Allow specifying a port in ReceiveMessage

[BenGood]

Listening on port 44818 from a PC running RSLinx is a pain. Could we allow specifying a different port to listen in the new ReceiveMessage feature to avoid the conflict? Maybe default it to 44818? ReceiveMessage(ipAddress, return_function, port=44818)

[dmroeder]

Sounds like a great idea. I'll test this and consider it for the next release. I have a few things I want to add.

[dmroeder]

Thinking about this more, I don't think you are able to specify the port in the MSG instruction are you? In other words, I think the MSG only sends the payload to 44818.

Are you having an issue with this or are you assuming there will be an issue? I'm running it on a system right now that has RSLinx and is online with the PLC producing the MSG without issue.

[BenGood]

Oh you can absolutely specify the port in the MSG instruction. For example "ENET, 2, 10.185.53.51:51000"

I don't know if it's something with my configuration but I find RSLinx binds to port 44818 and my test app can't get that port. If I start my test app first and then RSLinx everything works fine since RSLinx doesn't need the port for normal operations but as soon as I restart my app to change something RSLinx has stolen that port and I have to shut it down and restart Studio5000.

[dmroeder]

Well there you go. After 25 years of using Rockwell products, still learning new things.

[kyle-github]

I did not know this either! I will need to support it in my code. I support bridging but I do not support the port number. How is it encoded? Just one big string?

[BenGood]

As far as the message.path it's just part of the IP address string, yes.

[kyle-github]

Good to know. Thanks! I was confused by the length of 22 for a second. I thought it was in hex. Nope. 22 decimal aligns with the path length. Phwew.

[BenGood]

yeah there is a trick though which you probably already know:
$01 $03 refers to backplane -> slot 3 of course
the first $12 seems to identify the start of an IP address string
The second $12 (is decimal 18) calls out the length of the IP address string.
The catch is, this length has to be an even number. If it's odd, add one to the length by putting a null ($00) character on the end.

At least that's what I do and it has always worked

[kyle-github]

It is actually a special part of the CIP path elements. 16 means something like "extended path element". Then we add the port to it. port 1=backplane, 2 = first ethernet out port (A), 3 = second ethernet out port (B), 4 = third ethernet out port (A2 apparently),...

16 + 2 = extended path element using port 2.

Yeah, the padding is a fairly common thing in CIP/EIP. But it is not always consistent.

[dmroeder]

I pushed a branch to accept a port @BenGood if you want to try it out.

[BenGood]

I will as soon as I'm back in front of a PLC and not dragged into meetings.... might be next week at this point