RS485 Multidrop Framing Error

[Jed_Davidow]

Hola!

I have an application with 4 identical 2-wire RS485 instruments, all daisy-chained.  Admittedly, I am trying to get this to work with a low-cost RS232 to RS485 adapter.

I have a total of about 5 feet of wire.  4 feet to the 1st device, then about 6in from each device to the next.  Settings are 9600/8/N/1.

With three of the four controllers powered on, all things work perfectly.  No errors.  I can run at 2400, 9600 or 19200 flowlessly.

The moment I turn on power to the 4th controller, I start to see intermittant -1073807253 (0xBFFF006B) framing errors.

It does not matter which of the controllers is turned off, 3 work, 4 do not.  The order in which I scan them also does not seem to matter.

I attempted to put a 120Ohm resistor across the final terminals, the controller terminals, and then both sets.  Each time this caused all communication to die.

I have also tried 9600/7/Odd/1 and 9600/7/even/1, these settings produce the same situation I have described.

Any ideas?  The next step is just to use a second RS485 bus and see if that works, but I'd like ot figure this out, if possible.

[K C]

Hi,

Do you have terminators on both side of the RS-485 interface ? Preferable an active terminator at one side and a passive terminator at the other side.

Are one or more of the interfaces (galvanic) isolated ? If yes adding the 0V connection might help.

Yes, also your RS-485 could be defective with higher loads.

No real solution here but it might help you.

Kees

[K C]

Oh, I forgot. How do you address the different slaves (?) Sure that this correct ?

Kees