Modbus TCP Master Limitation

I have a Waukee temperature controller that has an internal limitation of only allowing a single TCP master connection. Problem is I need to have two TCP Masters talk to it and request it's atmosphere value.

My two TCP Masters trying to talk to the Waukee controller as a slave are a Proface HMI and an IBA data logger. Ideally I think if I had a micro PLC, I would have that read my device data in and then the other Masters in the circuit can request the data from the PLC...perhaps that would be best?

Wondering if there is a cheaper aleternative like some kind of Field Server, IO controller or Modbus gateway perhaps from MOXA or another manufacturer that would perhaps provide this function; brokering requests from two masters while said gateway device only requests from Waukee controller (slave device) as a single TCP request and duplicates the data received back to dual TCP Masters...

Would anyone be able to recommend devices for that purpose or am I best implementing a small Siemens 1200? Thanks ^^

ZestyMozzarella said:

I have a Waukee temperature controller that has an internal limitation of only allowing a single TCP master connection. Problem is I need to have two TCP Masters talk to it and request it's atmosphere value....

Click to expand...

Caveat: I always go for the complicated solution first; you should check if someone has already created a product for this niche first e.g. see here.



What is the protocol riding on top of TCP? Modbus? If it's simple enough there are any number of devices that could do this. Certainly a PLC but that is the wrong tool. if the protocol is simple enough then a cheap SBC (sub-$100) could do it, depending on the environment. Linux-based would be nice because then it's easy (at least for me ) to prototype with a laptop first and port to the SBC later. That said, that development time is probably well past the cost of a PLC if you already know how to use that particular wrong tool.



Pie in the sky: leave the (presumably current) one connection alone and sniff the data (e.g. use promiscuous mode to see all packets) on a node that then serves the data to as many other nodes as needed.


Can the data logger talk to the HMI, or vice versa?

This little PLC would probably be the easiest and least expensive way that I know of:

https://www.automationdirect.com/ad...(stackable_micro_brick)/plc_units/c0-10dd1e-d

Software is free and pretty good. There are gateway modules that could do the job and might have a narrower footprint.

https://www.automationdirect.com/ad...unications/serial/modbus_gateway/sgw-mb1511-t

Never seen datasheet of Modbus TCP to TCP multiplexer.
TCP to multiple serial lines is common, if you can go to serial communication on other end, then moxa, adam etc. have gateways.

What is the data rate (sample/s or sample/h)?

I think the Red Lion Data Station Plus can be a Modbus client/master AND simultaneously, a Modbus slave(server), doing what you suggested with a PLC, reading the Modbus value from the Waukee, moving it to a slave register, letting the either Modbus TCP master read the server/slave register.



Trade off whether the time to learn the programming for a "blank slate" box is competitive your 1200 option.

A custom software running on a PC, Raspberry or even a mobile device could do the job, accept multiple connections on one side, replicate requests on the other side through a single connection, and redirect responses to the source connection.

Modbus/TCP protocol includes a transaction identifier that could be used to identify which source connection the responses correspond to.

I don't know if there is something like this available on the internet ...

Multiple Clients

If your two devices can have their read rates slowed down - and if they close the TCP socket connection between reads - and you can accept a slower update rate - then you don't have a problem.

Contact me if you are interested in a Raspberry PI solution.

Update Rates - Clarification

I've had several cases with power meters where I hit them with a Modbus TCP read from a couple of client locations at around a 1/2 second interval. They only supported a single connection at one time. Since I was shutting down the socket between reads they'd only have a couple of missed reads per hour or less. My guess is if you'd go for a 1-second interval you'd rarely ever see an error. What is your required update rate?

I'm not familiar with the devices that you are using. It appears that a lot of manufacturers don't document what they are doing with TCP sockets very well. It's hard enough to find socket count limits on the server ends. I suspect with a little investigation your problem may go away.

I have never seen a commercial device acting as a Modbus master that opens a connection and closes it with each request/response.

I think that can only be achieved with custom software.

I've had to do something similar in the past with a different device that only allows a single Modbus TCP master connection and have found that a basic PLC is the cheapest way.
I would second the Click PLC as suggested by OkiePC above. Simple & inexpensive.

Many HMIs allow to be Modbus Client and Modbus Server simultaneously.
So you can poll the Waukee controller using the built in Modbus Client and then share the data with Modbus Server so it can be polled by the Modbus Client of the data logger.
Exor HMIs we use can do it easily and it's very likely that Proface can do it as well.

Another solution is to install Codesys on a Raspberry Pi and then set up both Modbus Client and Server.

Click PLC is also a great solution and you could use Schneider M221 as well for the same purpose.

lfe said:

I have never seen a commercial device acting as a Modbus master that opens a connection and closes it with each request/response.

Click to expand...

We added a feature to open, read or write, close, repeat because we had customers needing to very slow poll devices. The devices would timeout, drop the connection between polls, causing us to log errors. Slow poll because the data was flow/counts/etc. and the connection was satellite, charged per byte.

lfe said:

Many HMIs allow to be Modbus Client and Modbus Server simultaneously.

Click to expand...

We do.

We have only started to install ibas in to our factory floor, so my knowledge of them is a bit limited. At least for Siemens PLCs it is possible to also write values from iba, not only read. Maybe iba would be possible to set up as TCP/IP server? It is also possible to set up iba as OPC UA server and publish variables that way, can the HMI act as OPC UA client?

Phoenix fl com server is bad a**.
It handles 7 open sockets at one time.
But last time I used it, it was a bit pricey. like $700