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RS485 driving me crazy

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GrandAlf

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We have an installation with around 70 units with a PIC16f648 driving a low power rs584 chip. These run via a a central 12v psu. Each unit has onboard 5v reg, filtering and surge suppression and is wired in various star and series configuration. These transmit back to a single receiver in an indicator panel. Voltage is down to about 7v at the furthest point. The problem we are having is that we are getting is that the pic chips are failing, measuring around 15ohms across the power pins when failed. There has never been a failue of any other component on the board. Other than RS485, all inputs are opto isolated. We have many almost identical systems, with no issues at all. Only thing I can think of is the difference in voltage is causing a problem. PSU and various other things have been changed to no effect. This is really getting difficult and I am at a complete loss. Any advice would be really very much appeciated while I still have some hair left. Have thought of opto isolation in sections, or extra power cables, but not sure if it will help. Thank you
 

As a basic protection means, I would supply transsil (power zener) diodes, at the 12v and 5V node. External signals connecting
to the processor should also have effective overvoltage protection. Of course, protection can only work, when the processor
board has a clear, low impedance ground plane.
 

If all, from FvM proposed transient supressors are installed, eventuell sourch pls for GND-loop(s)?
For that drive the sources or your central system over isolated main.
K.
 

They are pretty well protected as is, never a problem elsewhere with similar voltage drop. I have long suspected that it may be an earth impedance issue and am going to try a direct power run with thick cable to the far end as I have no access to local power.
Thanks for your input.

Added after 3 minutes:

Forgot to mention that the zero is not connected to ground, have tried it that way though, makes no difference to the problem.
 

It's basically simple. If the circuit is effectively protected, it can't be damaged by external influences.
The protection may be either ineffective, because it's too weak, or because the interferences are bypassing it.

You didn't actually tell, which chips are failing. Of course, this matters.
 

@FvM,
He wrote it:
"...we are getting is that the pic chips are failing, measuring around 15ohms across the power pins when failed..."
@GrandAlf,
What about such RF disturbances/RFI!?
Maybe are your bypass capacitors wrong or such similar...
K.
 

the pic chips are failing
Thanks, I didn't read thoroughly.

However, you can't damage the PIC chips without supplying overvoltage with sufficient energy, either to VDD or any
signal pin. No doubt, that damage can be prevented by effective protection.
 

As I said the inputs are opto isolated, I would expect these to fail as well. Likewise with power, non of the regulators etc have ever failed. Only way I can think of, is for the supply to dip below ground and cause the pic to conduct, as it is a battery backed supply, it seems unlikely that is the case. There appears to be very little RF that I can measure. I know the circuits are ok, as we have many near identical system, that run without issue. There is just one change in that data gets corrupted without termination at the control and the end of one of the runs. This is not normally needed on other systems as the rate is only 300 baud. Cannot think that this is causing that sort of problem though. maybe I should consider opto isolation in sections. Baffling!
 

@GrandAlf,
"Cannot think that this is causing that sort of problem though."
I belief, all possibilities are to check, in a problem situation more: the inpossibles too:-(
K.
 

the probem faced by you "GrandAlf" reminded me of one of our 3 years old similar project which was dropped out due to similar problems. Its almost 90% of my personal experience with a similar project of a Room bell system using MAX485, where we had a pile up of 89S52's with the communication pins burnt from inside. We were not in a position to venture in the depth of the cause to to our limited schedule and we had dropped the project. Despite we stuggled to provide both side with various protections, the problem was recurring and I have still Master PCB's built with 89S52 ans the slaves with 89C2051 in hundreds...in my junk box with some passive components where all micros are utilised for some other projects later.
 

I appreciate all your input, guess I will have to keep battling on with it until I find a solution. Many thanks
 

Just had a thought. I am considering using 4 IL610 isolated 485 hybrids. Four seperate inputs to one differential output. No sure what the fanout is on the single ended data output, was wondering if I could common the 4 outputs and feed to a standard RS485 chip to get back to differential, or would it be better to use 4 chips enable and common the A/B outputs. Circuit is unidirectional only. Cannot see what the fanout is on data sheet.
 

Hi GrandAlf,
On pp2 is "abs. max Iout= +/- 10mA.
K.
 

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