Hello

I am trying to communicate between cm 1241 RS422/485 to a slave device, I have done all configuration from plc side and slave modbus module. MB-COMM-LOAD is OK but MB-MASTER block is always busy.I am going to send a common from master to slave in order to start it.

I attached my program and configuration. Please help me.

Thank you .

Hello,

in case of write to Holding register, you need probably define your memory as WORD, or try send 2 BYTES. What are next lines of text about that Holding register, you show us only first Byte. Look to also MODE parameter value due register.

Dear Duris_I

Thank you for your response. 

unfortunately, I try it with word but it still does not work.

I attached other registers here. 

Hello,

Can try one thing,

Use Modview standard modbus rtu communication tester software.Define modview as modbus slave and communicate with CM1241 .

On error bit of master move the status of master block in other memory location to find out exact reason 

If master is busy then there might be different reasons, cable wiring need to be swap,  may be slave is not responding etc.

So we need to confirm that PLC as modbus master is ok with help of modview software.

Post the error code.

If you feel this post is useful then don't forget to rate it.

thank you DJ-1

I bought a USB to RS485 converter and monitored data on the bus.The data is available but I set slave address on 5 and when I send read request data from plc to slave address 5, done pin in MB_Master block is false but in similar configuration when I chose it 0 in MB_Master block it become True!

is there any problem from slave device? Because Plc does not response to selected slave address?!

Hello,

Now tell me which modbus simulation software are you using.

Lets say if it is Modview, then you need to define Modview as Modbus Master mode, and your device as Modbus slave.

Once everything is set properly then you should get values in Modview software. If it is communicating with simulation software then only try with PLC else contact to device vendor.

Some of the devices are having fix device id, you need to verify the all details from the device vendor or manual.

Now with same setting you have to define in PLC modbus comm load and master block.

Baud rate, parity , stop bit , device number and memory address.

Also do some changes in PLC side,

On modbus master error bit , move the status word value in to some other memory location to get exact error code.

Thank you DJ_1

I changed slave device and my problem have solved.

Now I have a problem with DATA-PTR, when I select it from memory bits it works correctly but I need to save data in a data block and it has an Error and does not work  !

I attached a screen shot of my program.

Hello,

DB should be Standard DB -compatible with S7300/400 for Modbus communication.

It means DB should have absolute address.

Just right click on DB3 and go to the property , click on attribute , uncheck the optimized access .

After compilation of DB3 , you can the offset.

Then download program again in PLC and warm restart the PLC.

After doing this , error should dissapear.

New question published by arunmp11 is split to a separate thread with the subject MB_SLAVE error 8187.

Best regards
Jen_Moderator