On Friday, April 5, 2013 8:01:34 AM UTC-5, GastonP wrote: > Zitt, > The problem is not that it is an open drain or a CMOS (it is a CMOS and > I put it as so in my first email). The problem is that the base MPSA42 > conected directly to the output effectively shorts the output to ground > unless you put some resistance into that base circuit. > You say you tried putting a resistor in the base of the transistors to no > avail. To avoid unpleasant sudden malfunctions, let the resistors there. >
Gaston, Thanks for the feedback. While valid (regarding shorted A42s); this turned out not to be the cause of the problems. More debug seemed to point to the 4555 1to4 decoder circuit. I pulled the tristate buffer (u4) and solder bridged the outputs of the 1to4 decode straight to the transistors thru a 9.1k resistor. The display does the same thing without U4. This points to U3 (cd4555) being the culprit. Now; given I’ve already verified 7 digit emulation with the arduino which doesn’t work in the machine; seems to be a clue. I’m leaning toward the output of the MPU being TTL compatible; but not CMOS compatible. I don’t think the signal to the displays is meeting CMOS high requirements for digits 5&6. According to my mpu schematics; the digit enables are driven directly by the output of a 6821 PIA. So; next step is going to be to wire in an oscope to see what D5’s levels are. This pretty much sums it up: http://www.flickr.com/photos/52968060@N00/8632599249/sizes/o/in/photostream/ PeakToPeak; my displays (digit enables) are only getting 1.6V for a high. Well below the cmos “good” for a 1. This is measured at R44 and R43 closest to the connector on stock displays. This is the reason why my Nixie design isn’t working. I can design around this with either a transistor or some other translation logic; just not sure why the value is so low. I see why it works on stock displays as all we need is for the high to be > ~0.7V to turn on the digit enable transistors in the original design. Other pinheads confirmed their machines were also outputting ~1.8V for a high instead of the 5V I was expecting. I did some "dead bug" rework to the existing display; basically epoxying two SMT 2N3904s to the top side of U3. I cut some traces and soldered a 20K ohm pull up between VCC and the collector of the transistors. Emitters were tied to ground. Bases were connected to the series resistors currently present in the schematic. http://www.flickr.com/photos/52968060@N00/8638427889/sizes/o/in/photostream/ The goal here is to work out the kinks in the design so they can be incorporated into a FAB B board and be relatively confident the design will work out of the box. Because the BJTs are single transistor inverters; I needed to rewire the input to U3. I wanted to reuse the existing CD4555B chip already present to keep the design simular. Turns out that by swapping A & B inputs vs the schematic; the following boolean logic become obvious: U3A Q1 = !B*A = D5 U3A Q2 = B*!A = D6 U3A Q0 = !B*!A = D7 Remember that when the MPU is driving A5 or A6 high; the BJTs invert that to be an active low. So the boolean math makes it logical. The result (finally): http://youtu.be/c7Q9Ry00e2o Here are some pictures of the display installed in my Bally Star Trek<http://pinside.com/pinball/archive/star-trek-dataeast>machine: http://www.flickr.com/photos/52968060@N00/8638427991/sizes/o/in/photostream http://www.flickr.com/photos/52968060@N00/8638427933/sizes/o/in/photostream Known issues: 1) Digits are too light when running in multiplexed mode (in a real machine); plan to drop the anode resistor to ~2.7k from 22k to brighten the digits. Not a good idea for non-pinball machines which aren't multiplexing the digit enables; but should be fine for more machines. 2) Need to "fix" the native 7 vs emulated 7 digit jumper selector given the need to invert D7 in native mode. -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msg/neonixie-l/-/Oe9o40rhjqQJ. For more options, visit https://groups.google.com/groups/opt_out.
