Getting started with Modbus on a DL06

[wolverine1981]

In your comms port setup ensure that the modbus protocol is the only one checked. And check that your slave address is the same that you are attempting to read or write to. I would change the comms port address at 1.

Tom

 

[bluffit]

Ok, Well the station number is 1. Is this what you mean? I then went on to the CIT device, and set its Modbus Slave ID to 2. Double checked the baud (set it to the slowest 2400), no parity and 1 stop bit. Then went the to the ladder logic mrx command and set the slave id to 2.

I then tried removing the number of bytes read to 2 just to get the first sensor.

Still nothing. SP116 states are toggled, but only the 1st bit is set on registers V400 and V2000. (Of course I'm checking these values via XRef once the program stops - is this wrong? )

If you look at the CIT manual, there are some debugging function codes, but these are 8 for Diagnostics, and 11 for to report slave ID - both of which can't be used when using MRX.

 

[bluffit]

Ok, now I'm totally confused.

- I went the "Change Value" Dialog, to have a look at V2000 and V400, then steped through the program once in Test mode.

- Then checked the bits in Change Value and they were all zero

- However, the xRef grid showed the first bit to be set.

- So I wrote a value to the PLC using the Change Value dialog, and the change did not show up in the xRef grid.

 

[bluffit]

Did you happen to get the zip-link connector that goes on the 15 pin VGA style connector? It does a nice job at breaking the RS485 out into terminals. It's dooable other ways but it gets to be a real pain. Secondary commport setup is OK and all that. Attached is an example of the same type. You usually need to put a inter-message delay (0.100sec perhaps) because of the RS485 two-wire. If you don't the you may have devices staying in transmit and wrecking the next messaqe. Anyhow, see attached.

You know, the cable might be the problem. We're using an adapted video cable, when we should be using shielded, twisted pairs. Not sure if it will make that much difference but worth trying!

 

[lee1968nh]

Watxh out. those error codes (V400 etc.) are in Heex and 16-bit word reversed and not in order even after that. They are a puzzle to solve and I'd generally ignore them.The FC03 seems to contain function code "3" but again, You'll want to see the the RX/TX LEDS blinking before any of that...notice they all blink at the same time...RX and TX being the same.
The part # for the terminal breakout (a wise investment) is AutomationDirect ZL-CMA15 and is $10.50. Terminating resistors are almost never needed no matter what some instructions and/or peple would say. Feel free to put in a couple of 120s if you feel like it. Send a picture of your secondary COM port settings in DirectSoft.

 

[bluffit]

Thanks for that! I see there is also another part - the ZL-CMA15L - with flashing lights, whistles and bells on it. Has to be good. Will get the cable sorted out with one of these fancy 15pin adaptors, then mount another attack. Appreciate your response.

 

[bluffit]

Update. After much soul searching, we found the problem. The simple A and B input selectors from the RS485 cable had be to reversed. B was positive and A was negative. So simple, so stupid, but there it is.

Thanks to all who assisted in this thread. Despite the problem being stupidly simple to fix, I've learnt a lot along the way. Faith in hardware partially restored - until the next crisis no doubt!

(Still reckon they should make putting a C compiler into PLCs mandatory!)

 

[bernie_carlton]

Another repetition of the rule regarding RS-485. "If it doesn't work at least try swapping the wires."

There is virtually NO AGREEMENT between RS-485 communication device suppliers at to the simple meaning of + and -. Which may by why Allen Bradley calls then 'A' and 'B'.

The simplest I've heard is, when no communication is taking place the more positive wire should be labeled '+'.

'Swapping wires' is nearly our first tro0bleshooting technique after verifying communication parameters.

Welcome to Modbus and RS-485 Bluffit!