Compactlogix L35E Serial MSG


Lifetime Supporting Member
Join Date
Mar 2005
ENE of Nowhere Oklahoma
Customer has a radio telemetry master using a Compactlogix L35E Serial port set up as DF1 Half Duplex Master and has been adding remote stations. He believes that he has reached the connection limit of 100 for that CPU. He can disable one old RTU and get the new one to work, but then can't get the old one re-enabled. I have not yet seen the code (he's supposed to email me the acd file).

I didn't think that a MSG connection using the serial port for the path should count toward the connection limit, but I am more of a Micrologix guy when it comes to telemetry systems and some of the literature and technotes I have researched seem ambiguous. So maybe they do?

That is the question: If there are 100 messages configured to 40 different DF1 half duplex slave IDs from a L35E serial port, will we run into a limitation that I am not thinking of?

I told him to use the task monitor tool to examine the running CPU to find out how many connections are in use, but haven't heard back yet. I think he is fighting multiple other fires today.
Last edited:
The serial port does not have any limitations. The Ethernet modules do have a CIP connections limit. It should be able to talk to as many as it wants. But it might just be getting overloaded with to much traffic. There is a limit to how much data you can squeeze through a serial port.
Might be better off to move all that to a Gateway device that can do all that work for the PLC.
Thank you. The customer reported back that the Task Monitor Tool shows plenty of free connections, so the root cause of his issues is elsewhere.
DF1 Half Duplex can get a little complicated from the Master side of things. There are two types of polling modes (Standard and Message based) both of which have their pros and cons and both of which could cause the symptoms you are describing. I'd be looking at how his master is setup. More than likely adding another group of remotes will require at least some consideration on changes that may or may not need to be made to the master.
P.S. One thing that stands out in my mind (from what you've described), if he's using Standard messaging, is the polling group size set to 100? If so, that will be the limit of active nodes.

Similar Topics

I have a problem and I can't find anything about it in the Rockwell Knowledge Base. The problem is that I am losing communication with a...
Hi all, I am an amateur PLC user. I am using a Compactlogix L35E. Some time in the past week, the I/O light began blinking green. Whereas...
Hi all. I need litle help. I need connected Micro 850 ( 2080-LC50-24QWB) to a Main PLC (CompactLogix L35e) and send/recieve data throught...
Dear All , I need help to resolve the issue of the RPI that has been not changing on my configured L35E processor ? it was running since but now...
Hi, I am new to PLC and seeking advice here. We have a compactlogix 1769-L35E. The L35E has been situated in shelf for a while, therefore, the...
Top Bottom