RS-485 and RS-232 simplidied testing method - Page 1
Forum section: electronics / repair

author: gildasd
author_rank: Frequent Contributor
timestamp: October 24, 2021, 05:52:16 am
content: 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 +3.5V2b) B to Vcc +1.5V to +1.75V2c) 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 thisedit: Changed the image, I had put Y on both sides.

author: gildasd
author_rank: Frequent Contributor
timestamp: October 24, 2021, 08:17:48 am
content: It's one of these, 7 tons of tightly packed stuff and over 20 years old.

author: gildasd
author_rank: Frequent Contributor
timestamp: October 24, 2021, 01:38:31 pm
content: 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 = 0V2c) 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.