Hello,

I am trying to set up modbus RTU connection between PLC and led display board, which operates on ModbusRTU. PLC is slave, board is master.

First of all - wiring.

I have CM1241 module with DB9 socket.Since I use 485 port, I need to use GND, A and B wire.

GND->5

A->1

B->2

or A->2, B->3? (like quasi 232?)

Secont thing im dealing with now is program configuration. Modbus_comm_load is in OB100 and triggered with M1.0 'first cycle).Seems to be OK. Port is 269 'Local-CM_1241...' from tag list. MB_DB is data block created after placing Modbus_Comm_Slave.

Modbus_Comm_slave is in separate FC. the problem is with MB_HOLD_REG. I created new Data_Block_1[DB8], unchecked all atributes, with 'optimized block access'. And... im not sure what to do next. I tried simply to define DB variable 'data' int, I tried to create struct... each time ERROR occurs and STATUS 16#8187. I saw examples from Siemens website and others. How should I declare this pointer?

Vulpes,

Here is the S7-1200 manual for the wiring of the CM1241, see page 1265.

• 1 Logic or communication ground

• 2 TxD+ 1 Connected for RS422

  Not used for RS485: Output

• 3 TxD+ Signal B (RxD/TxD+): Input/Output

• 4 RTS 2 Request to send (TTL level) Output

• 5 GND Logic or communication ground 

• 6 PWR +5 V with 100 ohm series resistor: Output

• 7 Not connected

• 8 TXD- Signal A (RxD/TxD-): Input/Output

• 9 TXD- 1 Connected for RS422

• SHELL Chassis ground

Best regards,

Hugo

vulpes

Secont thing im dealing with now is program configuration. Modbus_comm_load is in OB100 and triggered with M1.0 'first cycle).Seems to be OK. Port is 269 'Local-CM_1241...' from tag list. MB_DB is data block created after placing Modbus_Comm_Slave.

Modbus_Comm_slave is in separate FC. the problem is with MB_HOLD_REG. I created new Data_Block_1[DB8], unchecked all atributes, with 'optimized block access'. And... im not sure what to do next. I tried simply to define DB variable 'data' int, I tried to create struct... each time ERROR occurs and STATUS 16#8187. I saw examples from Siemens website and others. How should I declare this pointer?

 vulpes,

You don't have to put the whole "MB_COMM_LOAD" instruction into a startup OB.

You can simply use the M1.0 bit into the REQ input of the instruction and put it into your cyclic main OB1.

You will find a printscreen of a functionnal project I did to communicate Modbus RTU between two S7-1200.

I will add a printscreen of the MB_SLAVE instruction after.

Best regards,

Hugo

Hello,

first of all, thank You for your anwser

So wiring is now clear:

pin3 B ->B

pin8 A ->A

pin5 GND->GDN

As for datablocks, I've read more and it bothers me because I see that in your project You simply created Slave_data[DB3] and in it You have MB_SLAVE struct? with MB_HOLD_REG int?

I tried it but if I create my datablock MB_SLAVE returns STATUS 8087 "invaild pointer to MB_HOLD_DB-REG".

If I use, as in manual, MW100 (then modbus master addr is 40001, I get ERROR and STATUS 8280...which is not described, so I guess its a P2P status which I still didnt find out.

Could You take a snapshot of Your DB3 block?

p.s.

I noticed that You and some otehrs that show examples use Modbus blocks, not Modbus (RTU). Anyone know what's the difference?

vulpes

Hello,

first of all, thank You for your anwser

So wiring is now clear:

pin3 B ->B

pin8 A ->A

pin5 GND->GDN

As for datablocks, I've read more and it bothers me because I see that in your project You simply created Slave_data[DB3] and in it You have MB_SLAVE struct? with MB_HOLD_REG int?

I tried it but if I create my datablock MB_SLAVE returns STATUS 8087 "invaild pointer to MB_HOLD_DB-REG".

If I use, as in manual, MW100 (then modbus master addr is 40001, I get ERROR and STATUS 8280...which is not described, so I guess its a P2P status which I still didnt find out.

Could You take a snapshot of Your DB3 block?

p.s.

I noticed that You and some otehrs that show examples use Modbus blocks, not Modbus (RTU). Anyone know what's the difference?

 vulpes,

You can find a zipped library attach to the post where you will find the UDT I created, DB3 and OB1.

Best regards,

Hugo

vulpes,

Here is the whole PLC Modbus slave.

Best regards,

Hugo

vulpes,

For your question related to why I chose this instruction, I can't remember why but I can assure you that this project work perfectly!

Best regards,

Hugo

Hello,

SIMATIC S7-1200 Update to the S7-1200 System Manual, edition 06/2015

Modbus addressing supports a maximum of 247 slaves (slave numbers 1 through 247). Each Modbus network segment can have a maximum of 32 devices, based upon the loading and drive capabilities of the RS485 interface. When you reach the 32-device limit, you must use a repeater to expand to the next segment. You need seven repeaters to support the 247 slaves connected to one master for RS485.