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

- R
- rickman
  
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posted
18 years ago

Fri, Mar 31, 2006 3:24 AM

I need to interface to MIL-STD-188-114A which is electrically similar to RS-232. But I am finding that while MIL-STD-188-114A specifies a driver output impedance of 50 Ohms or less, RS-232 does not seem to spec an output impedance at all and can be met with a fairly high value. Looking at data sheets for a few parts, it looks like 100 to

200 Ohms is typical.

I have no doubt that I could use any of the many RS-232 converters on the market and get the job done. But my supervisor is being very cautious and wants to meet the full MIL-STD-188-114A spec. Since none of the chips I have seen will meet the output impedance spec, I am considering using transistors to make my own driver circuit. Anyone know of a better solution to driving MIL-STD-188-114A?

- A
- Al Clark
  
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posted
18 years ago

Fri, Mar 31, 2006 4:38 AM

"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

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Al Clark
Danville Signal Processing, Inc.
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- rickman
  
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posted
18 years ago

Fri, Mar 31, 2006 12:14 PM

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.