I'm working on a design where an RS-485 transceiver IC is connected to a Cat5e cable, hopefully at 96k baud at a distance of up to 150m. I'd like to offer some protection in the design, but I've never really worked with RS-485 or TVS diodes, so I'm looking for some reassurance and advice around the protection implementation.
The data is sent over one of the twisted pairs. Another pin on the RJ45 will provide a ground reference to the other side, and the shield will be grounded.
My thoughts in the design above are as follows:
- The receiving end will have a matching protection and termination design, the only difference being that IC1 will be configured as a receiver (RE and DE tied to ground, RO outputting data).
- The termination resistor values (R3, R4, R5) were chosen fairly arbitrarily based on the five or six different conflicting examples I found online.
- R6 and R7 are intended to offer some minimal current limitation in the case of a short, and act as fuses for high voltage shorts (e.g. short to mains in a catastrophic failure scenario). With 10R on each side, I devalued the usually-specified value of R3 by 20R.
- R6 and R7 plus the others on the receiving side might help resolve the impedance mismatch between the RS-485 driver (expecting 120R) and the Cat5e cable (which I understand is 100R). Probably won't matter too much at 96k baud though.
- The positive and negative TLP curves of the TVS diodes show no significant conduction until around 6.3V, which should be sufficient to protect IC1 but shouldn't trigger until there's an actual overvoltage.
- I wasn't sure whether to leave the RO pin on the SP485 IC floating, since it's unused on this side of the circuit. Same for DI on the receiving side. I couldn't see anything in the datasheet that specified this, or showed what the IO circuitry looked like. I'll likely include pads for resistors on both, but mark them as DNP, unless someone has more info to offer.
Does this design seem sensible? Any major mistakes? Are there any improvements that can be made?