Help on setting up a MODBUS "lab"


Thread Starter

Tom Richards

My company has had many requests for interfacing our product to MODBUS, MODBUS+ and TCP/IP over MODBUS. I have a lot of information on MODBUS but would like to set up a lab. I would like to find some relatively inexpensive PLCs that would support the above protocols. For interfacing to my software (WinNT based) I am thinking of using the WinTech ActiveX controls.

Hardware wise I am starting from scratch. I would like to setup enough hardware to "play" with MODBUS and eventually test my software.

There seems to be a lot of PLC vendors but I was hoping to save a little time by getting some recommendations from this list.

Any help would be greatly appreciated.

Tom Richards [email protected]
MCM Enterprise Ltd.
11010 Northup Way Bellevue, WA 98004
I suggest you buy a real Modicon PLC for testing. I have found that many other products that act as Modbus slaves do not rigorously follow the protocol rules, especially when it comes to the inter-message 3.5 character time delay in RTU mode.

Take a look at the Modicon Momentum. It's inexpensive and can be ordered with a TCP/IP hat.

Jay Kirsch
Macro Automatics
2985 E. Hillcrest Drive, Ste 101
Thousand Oaks, CA 91362
[email protected]
I work with different electronic devices over Modbus protocol. (only Modbus for the moment).
Now, I use TSX nano PLC from Scneider Electric to add E/S to my PC (WinNT based). The modbus protocol is just implemented. Writing a modbus protocol is quite simple, with readfile or writefile functions or with the OCX MScomm32.
However, I have encountered timing problems due to NT. (when you need to wait for 3,5 characters to decide that the frame is complete), or after
sending, to put RTS high. If you write a real driver, wich can have a direct access to interrupt, these problems can be solved. The other
solution is to use an intelligent converter (RS232 / 485) wich can take into account the line management. It's a little more expensive than a
classic converter, but you can write a simple program in C or C++ to manage the protocol. For example, I can ask for E/S status of my PLC
every 60 ms.
My answer to the components that you will need to make this lab are going to be expensive, because you will need a PLC that will handle the MODBUS which can be a micro, nano, or something like a 484, 584, 884, 984, Quantums just to name the principal Modicon PLC's let see; if you use a micro you do not need a power supply, backplane or I/O, so use one of these, MODBUS+ can be run on either a 984, or Quantum PLC, fore either you are going to need a power supply, backplane and I/O the Quantums have an Ethernet Module that you can use to get to the internet with, you will also need a gateway, or maybe you could use a bridgemux to accomplish that and the list goes on, e-mail me and tell me exactly how much you are planning to spend and I can give an idea as to how far you can go with this.