General > General Technical Chat

Fault protected RS485, constant short circuit to power supply

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langwadt:

--- Quote from: nctnico on January 27, 2022, 05:55:07 pm ---I'd use the 36V TVS diodes only and call it a day. Putting fuses in series isn't necessary; these might cause impedance mismatch problems in case you are using long wires. The pull-up / pull-down are also not necessary. The receiver has build-in pull-up & pull down resistors.

--- End quote ---

is the build in pull up/down sufficient? Siemens sells an add-on fix for their some off their RS-485 stuff, without it noise on the idle line screws with communication

http://www.easycnc.org/media/catalog/product/cache/1/image/1024x/040ec09b1e35df139433887a97daa66f/A/5/A5E00151017-RS485.jpg

chris_leyson:
Hi Mikeselectricstuff. The LT2862 was a drop in replacement for a competitors standard RS485 driver and was used on a 50m or longer cable run to interface a laser vision system to a welding control cabinet. 48V was used for the supply to avoid any voltage drop in the cable, sometimes 100m. It got expensive because a laser sensor, PCB from the control cabinet and cable had to be shipped back, repaired, and shipped to the customer. That meant a few weeks where your customer has a welding system out of commission. The additional cost of a robust RS485 driver is tiny when compared to all the shipping costs and loss to production, not to mention having to scrap some PCBs because of vaporised tracks.

I will see if I can dig out a schematic to check on the exact LT part number. TVS diodes were retro fitted to the laser vision system but didn't fix a short to 48V.

Would be nice to see someone reverse engineer the LT2862 because I couldn't see how the driver could survive a short to 48V.

abomin3v3l:
I think I would like to have the option to have external-to-transceiver pull-up and pull-down resistors on the the bus, connected in one of the devices, in order the transceivers be less susceptible to noise, than just using the internal ones.

About PTC fuse, a PTC with I.hold of 200mA and I.trip of 400mA, in a part number I saw on Digikey, its normal/off resistance is around 0.8 ohms and the post trip resistance (R1,max) is around 5 ohms. If +24V connects to A or B, the TVS SM712 would burn, and maybe the termination resistor also. And I.hold and I.trip are very dependent on the ambient temperature. Really, I am not experienced with designs using PTCs.

Consider now a very wrong miswiring in the installation of the product, where the wires of the power supply were connected to A and B, and that the board is not powered (miswiring) and that then the GND of the board is floating, in this case the SM712 would not trigger the TBUs simply because there will be no current flow through it. That's the reason I have added the differential TVS diode SMAJ7.0CA.

Regards.

chris_leyson:
Don't think the ADM2587E driver will toletate an A or B short to 48V. Nice part though, has good CMRR

langwadt:

--- Quote from: chris_leyson on January 28, 2022, 12:04:52 am ---Don't think the ADM2587E driver will toletate an A or B short to 48V. Nice part though, has good CMRR

--- End quote ---

if the pair is floating it can't see 48V

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