RS485 "thermal incident" protection 6

[RyreInc]

We are using a USB to RS485 interface, currently a Gearmo USA485422. We recently had a "thermal incident" i.e. fire on this device when one of the 485 lines was shorted to 24V for a sustained period. For the other devices on this network, the RS485 chips are resilient against such assaults, but this USB interface is not.

What is a simple way to protect against such an incident? Ideally the solution would keep the USB interface operational after the fault condition is removed, but I really don't care; I just don't want a fire! Would a simple slow blow fuse do the job?

Baud rate is currently 1Mbps, but a solution that allows up to 3Mbps would be nice. Would e.g. a fuse cause impedance issues, or otherwise interfere with transmission or max node count?

I should also mention that there is no PCB involved, so any solution would need to be mounted and wired in with screw terminals or something. I recognize that wire routing will be important with any solution to prevent reflections and whatnot.

Thanks!

 

[RyreInc]

I think I'm heading to that same conclusion IRstuff.

I did come across this USB interface, which claims to be isolating. Seems like a good solution on the surface, but maybe a little TOO cheap... [URL unfurl="true"]https://www.waveshare.com/usb-to-rs232-485-ttl.htm[/url]. Thoughts?

 

[IRstuff]

Bear in mind that the 485 is still shielded and still likely connected to the chassis, and you cannot "isolate" the common. If this incident is something you're trying to eliminate, then you ought to be looking for a purely optical interface.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[LionelHutz]

Directly hook the shield up to the chassis if you're concerned about the shield being shorted to another wire. The ground fault current then won't go through the converter.

Another possibility is to float the whole device you're communicating with. Not too likely it can be done or you want to do it, but it is another way to isolate the port.

 

[JG2828]

I hadn't seen that before, seemed like a decent option, but after looking into it (you have to go awfully deep to find this!), that model only goes up to 128kbps...

RyreInc,
I noticed the website said the US-422i converter went up to 1Mb, but as you pointed out he PDF said it only goes up to 128Kb. I used the chat button on their website and received this response via email. According to them, their PDF is wrong and the device does actually work at 921,600 Kb/s. Worth giving it a try.

 

[RyreInc]

Wow, thank you very much JG2828! Although that doesn't inspire much confidence in Gearmo, does it... but I went ahead and order one (along with three other options) to try anyways - and only $50 on Amazon too.