Protocol Variants

Many devices from different vendors support the Modbus protocol. Unfortunately, addressing structures used by these devices vary. The following information will help you understand which protocol you should select on the I/O devices dialog box to make the addresses you enter into the variables form match those entered into the programming software for your device.

You will need to know the following:

• What is the lowest register / coil number in the device? 

  Some devices (such as a TSX Quantum or Modicon 948) have register / coil 1 as the lowest register / coil number. Other devices (such as the TSX Premium) have register / coil 0 as the lowest register / coil number

• When addressing an individual bit from a register, what is the lowest bit number? 

  Some devices refer to the bits within a register as bits 0-15, others refer to them as bits 1-16.

Based on the above information you can select a protocol using the following table:

If you are using a PLC that supports extended registers (6xxxxx addresses in a TSX Quantum for example) then the following table should be used:

Two special protocols exist for use with the TSX Quantum and TSX Premium PLC ranges.  These protocol options will select the correct addressing modes, select certain INI values and other settings to provide compatibility with these devices without the need to set any other special parameters.

These protocols also allow you to access memory digitals and memory words using %M7 or %MW3 or %MW4.6 style addressing. Select a protocol as follows: