rs485 bus on preamp not working

i suspect it will take the audience about 5 secs to realize I am a novice but I have an interesting problem and would like to see if experts can make a novice into a functional apprentice via the web.

In simple terms: I have a preamp that connects to keypad through a rs485 bus communiction port. My keypad's leds all are on red and the trouble shouting brochure states that this means that i have my rs485+ and rs485- misconnected. However I checked it 10 times and I am convinced that a portion of my preamp was knocked out by a surge. I found two burnt out resistors (toasty brown) and I replaced them. Also 3 optoisolators were in the area of the communications port so I replaced them also knowing that they are surge sensitive. So far all leds are on red and not green which is not good. Near by there is a voltage regulator and a lot of mini capacitors both the round and flat type. I was thinking of replacing the regulator but I was wondering if capacitors could be the problem. I am a novice. I just have a digital multimeter and a plain old soldering iron. I do not own oscilloscope or hot air soldering gun which would help with the smc components. So any takers to this challange? The preamp is an elan z630 and the keypads z100. The only warning is I may be a little slower than the group is use to. But I did take "shocks for jocks" and tools for fools in college

wkaye west palm beach

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B&B Electronics has a pretty good app note on RS-422/485 that is worth taking a look at:

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"Misconnected" could be due to swapped polarity but could also be because either the driver or receiver (or its upstream opto) has been toasted.

Since *some* PC serial ports will receive "TTL-level serial," you might be able to look at the data from the driver (from the keypad side?) by connecting the 485 A(-) line to pin 2 on a serial port's DB-9 connector.

You should be able to infer the regulator is working by checking its output with the multimeter. While you're at it, also check that the power and ground are at appropriate levels for chips, such as the optos, that you're able to identify and pull data sheets for.

Rich Webb     Norfolk, VA
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Rich Webb

If you had that much damage, you need to check the copper lans on both sides of the board. It'll possible you have a crack. Use a magnifier.

I'm guessing you have twisted pairs RX+ RX- and TX+ TX-

you can use your DMM in AC mode to get some kind of reading that should indicate activity on either one both pairs.

Make sure you're using the proper cable..

You most likely need a cross over cable.

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