That looks scarily like it's adding a pullup to 5V on the RD line, and the CM5 is not (officially) 5V tolerant. It also appears to be star wiring the RS485, when it should be a linear bus with 120ohm termination at the two ends (only really needed for speeds over 100kbaud).Is there a reference design for RS485 chips, especially regarding CTS/RTS functionality?
I am currently using this design, which has been adapted using Renesys (manufacturer of the RS485 chip) feedback:
The normal chip I'd turn to for a 3.3V RS485 transceiver is the MAX3485.
Their datasheet doesn't seem to cover the biasing of the line, probably as that is generic to any RS485 bus, and should happen in one place if needed. Typically bias resistors are only required if you need to guarantee the bus state between messages, and my (limited) experience is that that is only critical if you've got fibre transceivers or similar that will only turn around if the bus is idle.
Statistics: Posted by 6by9 — Thu May 15, 2025 7:42 pm