Hello,
I work on ROV Remotely Operated Vehicles (subsea robots), most of the transmissions aboard the robot use RS-232 and RS-485 either in simplex or duplex.
Most of our problems are due to intermittent faults on the external cabling and/or their connections to the different water/pressure resistant housings.
Twice last trip, we opened housings (up to a day of work for 3 people) convinced we had board level issues, only for it to be a cable pinched in a hard to inspect area...
So I am trying to make myself a simple testing method on the pins of the bulkhead:
So this is my proposed RS-485 testing method on the external pins on the bulkheads:
All standard cable continuity tests, pin + bulkhead cleaning and visual inspection done.
With duplex setup, it is simply 2 more A and B lines to test.
1) With the system unpowered, cable removed, VM on Ohms:
1a) Test the omhmages of the schematic (A to VCC ~720Ω, A to B ~130Ω etc)
2) With the system powered on the RX side, VM on Volts DC:
2a) A to Vcc +3.1V to +3.5V
2b) B to Vcc +1.5V to +1.75V
2c) A to B never below 0.2V, should be 1.35V to 2V.
If this is all good then start with the cable:
3) Repeat tests with cable:
3a) Bulkhead X (unpowered and powered).
3b) Bulkhead Y (unpowered).
4) In place test of disconnected cable:
4a) Megger.
Faults are hard to find on these cables, something that tests out fine on deck can fault at 40m deep, and be ok at 900m deep.
I think I have enough parts to make a testing cable for the standard 8 pin cable (used for most cameras, sensors and tools) so I can do a powered test on the Y side.
Last trip, we had time to totally remove all the cabling on the ROV and reroute it all so it passes somewhere accessible.
Normally, this is not 100% my job, but due to Covid rules, this trip our dedicated electronic wiz could not get his Visa on time...
This is on a Perry ROV so no I wont be posting things from the manual, they are touchy about this
edit: Changed the image, I had put Y on both sides.