Experimental Physics and
| |||||||||||||||
|
If the "random characters" tend to occur between normal traffic (i.e. when a device is "driving" the signal lines) or at the end of normal transmissins, that could be an indication that the RS-485 connection is not properly terminated and/or biased. I strongly recommend using digital scope to monitor the voltage levels of the signal wires to see if they are at a sufficient voltage level when nothing is driving the line. When we where having this sort of problem a couple of years ago with a couple of devices using RS-485 (or RS-422), I read several different how-to's and explanations regarding how RS-485/RS-422 connections work and should be set up. Unfortunately, there is one aspect of these descriptions where people seem to be roughly evenly split: Should have/do you need a common reference for the signal voltages. II suppose this is understandable, since what I read says the original standard said that insuring a "common return path" was optional, but the the newer ANSI/TIA/EIA-485 standard REQUIRES it for proper operation. In our case, the problem was caused by not having the signal wires sufficiently biased to keep the voltage difference > 200mV when no device is driving them. In our case the Moxas we use for the affected devices include pull up/down resistors that can be enabled and proved sufficient to satisfy this requirement. If it would be at all helpful, I have some notes I took when learning about all this as well as still pictures and some video of what I found when diagnosing such problems for a couple of the devices at our lab. Mark Davis On 4/6/2020 7:48 PM, Sobhani, Bayan via
Tech-talk wrote:
| ||||||||||||||
ANJ, 14 Apr 2020 |
·
Home
·
News
·
About
·
Base
·
Modules
·
Extensions
·
Distributions
·
Download
·
· Search · EPICS V4 · IRMIS · Talk · Bugs · Documents · Links · Licensing · |