Getting Started

To get started with direct Modbus TCP, 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.