Allen Bradley MicroLogix 1100 PLC - Page 1
Forum section: electronics / repair

author: olewales
author_rank: Regular Contributor
timestamp: March 31, 2016, 04:48:43 pm
content: Hi,I've recently got myself Allen Bradley MicroLogix 1100 PLC controller to play with. I bought it for next to nothing because it is dead - no reaction to power. As it turns out there is very good reason for that. Someone has opened it before and the entire power supply board, which takes 12-24V DC in and provides internal power rails is missing.I'll be posting complete teardown eventually because some interesting design can be seen inside (like using 3 different parts for isolating 10 seemingly identical inputs), but for now I'd like to ask you guys if you by chance happen to know what voltage rails are supposed to be there or better still: pinout for "P2" header connecting power board to the rest of the controller.I have not had much luck googling for it. The only (possibly) relevant piece of information that I found is this teardown of bigger model from MicroLogix family - ML1400http://steelcityelectronics.com/2015/02/09/micrologix-1400-plc-teardown/It mentions that PSU provides 3.3v, 5.2V and 24V. It seems reasonable (I know for sure that 24VDC must be there, because that's output relay's coil voltage) but it is a different device and I don't want to possibly fry it by randomly applying voltage to it (just yet).

author: jeroen79
author_rank: Frequent Contributor
timestamp: April 01, 2016, 03:32:47 am
content: I would lookup the ICs on the board and search for datasheets.Then you'll know their pin layout and maximum ratings you can then trace these back to the connector.

author: olewales
author_rank: Regular Contributor
timestamp: April 01, 2016, 08:37:49 am
content: Yep, already started doing that, but there is always a chance that I miss something. I think that PSU board was removed because it was damaged (plastic case is a bit melted in the bottom) but considering that those things are built to be quite tough there is a chance that the rest of the electronics survived this just fine. It would be a pity to fry it by accidentally apply 24V to 3.3V rail now...About 24V rail: I have a problem identifying IC in the photo attached. It is marked 305AG (I believe that 939 is a date code. It is pretty consistent across all components). I did not have much luck googling for it but maybe I am just not looking in the right places. This IC directly drives output relays (those SOT-23 devices are just protection diodes across coils) so it would be a good place to trace 24V rail.

author: olewales
author_rank: Regular Contributor
timestamp: April 07, 2016, 01:50:11 pm
content: I need a break from this so here are my findings do far. Maybe one of you will have any idea if there is anything else to try.I think I have identified most of the pins in psu board connector. They are marked in the picture attached below.Identifying ground, 3.3V and 5V rails was pretty straightforward - just look up few ICs datasheets. There does not seem to be any local regulation on the main board, apart from isolated IO section, which has its own LM7805.I had more problem with tracing 24V needed for relays operation. I was not able to identify TSSOP-16 driver IC but one of the pins was directly connected to protection diodes on the cathode side. This also connects directly to one of the pins on the external module header so I am pretty sure that this is indeed 24V rail.Two pins in the top row do not seem to be connected anywhere except for the screw header. On my unit (pn: 1763-L16DWD) those places are marked as "not used" but on the AC powered versions of this PLC they are used for providing 24V dc for sensors.However, after connecting power all I am getting are two LED lights: "Power" and "Fault". In the manual this condition's probable cause is described as "Loose Wiring" or "Processor Hardware Error". Normally I would just give up at this point but after reading some forums it seems that this state is often caused by power issues so I believe that main board may still be ok. (numerous) clocks are present and LCD seems to be initialized a second or so after power up (faint pattern appears) but no activity besides that. Like some brownout protection was holding device in reset.I guess it might be related to the one pin in P2 Header I was not able to identify - marked in red in annotated photo. Its location suggests that it may be separate 24V ground but after probing with multimeter it seems that it is only connected to one of the pins of Rockwell branded Atmel part on the main board (I suspect it's either ASIC or AT40K fpga. It has 64MHz clock on one of the legs and in a bigger PLC model in the line - ML1400 there is an Altera cyclone next to Coldfire MCU also present here) so it seems to be logic IO of some sort. I have two ideas for what this line might be for: "power on" signal from MCU to PSU, allowing for some kind of power sequencing or some sort of "power good" signal form PSU to MCU keeping it from booting until voltages are stable. This pin seems to be constantly in high impedance but applying 0V or 5V to it do not change device behavior.I'd be interested if you have any ideas how to bring it back to life. Of course, one of the ICs on main board may be fried in which case there is not a lot that can be done but playing with it was still fun.