Circuit reliability issues

[hkpatrice]

Dear All,

I have been reading the posts on this forum for the past 3 years, today is the first time I post a message myself. Hopefully, the community of members here can help me solve my problem.

I have designed a circuit designed to switch 16 outputs on and off. It is hooked up to a RS-485 network (not exactly but similar, using two lines idling at 24 volts and a third one for ground, each 24V line may be switched off to represent digital "ones" and "zeros"). Up to 128 switchers may be connected in parallel at any time. Each of these "switchers" is adressable so that the controller can "talk" to each and every one of them separately and switch on/off the apropriate output. So, in essence, the controller can switch on and off a maximum of 2048 outputs (128 switchers X 16 outputs).

This whole network is used in multimedia applications and the configuration of the network will change from one time to the next, for instance, one application may require only 10 switchers whilst the next one may require 100.

The prototypes I built all worked perfectly on my workbench and reliablitlity was 100%. Communication was rock-solid and, after much devellopment, I had 300 of these switchers manufactured.

Here is where the problems started, Now that we are using them in the field, we are experiencing reliablitity issues. Out of 100 switchers, there is always 2-3 that will refuse to start-up, causing all kinds of problems and sometimes short-circuiting themselves.

The problem is that the circuit behaves as if the Pic doesn't start at all, or hangs, causing the outputs to be in indefinite state and disturbing communication on the network. The circuit talks back to the controller by using a current-loop kind of communication system. Essentially dropping a 270-ohm resistor from 24 volts to ground to represent digital "ones" and "zeros". This is fine when communicating at 9600 bauds, because the timing of the "ones" is very short. However, if the Pic hangs, and somehow the transistor controlling this resistor remains switched on, the resistor will eventually burn-up, causing a short circuit in the network.

This behavior is unpredictable. I may have a network with 100 switchers working perfectly at 3 PM. Turn it off, Turn it back on at 5PM and have 1 or 2 switchers not working anymore. Or, I may have a system installed for 3 days with everything working perfectly at all times.

At first, I tought that the problem was related to manufacturing, that somehow the soldering of the Xtal, or trace contaminants on the boads were to be blamed. I had each board cleaned up, and then applied 5 coats of conformal coating to each of them, unfortunately, it did not solve the issue.

I then suspected power supply problems but it doesn't seem to be the case. The circuit is powered by a clean 24Volts DC line and the voltage is regulated to 5 volts by a low-dropout regulator (LM2931AZ-5.0) with a 100uf can type cap upstream and a 47uf tentalum cap downstream, as per manufacturer's recommendations. A check with my scope shows a very clean power rail.

The microcontroller is a Pic 16F877A, with both supply pins decoupled with a 0.1uf cap and the MCLR pin tied to the power supply rail through a 4.7K resistor. No output pin is left unused and they all have 10K pull down resistors.

I am using a 4Mhz Xtal, with 20pf ceramic-disc caps, as close to the chip as can be. The whole circuit has a ground plane.

More and more, I am suspecting that the problem is software-related. Specifically, I am wondering if I did not make an error in setting my configuration fuses.

The symptoms of the problem will be as if the switcher starts to communicate to the controller, and then just stops. If that happens, the 270 ohm resistor will stay between 24 volts and ground and will either short-circuit and/or disrupt the current-loop communication. Another way it can fail is that it will just stay non-responsive, as if the Xtal did not start up. Since this is intermittent, and happens in the field, it is almost imposible for me to take readings with a scope when it happens. Please note that the switcher only communicates when asked, however this problem will pop up at power up, even before I ask the switchers anything. I am sending a dummy message at the start of my program, I read on this forum that it was preferable to do this when using the USART.

I am setting the fuses in the Melabs programmer, not in the program itself. My fuses ar as follows:


Oscillator: HS
Watchdog timer: Enabled
Power-up Timer: Enabled
Brown-out Reset: Enabled
Low voltage programming: Disabled
Flash Program memory write: 0x1000-0x1FFF
code: Protected
Data EEPROM: Protected


I am wondering if a programming blunder somehow puts the pic in reset, and the outputs remain in indefinite state? But then again, the problem seems to only affect the pins driving the two transistors controlling the 270 ohms resistors, the 16 transistors doing the switching are never affected. Or maybe the crystal doesn't start up? but why? Or maybe the 74ALS08 controlling the two "communication" transistors fails? I do have one unused pin on there that I forgot to tie to anything...

I have attached the listing of the program to this message, as well as a JPEG file of the circuit.

Any help or suggestions, be it hardware or software related will be more than welcome as I have been trying to solve this for the past 2 months and am now running out of ideas.

Thanks and best regards,

Patrice