I have a project coming up that requires serial data comms across a large vehicle with two trailers connected in train. The total length of the vehicle is 100ft, and I need to install several nodes along the length of the vehicle that all need to communicate with the master node at the front.
I plan to use RS485, and have no problem using proper twisted pair cable throughout the body of the trailers.
My concern is connecting across the drawbars of the trailers. Ideally I want to use a standard 7 PIN trailer lights connector because of its robustness, and the availability of a curly/stretchy cable that expands and contracts as the vehicle travels around corners, but this is not twisted pair!
I can use a low baud rate as I’m only transmitting commands, not huge amounts of data.
Will it still be reliable if a section of the data line is not twisted pair?
Best Answer
It could be reliable or at least made reliable but remember that automotive and vehicle environments are pretty bad for both power supply cleanliness and interference.
If you only need slow data rates then it's quite feasible that if the first packet sent to a slave gets corrupted (or the return packet gets corrupted) this can be detected by incorporating checksums into the data stream (appended to the data payload) and a retransmission performed.
I would also consider it sensible to use RS485 interfaces that are galvanically isolated such as this one: -
LTM2881 in more detail: -
See the galvanic isolation barrier in grey. The barrier allows much higher common mode interference levels on the RS485 cable i.e. it is a more robust solution but does come with a price hike.
There are also offerings of a similar nature from TI, Analog Devices and possible Maxim.