Pro-tips: MODBUS installations

Pro-tips are just that: tips to be more professional. Professionalism includes (but is not limited to) proper planning for jobs.

MODBUS is a common industrial protocol that is ever more intwined into control systems. It may be surprising that issues with it still come up. Most have to do with how numbers are represented, although some have to do with register addressing. However, there are some easy ways to prepare for it.

My favorite is to use a simple table, with a simple verification of points at each end (customer and vendor) prior to the system's start up.

The table itself has the following columns...

1. Address- Where does this block of registers start? Where does it end? (A surprising number of issues come up from things being shifted up or down by one register)

2. Contents- What is each address?

3. Type- How is this being read? Binary, integer, long integer, double float? If it's a float, is it single or double? If it's a long integer, float or double float, is it least or greatest significant bit first?

4. Client* side check- Has this been verified with a simulator acting as the server?

5. Server* side check- Has this been verified with a simulator acting as the client?

As usual, I vouch for WinTECH's MODSCAN and MODSIM software for testing. If you use these together, you can check both before anyone ships anything anywhere. You can make modifications before leaving the factory. Also, you are always using the same terminology, which is often half the battle.

I hope this makes your start ups easier and more successful!

*- I have assumed you are using MODBUS TCP, in which case you have ethernet based server and client devices. If you are using MODBUS RTU, you have a serial network, which uses master and slave devices.