Getting Started

To get started with direct Modbus TCP when using any HMI, PLC, or a 3rd party Modbus Client Application, first try writing & reading to/from the test registers.

See the Examples section on the Modbus Protocol Details page.

If using COTS Modbus Client software, keep the following in mind:

• The LabJack is a Modbus TCP Server.  A Modbus TCP Client can send a command to the LabJack and get back a response.  Sometimes a Server is called the Slave and a Client is called the Master.
• We use a single register map with addresses from 0 to 65535.  Each address points to a 16-bit value that might be readable, writable, or both.
• The meaning of the registers are defined in our Modbus Map.
• Use function 3, 4, 6, or 16.  Choose "Holding" if needed.

If you don't have a better way to see the bytes written and read for each packet, you can use Wireshark to get a TCP capture.

Common Issues:

Below is a list of some common issues that customers tend to deal with when configuring & setting up Modbus Client Applications:

1. The client subtracts 1 from all addresses.  You tell the client you want to read address 2000, but the client puts 1999 in the actual packet.  That means if you want to read address 2000, you have to tell the client 2001.  This seems to be an attempt to give the user addresses that go from 1-65536 rather than 0-65535.  We use 0-65535 addressing.  If you want to read address 2000, then 2000 should be in the packet.
2. The software flips the order of the words within a 32-bit value.  For example, a read of TEST should return 0x00112233, but the client returns 0x22330011.
3. The software says it is adding 40000 to the addresses, but if you look at the actual packet it is not.  For example, if you ask the software to read from 2000, it will say it is reading from 42000, but in the actual Modbus packet it specifies address 2000.  Does not seem to cause problems, but can be confusing.

General Modbus Information

More getting started information and general Modbus resources can be found on the Modbus API Documentation page.