Controllogix Modbus Communications


Thread Starter

Ben Simpson

<p>Hello All,

<p>I have a controllogix PLC (L35E) using the ModbusMaster.acd file supplied with RSLogix5000 version 13.

<p>I have a Modbus network working on a RS232 network. The RS232 port of the compactlogix is connected through a DB9 crossover cable to my laptop with a modbus simulator (modbusview).

<p>I am simulating the PC as slave node.

<p>the PLC Master polls the PC slave and retrieves data from registers 0-100. This works fine.

<p>However in my application i wish to poll a device with address 2001. I am unable to do this because the following tags for registers 03 and 04 of the Controller Tags is limited to 250 words.
Mod_Data_InpReg3 Type: INT[250]
Mod_DataHoldReg4 Type: INT[250]
<p>QUESTION: when i try to read an address above 249 (03 or 04) the statement is not sent. I can see the statement is not sent because the PLC comms light stops flashing. When i change the address to 247 however it polls correctly and it retrieves data from my PC slave node. So to summaries I cannot poll data from a slave node above 248.

<p>Has anyone encountered this problem?

<p>Is this a limitation?

<p>I believe it may have something to do with setting the array larger than 2500 so that the data at address 2001 can be stored?
It sounds like you're mistaking the slave address field (valid values are 1..247) for the data address field (valid values are (0..65535).

Slave address is the modbus node, unique to each slave device in a multi-drop system.

If you would like to try a different slave, you can download a fully functioning trial version from

If you would like to try a different master, you can download a fully functioning trial version from