RS-485 and RS-232 simplidied testing method

[gildasd]

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.

[gildasd]

It's one of these, 7 tons of tightly packed stuff and over 20 years old.

[gildasd]

For RS-232 things are different:



My proposed RS-232 testing method on the external pins on the bulkheads:
All standard cable continuity tests, pin + bulkhead cleaning and visual inspection done.
The setup is 3 cable, also known as "Null Modem".

1) With the system unpowered, cable removed, VM on Ohms:
    1a) Test the omhmages of the schematic, everything has to be 0Ω.
2) With the system powered (but not transmitting) on the X side, VM on Volts DC:
    2a) Tx to ground =  -Vcc (idle state)
    2b) Rx to ground = 0V
    2c) To do a powered test on Y, I need to make a test cable.

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.


RS-232 is only used on two sensors that I thinik were grand fathered in.