MIL-STD-188-114A vs. RS-232

"rickman" wrote in news:1143775491.874277.176470 @j33g2000cwa.googlegroups.com:

I did a product many years ago that supported MIL-STD-188-114A (I think). As I recall the main difference was polarity. They had a few other slew rate issues I think as well. I'm fairly sure that we just used 189 and 188 type devices anyway and we never had an issue interfacing anything.

A typical 232 driver has something like a 300 ohm series R. This is to protect it from shorts. You could probably add some Q's then then use a series R

Thanks Al. The polarity is not important as it is programmable in the FPGA. I had considered adding a separate TTL -> 114A interface using discrete transistors, but your suggestion makes it simpler. I can just use a complementary pair, each device in an emitter follower configuration. With a Vcc of +-5 volt, this will pull to better than the 4 volt minimum and a 50 Ohm resistor will provide the current limit. We already have a SOT-23 dual transistor that should do the job. I will have to add a voltage inverter to generate -5 volts. That may be more difficult since I need a true -5 volts. A capacitor inverter circuit using diodes will not get a full -5 volts. I guess I could use FETs, or maybe I can find a very small IC to do the job. Space on the board is very tight though.