Allen Bradley Modem Communications


Thread Starter


I am trying to set up a modem connection with an A-B SLC5/03. There is a US Robotics Sportster modem on each end, and I can get the RSLinx driver to dial, and the modems connect...but I can't go online with RSLogix. Any tips besides A-B website?
A recent experience with this to a micrologix 1500 had me stumped, finally found buried in AB documentation these settings to be applied to both RSLinx & PLC - try them out and let us know if they also apply to SLC's

COM# (Modem Port)
9600 Baud
No Parity
1 Stop Bit
BCC Error detection
Full Duplex Handshaking


Larry Lawver


Actually, the notes about this matter in the A-B Knowledgebase online are pretty good, since this is a very common request and a lot of tech
support time is spent on it. Please look there for details; here, I'll sketch out what you have to do.

Two things are essential: The modems must be set to operate as "only 9600 baud" and all compression must be turned off. This may be, on your modems, soft commands, DIP switches, or a combination of both.

Of course, both RSLinx and the 5/03 must be expecting 9600 baud. Autoconfigure doesn't work through the modem, you have to set the baud
rate and error checking settings manually.

At the 5/03, Channel 0 must be set to System Mode (factory default).

If you have all the settings right, and you still can't go online, then you have a problem with the actual baud rate or with compression. The
Sportsters are excellent modems, but they were designed for the Internet and want to go fast. I personally own one that cannot work with RSLinx,
because it will not keep the 9600 baud rate if it can negotiate a higher rate with the modem at the other end!

Browse the Knowledgebase and see if the settings for your particular modem have already been determined.

Hope this helps!

Larry Lawver
Rexel / Central Florida

Curt Wuollet

Try asking on the or lists.
Ron Gage has done similar stuff for Linux.



Jason Valenzuela wrote:
>I am trying to write a program for Linux that reads and writes to data
>tables in a SLC 5/05 via ethernet. Similar to what I can do w/ DF1. I've
>glanced through the Ethernet/IP specification, but it seems that it is not
>specific enough to write a program to communicate w/ an existing product.
>Does anyone have or know where I can get specific information on how
>Allen Bradley implements the Ethernet/IP protocol in their SLC 5/05? I am
>looking for something along the line of their "Data Highway Protocol and
>Command Set" publication, but for ethernet.

The SLC5/05 can use either the CSP protocol (DF1 in a TCP/IP wrapper) or it can use the Ethernet/IP protocol. For simplicity sake, I recommend using the CSP protocol as it is worlds easier to implement, and is much better documented (from a usability standpoint).

In the meanwhile, might I suggest you look at my ABEL library (Allen Bradley
Ethernet library for Linux). It is freely available software and may be downloaded from

This library is mostly designed to talk to PLC5's but it has success with SLC's as well.

As for Ethernet/IP... I am working on licensing issues (Ethernet/IP requires a
license to develop software) but once those issues are resolved, I plan to release my CELL library (CIP/Ethernet Library for Linux) - probably commercially.
Currently, the CELL library is capable of extracting all tags from a ControlLogix
PLC (controller and program scoped) and it can read the tag values as well (for the most part, I seem to have some troubles with array'ed and/or structured tags right now). When this library is finished, it will also be able to write values to the tags on the ControlLogix PLC. The product will be an open-source commercial product, distributed as source code only. Target pricing for the library is probably $99.00 - to cover my licensing costs.

>Jason Valenzuela

Ron Gage - Saginaw, Michigan
([email protected])

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