MODBUS interface to proprietary / 3rd party interface


Thread Starter


We are in the process of basic design and have already finalized the control philosophy and DCS overview. We have several 3rd property systems like DM plant, Air Compressors, CW treatment plant that are to be integrated through MODBUS.

I need a format (form or questionnaire) to be sent to all vendors in order to have a unified approach for detailed design, So, i need:

1. Need info about protocol
2. What kind of interface (link redundancy)
3. Design of the local control panel, etc
4. power supply requirement.
5. Necessary signals to be hardwire
6. Any special requirement or constraints from the vendors

Can any on provide me with such a format that I can use as a template to seek all relevant information from the suppliers.

Thanks in anticipation.

Lynn August Linse

I can't offer the full list you ask for, but here's a few I'd ask for:

1) confirm if the vendor documents Modbus registers as 'absolute-zero-based' or 'off-by-one'. Some vendors document 'register 10' as meaning you send offset 0x0009 in the Modbus request, while others need 0x000A. I can't count the number of projects I've been on where every single HMI screen has to be redone because some junior engineer/tech didn't understand that issue.

2) ask if there are any size restrictions - such as one can only read 64 registers or only write them 1 by 1.

3) confirm the support for functions 5, 6, 15 and 16 (the write 1 vs write many functions). Some devices might require use of functions 15/16 even for a single coil/register, others might ONLY support the 5/6 single coil/register write at a time.

4) confirm how 32-bit values are handled - are they 2 16-bit registers? Which word (high or low) is first.

5) if RS-232, how many control signals are REQUIRED (older Modicon PLC actually acted brain-dead, ignoring Modbus requests unless their DSR input was high. Others answered reads with DSR low, but would not allow 'privileged' Modbus requests (like PLC run/stop) if DSR was low!)

6) if RS-485, confirm the wire count and if there is a signal-ground screw included on the device.

7) for either RS-232/485, ask if the port is optically isolated, meaning they 'float' with reference to the devices' power supply and ground. If not, you probably want to budget to install an isolator just to avoid catastrophic system failure during start-up or testing (Again, I can't count the number of times minor grounding issues caused dozens of people to stand around on-site during testing waiting for replacement parts.)