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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?

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  • \$\begingroup\$ This shouldn't be a problem as you have a quite "clean" environment. Maybe use 5V and not 3.3V drivers and add a checksum to your frame for the worst case. \$\endgroup\$ – Humpawumpa Feb 23 '18 at 7:46
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Will it still be reliable if a section of the data line is not twisted pair?

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: -

enter image description here

LTM2881 in more detail: -

enter image description here

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.

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  • \$\begingroup\$ Thanks. I've scanned through the datasheet for this... but I can't work out how I would power this. I'm going to be running this off the 24v vehicle battery, with probably a linear regulator to get 5v, but how would I split this into the two isolated power pins? I guess linking them and powering them off the same 5v would defeat the point of using the LTM2881. It might be too expensive as well as I need around 8 slave nodes. \$\endgroup\$ – BG100 Feb 23 '18 at 10:32
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    \$\begingroup\$ @BG100 you don't need to power the isolated section - internally it provides power through the isolated DC/DC converter - the isolated 5 volts is brought out on a pin and can power "line-side" circuits (if needed). Don't try and power your carnival lights from it though!! \$\endgroup\$ – Andy aka Feb 23 '18 at 10:34
  • \$\begingroup\$ I see... Thats very clever... I dunno how they manage to fit all of than in a single package with no external components! I don't think I can use it though, because of cost but also this is just a hobby-type project, and I don't have access to specialist PCB and soldering kit that I think is needed to solder the types of package this comes in. I may just try it with a cheap basic rs485 transceiver, slow baud, and error checking in the firmware, and see how it goes. Thanks for your suggestion though, its interesting reading the datasheet on this. \$\endgroup\$ – BG100 Feb 23 '18 at 10:41
  • \$\begingroup\$ If the package and the price is a problem, TI has an alternative design that uses ISOW7841 signal/power isolator + any RS485 driver you like. You can use SOIC packages in it and the solution still will be quite small and, probably, cheaper than the LT one. \$\endgroup\$ – GAttuso Feb 26 '18 at 8:20

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