AB uLogix 1000


Thread Starter

Mark Riche


We are presently experiencing problems with the AB uLogix 1000E powered with 24 V dc. We have fried 4 to date. We are using 24 V dc with a
grounded negative. Two were destroyed when a positive ground occured. The Chassis ground was tied to ground at that point and the 24 Vdc battery negative was ungrounded. ( I understand in the uLogix that the Chassis ground, 24 Vdc negative and the RS232 ground are all common). We removed chassis ground at that point on the remaining uLogix.

We then fried another uLogix when, we believe, a high resistance ground occurred on the positive supply. We then grounded our 24 V dc supply at
the negative. Everything was fine until yesterday. A technician attached an RS232 cable to the comm port and the uLogix lights went out.

Has anybody experienced similar problems or have any suggestions ??


Mark Riche, P. Eng.
Electrical Design Engineer
Northwest Territories Power Corporation
4 Capital Drive
Hay River, Northwest Territories
X0E 1G2
ph (867) 874-5266
Fax (867) 874-5264
We have experienced same thing at our facility, what kind of serial cable are you using?
Best to use the cable specified by AB, if you are not using the correct cable you can be blowing the internal fuse. Spend the $200 for the cable and it will work. (notice any communication
cable from AB is almost always $200) Also if you replace the fuse the unit should still work.

Gary Drake
Lead Process Automation Tech
Cargill Fertilizer
Riverview Fl.

Larry Lawver

My OEMs buy a lot of those, and I have not seen this problem.

The 24V MicroLogix1000 should be able to defend itself against most wiring errors if it is only exposed to one 24V source. Some ideas:

Are you mixing the voltages from more than one 24V source? This can work, but grounds and commons must be carefully thought out.

Are AC voltages entering the system? Again, this might be legal, but not on the inputs!

Are you using the inputs in sourcing mode? If so, please evaluate your input commons carefully, and remember that you can't mix sourcing and
sinking on the same input common.

Finally, have you asked Allen-Bradley for a Product Service Report (PSR) on any of the failed units? Contact your distributor or Rockwell
Automation rep to learn how to get an evaluation of the failures.

Hope this helps!

Larry Lawver
Rexel / Central Florida

Curt Wuollet

Hi Guys

I just wanted to let you know that I have added this to my 100 worst automation ripoffs list. As a reference point I buy serial cables for $3.98 for the LinuxPLC.


The uLogix 1000 serial port is a mini din/ps2 style connection. To save a little money we tried using a mini din to 9-pin serial converter that comes with a new mouse with a standard serial cable to connect to the unit. the problem was that pin arrangement is such the +24VDC from the uLogix is grounded through the serial port of the PC. After replacing 3 units we bought the correct AB cable (1761-CBL-PM02-series
B cable a 9 pin d-shell to 8 pin mini din serial cable). Only then were able to connect and program the unit.

The vendor neglected to point out the potential problem to either the project engineer or contractor doing the installation. Only when
the job was turn over to automation for programming did the problem showed up and the finger pointing started, was the problem addressed
and the proper cable bought.

So now our policy is to buy the recommended communication cable when ever a new style of plc is place in service.

(BTW AB recommends a optical isolator between the unit and any PC used for programming)

Gary Drake
Lead Process Technician
Cargill Fertilizer
8813 Highway 41 South
Riverview, Fl 33569
phone (813)671-6213
fax (813)672-6438
Doesn't AB still offer their book of switch settings, cable wiring, and other details? My copy is Publication ICCG-1.2 - February 1992. _Control, Communication and Information Reference Guide_

I would like to get a newer version of this book, but a quick search doesn't show that pub. # on the web.


Speaking for me, not for Starbucks.

Larry Lawver

The Allen-Bradley 1761-CBL-PM02 cable for connection between a PC serial port and a MicroLogix controller lists at $50 today. If you have the business credentials to set up an account with your local distributor, you will get a substantial discount off that price. If you have the technical credentials to build your own cable, you can do that as well. That should keep the price well below $200.

Hope this helps!

Larry Lawver
Rexel / Central Florida