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JBus Protocol vs. Modbus Protocol

I saw that you have a TOP Server Driver for the Modbus protocol, will this support JBus as well? If yes; Is there anything I should be aware of when configuring this in TOP Server?

The Modbus Protocol was initially developed by Modicon. April developed their own version of Modbus; later dubbed JBus. The two protocols are certainly compatible, and the TOP Server Modbus driver or the Cogent DataHub Modbus driver will be able to communicate with devices that speak either protocol.

There are a few considerations that need to be taken into account:

  1. Modbus is natively a 0 based addressing protocol (Reg+1), whereas JBus is natively a 1 based addressing protocol (Reg+0). The register offset of 1 can easily be configured in the device properties "Use zero based addressing" found under the Settings tab
  2. Modbus and JBus have different "limitations" when it comes to packet capacity
   JBus Modbus
 Number of Stations on bus  255  247
 Max number of bits to be read in single request  2000  1920
 Max number of words to be read in single request  125  120
 Max number of bits to be written in single request  1968  1920
 Max number of words to be written in single request  123  120

 

This limitation must be taken into account when setting up the Block Settings of the TOP Server device - since Modbus is lower across the board there should not be any problems with JBus Devices.

If you are communicating with a JBus device and experiencing problems, please contact our technical support staff and we would be more than happy to assist.