Electronic – RS485 selectable termination device

I encountered exactly the same problem: FTDI based USB<>RS485 converter (brand is "DIGITUS"), running at 57600bps, working on some machines properly and on some it send bigger chunks of data very very unreliable. What I found out (after wasting lots of time of investigations on grounding and bus termination), is that on fast machines it did not work, because the RS485 driver chip (not made by FTDI) did not release the RS485-bus fast/reliable enough in relation with the output of the FTDI chip. FTDI is releasing the bus after each and every byte (right after the stopbit) via the TXEN port and acquires it immeditately again for the next byte (which on fast machines is already in the buffer). The workaround was, to put a delay between every sent byte and it worked. Yes, pretty bad workaround. Wish FTDI would give an option e.g. in the driver to adjust the TXEN timing.

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.