Hi to all,

according to example programs on forum I made program for Modbus communication for slave S1200 (6ES7 214-1AG40-0XB0) and communication board CB1241 (6ES7 241-1CH30-1XB0) using TIA (Version V13 SP1 Update 9).

In order to test communication I used second  PLC as Master, and everything WORKED well. (parts of program for MODBUS communication can be found in attachment)

Now, I want to test communication additionally with Modbus poll software on PLC. I am using Moxa 1150 with DB9 connector to connect PLC with PC. (wiring can be found in attachment)

Settings of Modbus poll software can be found in attachment.

When I use function code 03, TxD and RxD on Moxa connector are blinking, but only RxD is blinking on CM1241, and I receive error "Checksum error" when trying to write on only one address. Or "Insufficient bytes received" when trying to read from more then one address.

When I use function code 06(write single register) I don't have errors but I don't receive anything on PLC.

Also, I tried to change in Modbus poll to PLC address(base1) but received same errors.

Can you please give me some advice? If anything else is needed let me know so I will post it later.

Regards

Anybody?

Or if someone could give me program example, previously tested with Modbus poll software.

Thank you in advance.

I feel like talking with myself, but I will keep talk :D

From attachment below it can be downloaded program example from this forum,

MODBUS communication between this two PLCs is working perfectly, but when I connect my PC to SLAVE PLC and try to test it with Modbus poll I receive errors like described in first post.

Please give me some advice.

Thank you

Hello,

only my opinion, if your info about 2 PLCs connected together was functional, I see problem probably in MB poll SW or Moxa adapter. I must say, I had one of this adapters (older type one) and was not satisfied with it's functionality, next Moxa adapters/converters (like NPort) are ok in my opinion, but this can be personal opinion. I don't know that MB poll SW functionality, so don't know if someone will answer exactly what problem can be, maybe with parameter of requested address of register (some SWs use offset/ some not), maybe also max  amount requested data (also must be checked in your DB if are sufficient). Hope you'll find problem so soon, good luck.

Duris thank you for your answer.

That is also my opinion, if 2 PLCs (Master and Slave) communicate throught MODBUS connection it shouldn't be a problem that other devices communicate with Slave.

But, I still didn't find what is the problem.

I tried with another software (Simply Modbus Master) but with no success.

I can not believe that nobody here didn't try to test Siemens PLCs Modbus communication with some software from PC.

I am running out of ideas what to do, stuck with this for a week already.

Hello,

I'm curious how you did that test with app from SimplyModbus. Simply if you send some request from PLC to Slave PC app (f.ex. simplymodbus RTU Slave), you must see in app error message or something depended to request. If you can, make again one test and post screenshot from Slave RTU app simply modbus, not that other SW (I don't know it's functionality). Look carefully to TX/RX lights of your CM module and also to Moxa's LEDs. You can also omit these MB SWs and connect simple Hyperterminal and after recv check incoming message consistency (also you can attach screenshot with of course description what you request from PLC - can be quickly checked). Simply you must try more options, no other way.

Dear Duris,

thank you for your answer. In attachment you can find new photos of testing with Simply Modbus software. I think the problem is with expected response bytes.

When I try to read 10 registers (20 bytes) expected response bytes is 25, as you can see on picture. I don't know for what are these 5 additional bytes?

I hope this will help to find a problem.

Regards

Kijo

Hello,

look to www.simplymodbus.ca for byte sequence description, also you have that in SW. Look, logically 20 bytes are only your needed 10 values but MB complete message must also include other needed informations that are described in help. Pay attention to not manipulate with standard settings in PC SW (offsets) if you don't know really what you do. This can be not easy then recreate back to functional state.

Hello,

ok, look to your MB address - Slave, you're request to addr2 and your Slave is 0.

Hi again,


sorry for not posting earlier, I was on the field last few days so I couldn't answer earlier.

Regarding address, that is not the problem. Just print screen taken in wrong moment.

I have error 16#80E2 (Frame aborted: Character frame error Check the settings for start bit, data bits, parity bit, data transmission rate, and stop bit(s).) so I tried to change something of this to make thing work but with no results.

In attachment there are new picture with marked settings on PLC and on PC.

I really don't see what is the problem.

Is there anyone with PLC program tested with some PC software for MODBUS testing that is working?
 

Regards

Kijo