1
\$\begingroup\$

I have a question regarding a scenario I found myself in, hoping that someone can clarify if the scenario I'm going to illustrate is possible (short premise: I'm a programmer, I'm not an electrician nor an electronic expert).

In my company we have 5 badge readers daisychained using RS485 and connected to an RS485 to ethernet adaptor which is connected to our LAN.

While working on renewing our server room the RS485 cable got cut by mistake by a tecnician of company A.

The day after, a tecnician from company B (the company that installed the badge readers and the software that collects the data) went to the office and said that the rs485 board on those 5 readers got burnt by a short-circuit caused by the cut cable while the devices were powered on. Since the readers now have to be changed, they suggested to move on ethernet ones.

Cutting the cable without being sure of it not being useful was certainly a mistake. However here's my thoughs: On one side company B may have exagerated the damage just to convince us to buy 5 new badge readers (I cannot know if they are actually burnt and I was not there when the tecnician came). On the other side I have company A that isn't keen to pay for 5 new badge readers for a damage that they think is impossible to be caused the way it was described.

I too, to be honest, find a short circuit surprising, but again I'm not an expert and In my programmer experience I've seen plenty of very unlikely things happen. I've also found on the internet different people burning their CIA chip on a C64 because of a cable unplugged while the machine was on, so to my unexperiend mind the short-circuit board-burning story may hold as well.

Of course I don't expect a black and white answer, rather I'd like to know similar experiences or an advice from someone that is experienced on this field.

Thanks in advance.

\$\endgroup\$
10
  • 1
    \$\begingroup\$ Why do you find a short circuit surprising? If you cut through a cable with cutters the metal blades are pretty sure to connect the conductors they are cutting. I once saw a cutter that was used to cut a 240v AC mains lead. About a quarter of one blade was missing. The user was lucky to only get a burned hand. \$\endgroup\$
    – Michael Harvey
    Nov 4, 2020 at 18:51
  • 3
    \$\begingroup\$ Is power supplied along with the RS485 wiring (such as additional wires in a DE-9 connector?) If so, cutting that cable could damage things. But if only the RS485 wires (ground, A, B) were cut, then no. \$\endgroup\$
    – rdtsc
    Nov 4, 2020 at 19:01
  • 1
    \$\begingroup\$ If RS485 cable contained only the two data lines (A/B) and ground, then it is quite exceptional that all of them broke. But if the ground wire did get cut first, then it is possible that the power supplies were balanced to common level via the ground wire, and when it broke, the data wires (A or B or both) could have voltage levels exceeding the protection on RS485 tranceivers. \$\endgroup\$
    – Justme
    Nov 4, 2020 at 19:18
  • 2
    \$\begingroup\$ @Justme I can imagine the voltage on data lines going beyond common mode limits in your scenario. I cannot imagine it going beyond protection limits, which are typically in the 10kV range when built-in into the transceiver and even more than that if external TVS is used. Unless, of course, the readers in question are badly designed. \$\endgroup\$
    – Maple
    Nov 4, 2020 at 19:42
  • 1
    \$\begingroup\$ @Justme What I meant, is that while TWS diodes designed for very short high voltage pulses they can also protect data inputs from slightly longer lover voltage pulse at the moment the wire is cut. And after the cut there should not be any applied voltage regardless of ground potential differences. This of course assumes that diodes are connected between data lines and local ground, as they should be. \$\endgroup\$
    – Maple
    Nov 4, 2020 at 20:15

0

Reset to default

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.