daylight Time, >> [email protected] writes: >>> >>> Hi Bert, >>> >>> I am sure your circuit is clear in your head, but would you mind >> attaching detail? >>> You have perked my interest with the "low-noise" keyword ;) >>> >>> >>> -marki >>> >>> -----Original Message----- >>> From: [email protected] [mailto:[email protected]] On >>> Behalf Of [email protected] >>> Sent: Thursday, 25 July 2013 11:39 PM >>> To: [email protected] >>> Subject: [time-nuts] RS 232 >>> >>> Since joining time nuts over four years ago I have not used a single >> MAX >>> 232 chip. Two reasons MAX do not give me isolation and do generate >> noise >>> in critical applications. I prefer the use of two H11 opto couplers >> which >>> work perfect. On the receiving end the diode along with a current >> limiter >>> and blocking diode for the negative level works perfect. On the output >> side >>> a power source is needed. If one uses an USB adapter it does have the >> + 5 >>> volt which again works perfect. How ever many prefer to use RS 232 >>> direct and that is why I hope to get some comments and suggestions from >> the >>> list. Corby used on our counter circuit that he described a separate >> power >>> source. >>> Present MAX circuits use a + 1.4 volt threshold but considering legacy >>> the question is what should the voltage swing be to make it compatible >> for >>> most PC's and what options exist to get the necessary voltage. In the >> past >>> other pins on the RS 232 port where an ideal power source. Still an >> option? >>> Bert Kehren >>> _______________________________________________ >>> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
