Modbus Comms between 2 devices

F

Thread Starter

Fox

Okay, I have a Distributed Control System (DCS) and I want to communicate with a new piece of equipment via Modbus. There is no other equipment between these two pieces of equipment i.e. PLC. DSC is currently setup as open Modbus, but I have other options.

I'm using Cat6 cable to communicate with. I can ping the new device from DCS so I feel the Modbus setup is my problem.

Q1. How do I know who is the master and which is the slave? Do I decide? Would DCS be the master or it does not matter?

Q2. RTU or TCP? Currently TCP.

Q3. I'm asked data type and I have several options i.e. byte, Lword, short something and I tried several but no data transfer.

Q4. Do you feel Free downloadable Modbus software would be helpful? Recommendations?

This is a new install so I don't have another to refer to.
I feel like I'm throwing darts in the dark and the correct combination of settings will hit the jack pot.

Thanks in advance for any suggestions.
 
Q1. How do I know who is the master and which is the slave? Do I decide? Would DCS be the master or it does not matter?

DCS usually has Modbus master capability. PLC's might or might not, it depends. A Modbus master is usually described as such. Devices lacking a a specific functional description as Modbus master are usually Modbus slaves, but the key is whether there is a 'map' of Modbus registers. Only slaves have a Modbus register map.

If either can be a master then you decide which has the most flexibility to either get the data or to handle the data once you have it.

Q2. RTU or TCP? Currently TCP.
There are fewer comm settings to go wrong with ethernet (TCP).

Q3. I'm asked data type and I have several options i.e. byte, Lword, short something and I tried several but no data transfer.

The data type or format is defined by the slave. In some cases, it is 'assumed' that you know that the data is 16 bit integer data. Most slaves mention whether what the data format is in the documentation, typically on the slave map table. With PLCs the slave map is frequently custom defined, in other words, you'll have to put the data at some location in the custom map so that it can be accessed.

Q4. Do you feel Free downloadable Modbus software would be helpful? Recommendations?

I always talk to the slave with a PC Modbus master first, to prove exactly which addresses are used and what the data format is. It's easy to change formats, connect, disconnect, change function codes, etc on-the-fly in a PC master like Modscan32 or Simply Modbus.
 
Top