5i25 with 7i84 and THCAD

Hi PCW,
I have plenty of Mesa boards sitting around and while looking through all the firmwares for the 5i25 i ran across the 5i25_7i78x2.bit with the attached pin configuration:

IO Connections for P3
Pin# I/O Pri. func Sec. func Chan Pin func Pin Dir

1 0 IOPort StepGen 0 Dir (Out)
14 1 IOPort StepGen 0 Step (Out)
2 2 IOPort StepGen 1 Dir (Out)
15 3 IOPort StepGen 1 Step (Out)
3 4 IOPort StepGen 2 Dir (Out)
16 5 IOPort StepGen 2 Step (Out)
4 6 IOPort StepGen 3 Dir (Out)
17 7 IOPort StepGen 3 Step (Out)
5 8 IOPort PWMGen 0 PWM (Out)
6 9 IOPort PWMGen 0 /Enable (Out)
7 10 IOPort PWMGen 0 Dir (Out)
8 11 IOPort SSerial 0 TXData1 (Out)
9 12 IOPort SSerial 0 TXEn1 (Out)
10 13 IOPort SSerial 0 RXData1 (In)
11 14 IOPort QCount 0 Quad-IDX (In)
12 15 IOPort QCount 0 Quad-B (In)
13 16 IOPort QCount 0 Quad-A (In)

so i had an idea that i need confirmation for, namely wire a 5i25 to an 7i84 through sserial and use the encoder inputs for the THCAD.
Now the uncertainty, the 7i84 uses two sets of TX/RX wires, but the 5i25 has only one set and a TXE, this is one one port. There are the same pin outs on the second 5i25 header, but that seems like the second sserial, not to be combined with the other one.
My guess is that wiring TX0 (5i25) to RXA (7i84) and RX0 (5i25) to TXA (7i84) should work but i have doubts about the missing second communication channel.
I am pressed for time, that is the reason for asking this, as i would just wire it and try.
Thanks in advance.

In general you cannot connect a 5I25 directly to a 7I84, The reason is the that the 7I84 communication is all differential (RS-422) and the FPGA signaling is single ended

Its possible to get differential inputs and outputs with some bit file mods and external resistors but you will lose the common mode range and ruggedness of an actual RS-422 driver/receiver, that is the RS-422 receiver can pick up 200 mv of signal riding on +-5V of common mode noise.

Thank you PCW, that is exactly why i asked.
I could use some IC to do that but that would defeat the purpose.
Back to work.

Yes its one IC like a SP491