I am using an Analog Devices ADM483E RS485-chip to build a device with RS485-capability. Datasheet here: http://www.analog.com/media/en/technical-documentation/data-sheets/ADM483E.pdf .
The finished apparatus is supplied with 24V DC, and now I'm looking to design it so that it isn't destroyed if a technician accidentally connects the supply 24V +/- to the RS485 inputs (which are connected to the RS485 input/outputs on the ADM483E).
The ADM483E is specified for +/- 14V applied to the RS485 inputs/output.
It would be perfectly adequate for the apparatus to simply short-circuit the power supply in order to protect itself. The power supply is 24V DC and I can arrange for it to have a fuse of 2A.
I can't use reverse biased diodes to ground, since -5V is perfectly acceptable RS485 signal level.
I've considered using transient suppression diodes, but the typical ones are only rated for 1-2 joules of transient energy, and I haven't found any 2A fuses that will reliably break at such low energy.
Do I have to build my own crowbar device, or is there some best practice for what I'm trying to do?
Best Answer
Rather than the brute-force method which is fraught with challenges & compromise, I'd put a couple of PTC resistors (aka polyfuse) in series with the 2 RS485 lines, & probably place them the last thing before they reach the connector to go off-board.
PTCs aka polyfuses have a near-zero resistance normally, but when you exceed their hold current & approach their trip current, they go fairly high resistance, and stay that way until the 'fault current' is removed. So if you apply 24V to the 485 pair, current will go through your termination resistor (120 ohms?) & fairly quickly put them into trip.
For RS485, depending on what source & line & termination resistors & impedance you've got, something around 50-100mA 'hold' current should be adequate for them to stay low-resistance at normal operating conditions.
PTC/polyfuse examples: http://www.littelfuse.com/products/resettable-ptcs/radial-leaded.aspx