BRX Serial Communications Troubles

[Mike Nash]

I am having not much success with getting a BX to talk RS-422 to a device that should work fine (Contrex M-Trim).

I have previously done a very successful project that used a BX-DM1E-36's built-in RS-232/485 port (RS-232 mode) to a FA-ISOCON to a Contrex M-Cut (RS-422) at 9600 baud.

I wanted to skip the FA-ISOCON and use a BX-P-SER422-TERM, so I got that from AD last month late and gave it a try with a M-TRIM (which I have on my desk working with an H2 and F2-UNICON.) I don't have access to an M-CUT until on site, but the comms are the same format.

Problem 1 - the BX-P-SER422-TERM is consistently sending garbage to the M-TRIM. I can play around with the port settings in the BX and get it to work somewhat, but the M-TRIM does show it is not getting a good request frequently. If I try to scope the TX signal, it hoses the comms entirely. I have tried 3 different types of cable/wires, none over 6 feet long.
9600, 7, 1, E, Unconditional, On - should work but doesn't.
9600, 7, 1, E, Delayed 50mS - works sporadically

Problem 2 - I dropped back to the "tried and true" and ordered a FA-ISOCON and used the BX built in RS-232 and THAT also fails to be reliable. This is with the port settings identical to the known good project. I can get it reliable only by putting a 120 ohm or 150 ohm resistor across RXD and COM A on the FA-ISOCON (or TXD and GND on the BX). I should not have to do this as this is RS-232. Note that a 1K resistor is not enough.

Problem 3 - I can get super reliable and clean comms by using a BX-P-SER2-RJ12 and plugging the cable into the RJ12 port on the FA-ISOCON. I can also get perfect comms by straight wiring into the RS-232 terminals on the FA-ISOCON (just proving they are parallel connections.) The problem here is just that I now have 2 additional items required where either of the first two should have worked. 9600, 7, 1, E, Unconditional, On

Yes, all grounds are connected and all of this is on my desk.

It's been over a week since I last messed with this and I don't recall all of the port settings I tried offhand.

I have code to allow the PLC to figure out what the port settings need to be to talk. This allows me to
'maliciously' change the port settings in the M-TRIM and the PLC will self adjust. This works great with the last setup, but fails with either of the first two because the M-TRIM isn't receiving good input even when it should.

The BX is OS 2.8.3 Booter 1.0.6

The original working system was 2.3

Any ideas?

[franji1]

Try updating BRX MPU Booter/FPGA and Firmware to latest (1.0.10 and 2.8.4, respectively)

There are some fixes related to serial comm in both of those compared to what you currently have.

[Mike Nash]

Follow up on this topic. I did update booter and firmware with no success.

I was getting a PWM of one of the levels, but I don't recall which. The resistor loading was helping with that.

That BRX got sold on a job when we couldn't get any due to all those supply issues. I recently received the replacement BRX and just tried it with the same BX-P-SER44-TERM POM and I get good comms and great waveforms and scoping doesn't cause any issues either.

That old BX-DM1E-36 had always had an audible power supply whine. The new one does not.