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diagnosing non working rs485 bus

I am trying to figure out why my prsamp and keypads do not communicate. I have an elan z630 and z100 keeypads. they communicate through an rj45 jack that has two wires labelled rs485+ and rs485-. The key pads have power and the led light up red. It is suppossed to light green. I opened the preamp and found two toasted fuses near the rj485 and i replaced them. I also replaced 3 old optoisolators because i know they are surge sensitive. Still no communication up and beyond powering up. I am trying to figure out what critical components there are in the rs485 circuitry so I can simply remove and replace. The preamp is part of a larger system and is 10 years old. The company will not repair anymore. So I realize this is a naive question, but should i just pull every small resistor, capacitor, and diode in sight and replace or is there a method I can use. I do not have an oscilloscope all I have is a multimeter.

Thanks

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bill13
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There should be an RS-485 transceiver chip at each end of the link; it's probably got eight pins and the popular one has '485' in the part number.

Each wire of an RS-485 link is going to be at 0V or 5V if there's traffic, and will be floating if there isn't. If there is traffic then the rs485- line will _always_ be opposite of the rs485+ line. That "floating if there isn't" is going to make it nasty to debug without knowing how your specific system behaves, though. Put a voltmeter on it

-- if one line is at 0V or 5V and the other is wiggling then it's a transceiver problem. Both lines being stuck at 0 or 5 doesn't mean much (because they float with no traffic) unless you can determine that they definitely don't wiggle on power up, which is practically impossible to do with a voltmeter.

Ideally you'd look at both lines with an oscilloscope at power up, to see if they both wiggle. If you have a friend with an oscilloscope, now may be the time to buy a pack of beer or a bottle of wine and show up on their doorstep.

If you're just going to replace parts, start with the '485 transceivers

-- they'll be the chips that the '485 lines go to. Success is _not_ guaranteed.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Reply to
Tim Wescott

I

jack

and

preamp

figure

10
a

and

have

I am desperatatly trying to find the transceiver. I googled the id number of every chip on board and non are identified as transceiver. do all rs485 cirucuits have transceivers? thanks bill

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bill13

power

also

is

capacitor,

number.

number

rs485

which carries my rs485 data. it is a max 1483 surface mounted. I will replace as you suggested. Is this a very vulnable part? do i need to locate a driver chip? thanks for adding your expertise, I really am a novice on an uphill learning curve.

again thanks

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bill13

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