MAX485 wrong data output

Martin,

I'm by no means an expert at RS485, but I have run into some of the same problems you've had with independent power supplies on multi-drop RS485 networks.

Most frequently, in spite of claiming fail-safe operation, they don't play nice with ground loops/DC offsets. For me, in the past, the most direct route is to use fail-safe biasing on the two lines. E.g. .

By the way: Most of my learning has come from others here as well as some pretty good application notes from Texas Instruments (PDF), Maxim (HTML), and Analog Devices (PDF).

Hope to hear if this gets you in business.

Josh

You're running in half duplex mode, joining Y to A, Z to B right? Why do you mention A- and B+ and A- and A+? I'll assume you just mean A and B, and that the termination resistor is across them.

Each receiver at the end of the 485 wires require 120ohm termination resistors, so in half duplex mode you need one on either end since both are receivers. Do you have one on the FTDI end too?

485 transmits 1/0s by separating the pairs by a voltage, why do you want 'failsafe' resistors that will tie up the bus? In an idle state the voltages should be the same between the two, which is what the 120ohm resistor also helps to do, you're working against this by pulling the separate lines apart and wasting power. You have 360+360+120ohm from VOUT to GND permanently.

What should the polarity of the stop bit be? Check the TX pin polarity of the UART and make sure its idle is 0 during the delay period(s). If it's transmitting a 1 it could be erroring because the FTDI receiver is never seeing the stop bit?(see update below)

The voltage levels themselves don't matter so much as there is a >200mV difference between A/B. The FTDI chip should have no problems determining what you're sending with a 3v difference, just as your receiver did with a ~2v difference.

Update: Since realizing that you are able to actually receive correctly the first few bytes before the bit error, this looks more like a timing issue. Try run at a lower baud rate and see if you get any more received data before the error.