Dear Forum Members,

I want to read a modbus RTU holding registers starting address 100 via the Modbus_Master function for a S7 1500. When a read the help file, this seems impossible because the function code 3 requires a address between 40001- 49999.

Does anyone have a idea if this possible or not ?

Best Regards,

Sander

Hello,

look pls to that table about MODEs, addresses and standard Modbus FC codes. Siemens use in this case some different system of addressing which with MODE value then define output standard Modbus frame. In this case you want read Holding register and address is 100, due table (MODE=0, Hold. reg. start 40001->first register) you need to your standard register address 100 add that 40001. Then your required address is 40001+100->40101. This address you need put to your Master block.

I tried to say : The modbus_master block will alter the register address which is sent to the modbus slave.

So 40001 + 100 will become address 100 with FC 3 and this is sent to the modbus slave.

The data send would look like this eventually

tx-> 65 03 00 64 00 02 8D F0

New question published by dynamite098 is split to a separate thread with the subject How to read the address if it is already starting from 40101.

Best regards
The Helping Hand