Hi
I'm trying to understand how to write to a modbus register.
I purchased an Active X control from Modbustools.,com
My goal is to read/write data values in an Automation Direct productivity 2000 P2-550 cpu. via Ethernet.
Once installed in my Visual Basic 2010 application I was able to connect to the PLC and read bits, & registers with no trouble.

When I try to write however I'm having difficulties.
The command I'm sending is
Result = WsmbtControl1.WriteSingleRegister( UnitID, Address,Register) where the UnitID is 0, the Address is 5 (I'm trying to write to modbus 40005) and the data is 0
I keep getting a Popup box displaying "Illegal data address"

I've contacted the company I bought the activex control from several times and they have not been as helpful as I'd like. I don't want anyone to write my application I need to know how to format a correct write to register command.
What about the command I'm sending is wrong with the address?
Does anyone know what I"m talking about?
Dana

 

Hi,

try with PDU address 4 since register 40005 may translates to:
4: indicate holding register
5: register number

Addresses used in the telegram (=PDU address) start counting with 0 but documentations sometimes state register numbers which are starting at 1.


see also section 11 at https://www.modbustools.com/mbaxp_quickstart.html

 

Hi
I'm trying to understand how to write to a modbus register.
I purchased an Active X control from Modbustools.,com
My goal is to read/write data values in an Automation Direct productivity 2000 P2-550 cpu. via Ethernet.
Once installed in my Visual Basic 2010 application I was able to connect to the PLC and read bits, & registers with no trouble.

When I try to write however I'm having difficulties.
The command I'm sending is
Result = WsmbtControl1.WriteSingleRegister( UnitID, Address,Register) where the UnitID is 0, the Address is 5 (I'm trying to write to modbus 40005) and the data is 0
I keep getting a Popup box displaying "Illegal data address"

I've contacted the company I bought the activex control from several times and they have not been as helpful as I'd like. I don't want anyone to write my application I need to know how to format a correct write to register command.
What about the command I'm sending is wrong with the address?
Does anyone know what I"m talking about?
Dana

Hi

As you are getting "Illegal data address" which occurs if you are reading or writing registers which are not in slave. So suggest to use address 4 for writing holding register "40005" and check.

Regards,