RS485 bug

[Saxony Thermal 0]

Hello,

I was wondering if someone here who is experienced with RS485 can give me an opinion for why my RS485 line does this bug; shown in the figure.

The first batches of thin lines are good communication. The slightly higher voltages are the machine I am talking to, and the slightly lower voltages are my PLC. You'll see this quiet time, then wide repeating square waves. Not visible: the end of each square wave is the machine sending several messages butt to butt (garbage message), and my PLC responds to it with an appropriate error message. This argument repeats. If I disconnect the PLC, the machine repeats its squarewave + garbage message until system restart.

The PLC is, Mitsubishi FX5UC using its RS485 built in port, GXworks3 environment.

4800baud

start bit, no parity. header and terminator being used on receive, disabled on send so I can send CRC (which mitsubishi only did Sumcheck so I do my own CRC calc and appending). Average message response time is about 15ms. Pretty nice with a non-lookup CRC generator.

The below is just discussion. Mainly does this seem familiar to anyone? easy fix?

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I have the PLC connected to some equipment and they will sometimes talk for a good 5 minutes without issue and then the machine's chip enters a state where it pulls the line high and spits out multiple messages butt to butt as though it was one message and sometimes missing CRC or bytes just out of order. I know the machine is inherently fine because there are other off the shelf items that communicate with it just fine. When the machine enters this state, it stays like that until I restart it.

Sounds like I am talking a lot about the machine which is not mitsubishi but the cause is my PLC and the machine is the key of observation. My PLC appears fine but is the cause. I am using a scope to check my bit-bashed messages and they are all great and within timeout etc.CRC is all good. The messages just prior to the bug are all clean. And this bug has happened after many different messages have been sent (so its not like a specific message is causing it. It can be any message.)

 

ALso it can bug after the very first message. Its quite random.

I thought I was getting it when adjusting termination: such as 110 ohms, open, or 330 ohms. I used a power suppy and 1k resistor to pull up and down the SDA and SDB respectively, it seemed to influence it but then the machine still enters its bug after I felt like I solved the issue. ANd thereafter it enters the bug repeatedly.

 

One thing that is consistent is that the bug comes in waves. It will be happy for a while, then bug. Then after several restarts the bug is quick to return (within seconds), then after enough repetitions it starts to go away and its fine for a few minutes.

It can deceptively seem like powering everything off for a while and coming back (like 10 minutes to an hour) can make it happy for 5 minutes before the bug. But thats not always true. I have come back in the morning and more often than not it works for a few minutes, but I have also had it bug first thing in the morning too just not as likely it seems?