RS-485 Question (Troubleshooting)

Sounds like DMX-512, which is a simple 250kbaud protocol.

Ideally you'd use two channels in A-B mode and connect the ground on the scope ot the ground lead.

There's a good chance that at least that one is no longer usable. ;-)

Best regards, Spehro Pefhany

A 120 to 220 ohm resistor would terminate the line for troubleshooting. The data is balanced, so if you have a dual channel scope look at both lines at the same time. National Semiconductor has some useful information on RS485:

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Thanks Mike. You and Spehro have the same basic approach.

It's a Strand controller, by the way.

-mpm

I have never worked on a state of the art lighting system, but I have troubleshot some RS422 & RS485 problems. If you have some spare interface chips you should build an interface to convert the bi-directional data to single ended. You could also add a transmit/receive switch, and a 0/1 data switch to see if yo can take control of the line. Add a 555 to generate a string of pulses, so you can identify your data stream. That makes it easier to troubleshoot the input of the failed equipment. (Maybe there would be a market for a simple, and reasonably priced test set?

I think that DMX-512 protocol was brand new while I worked at Ch. 55, and never had to work on it. It as a turnkey install, when the studio was built and just a couple months old when I was hired in 1987.

rked at Ch. 55,

WCNC-TV (6) Charlotte had an ugly, old Kligel system. (The kind you had to plug in by hand - patch cables everywhere...)

Was still in use when they moved to their new studios. Back around

I don't think the Strand uses DMX, but it might.? I am just going to assume that if it's spitting out any data at all, then is OK. If it's not, it won't matter because it's all surface mount, and I don't have that much time to donate to the Church. It's a really nice controller, though. Strand wants a fortune for a service call. (Out of Atlanta, daily minimums, etc...)

I did manage to get their fire alarm, burglar alarm, and PBX back up today. Mostly just blown fuses and an occasional reset here and there. (Don't tell anyone.)

The Strand has about a 200-foot antenna, er, I mean RS-485 link. It's also in outside conduit - a clear sign of an afterthought if ever there was one... The Strand looks to have taken the brunt of the "non-easily-fixed" damage.

I'm kinda hoping the new 485 transceivers do the trick. If it's a burned cable, that's gonna be a bitch to pull...

-mpm

That sounds like the damage to the original Ch 55 studio & offices in Leesburg. I lost the 11 GHz CARS used as the original STL, (long story) all the computer terminals, the KTU cards & power supply in the 1A2 phone system, some U-Matics, four LNAs, electrical outlets & switches, and other odds & ends. It took all day to get the phones & computer system limping by with one terminal, and arrangements to hand carry tapes to the transmitter site. I managed to borrow a bucket truck from the local CATV office to change the LNAs. It was six months before everything was repaired, and back in full service. It was day two when I learned to troubleshoot bad SMPS 'non field repairable' power supplies. :( I didn't have much choice. We couldn't find any available replacement terminals, and the ALTOS 986 computer was running the entire accounting & membership database under North Star Basic.

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Well, here's the update (after several hours working today..)

It's a Strand model 300, which they don't make anymore. (8 years old) It does use DMX512. I scoped the bus and nothing coming out the XLR's on the back. Programming looks good.

On the dimmer packs, I shotgunned all the LTC485 transceivers, but (of course!) that did not help. So, I un-did all the screws and plug-in jacks to peek at the bottom of the board for lifted traces, etc.. No problems there.

Did a continuity check on the cabeling - everything looked good there too. I was able to control the lights manually (panic mode), and also dim them with the analog controls. So, it looks like the dimmer pak might actually be repaired now - even though I can't test it. (I wish I had another DMX transmitter of any sort..!!) Do they have test gear for this??

So, back to the 300 controller. Opened it up, took out the XLR card, and starting looking back "earlier" in the circuit. Low and behold, just in front of this output card, I can see the data every time I move a slider...!!

So, it looks like the daughter board has a problem. It's a simple circuit, but too much time to trace out by hand. Also, the parts are surface mount, etc.. You know the drill.

There are surface mount components labeled D1, D2, etc, and some marked Z1. I'm guessing these are diodes. The seem to meter out OK in-circuit, but that might not mean too much. My guess is the protection is blown on this board, and that what's killing the comm-link to the dimmer pak.

Either that, or I am about to send back the wrong component to the repair depot. ;?

-mpm

Actually after reading a few more posts, my tech skills kicked in. Measure DCR from A to B Also from each to ground. A to B should be about 120 ohms. It is a differential bus. Either to ground may be half that (the best case) or very high. Since it is designed to be a linear bus, split in method half yields log(2) of n (where n is number of nodes) for single faults and finds multiple faults quickly as well.

HTH

I used to do a lot of surface mount rework, under a stereo microscope. After having a palsy in my right eye, and nerve damage in my left arm I'm not sure I would be able to do it, anymore. I routinely hand soldered 288 pin packages with no problems.

Are the supplies all working to that board? It may be fused, or a separate supply to provide some isolation to the other boards.

BTW, email me & let me know what part of Florida you're in.