Hello,

I have similar problem. Im trying to make master Modbus RTU Communication via RS485 Half duplex 2-wire operation in PLC with some microCPU device. I m using ET 200SP , CPU 1512SP F-1 PN, 6ES7 512-1SK01-0AB0 (firmaware 2.1.0),  with CM PTP 6ES7 137-6AA00-0BA0 (firmaware 1.2.0 and I used 12,14,16 pins)

microCPU Device is set to :

Baud:115200; Data bits: 8bits; Parity: none parity ;

The modbus_master_rtu status go only: 7001- initialization, 7002 -transmission, 8281 - Negative aknowlage and I use Done, Error and State tags .

You can view the program in zip file, (From program_Img1 to Program_Img7 , is just one OB30 everything else is diabled so only OB30 is running.). Slave ID and Data addres are correct as u can see at Simple Modbus Master Images where I directly connect from PC to microCPU device.

I dig even deeper into modbus_master DB and I checked SendBuffer, Its shown on SendBuffer img and when u compare it to Simple Modbus Master Read 2 u can see that CRC bits are on starting position in PLC and in PC program in Last position can it cause that 8281 error?

If not pls, pls help experts :) I m programing in SIEMENS TIA (any siemens) only for 2 months. I m AB person xD

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Split from Error "16#8281" in Modbus RTU Communication in S7-1500 PLC.

Hello,

for first tests better prepare only one request to Slave, then you can better check all statuses. Also for better check you can make pictures in Online mode, better then see what value is set to MB_MODE, what address is set for register. From my point of view:

1. make sure (look to instance DB of comm cfg) that 2 wire RS-485 is really set, what I remember some kind of bug was reported and therefore not correct value of interface was set in DB via module cfg.

2. I recommend not break EN input line for all instructions, leave it permanently connected and change REQ with your conditions only.

3. make as I already sugested only first request for test, you can change REQ and other values basically from Online status in LAD and see what will happen.

4. in case of check correct message format outgoing from PLC you can connect also Modbus Slave PC application or other simple serial Terminal program (not strictly Modbus version). After sending request from PLC you can see in your PC Slave app what message was sent and you can easy check byte per byte if some problem exists. Looking to send instance DB isn't good solution now in my opinion.

Update: hope you've check for sure output DONE -> true in COMM_LOAD, without this is next step with communication not sure... Also check correct Module HW address set in COMM_LOAD, this error can point also to this problem. In this case your Master block can't working correctly. All sequences must be executed correctly one after other.

Hello

Thx Duris for fast response.

I corrected EN inputs and the DB pointer to word changed to symbolic adresing to  word. Installed  Simply Modbus Slave and connected PLC to PC via CM PTP MODBUS RS485. The instruction MOdbus_Comm_Load is executed without any problem the done bit is save to Comm.Hold_Done bit and status in that state is 16#0000. The Comm port parameter are correctly set as in HW config as shown on IMG Modbus_Comm_Load_DB_Port_Records. Only MOD and RETRIES are set at start of OB. Next is Modbus_Master which is not executed till Comm.Hold_Done is not set. There is tag read. When Read is set the status of Modbus_Master go to 16#7001 or 16#7002 and Busy is set. After second latched on of Read tag the status go instantly to 16#8281 (Negatic aknowladge) and Error bit is set. I checked MB_DB and its set correctly. I tried 40001 and 400001 data adrresies and swaping D- and D+ and chenging version of modbusRTU from 3.1 to 2.1 but nothings helps :(

So I think that real problem is in Modbus Master Send P2P because lot of parameter are missing, I tried set Send_buffer before instruction but its set back to som bulls... inside the instruction.

Hello,

maybe really better can be now omit cyclical OB and put your code to some FC/FB with OB1 execution. Nothing special for this test. Via your picture with MB Slave PC app I don't see any request outgoing from PLC. Maybe via standard OB1 execution it can be test better now. Also set that REQ as simple memory one condition. You can also make next simple condition that if Master DONE or ERR is true you can reset your Master REQ request. Really I recommend for first tests make code very simple with 2 instructions only - Comm_load and Master. Without any special conditions you can easy test functionality.

Duris_I

Hello,

maybe really better can be now omit cyclical OB and put your code to some FC/FB with OB1 execution. Nothing special for this test. Via your picture with MB Slave PC app I don't see any request outgoing from PLC. Maybe via standard OB1 execution it can be test better now. Also set that REQ as simple memory one condition. You can also make next simple condition that if Master DONE or ERR is true you can reset your Master REQ request. Really I recommend for first tests make code very simple with 2 instructions only - Comm_load and Master. Without any special conditions you can easy test functionality.

 Thanks Duris for help, the problem was bad power supply I change it and the errors 8281 and 8280 disappear.

Bad power supply of what??....CM Ptp module?? ...PLC ?? ...modbus device??