Port0 268h
Read:
bits 0-6 unused by TC.EXE
bit 7 Index signal. High for duration of index pulse, low otherwise.
Write:
Not used by TC.EXE.

Port1 269h
Read:
Not used by TC.EXE, probably write only.
Write:
Unknown purpose. I have only observed TC.EXE writing 20h to this
port, but the disassembly suggests other values are possible.
If output to this port is disabled, TC.EXE fails to identify
the type of disk and suggests using TCM. TC.EXE writes this port
many times during operation, but only the first time is needed
for success.

Port2 26Ah
Read:
Used to detect transcopy board. My board always returns 23h.
Maybe board version?
Write:
Unknown purpose. TC.EXE is coded to always output 00h followed by FFh.
Disabling output to this port seems to have no effect.

Port3 26Bh
Read:
Not used by TC.EXE, probably write only.
Write:
bit 0 GO
bit 1 RDENBL
bit 2 WTENBL pin 55 of ASIC.
bit 3 HI_DENSITY_SEL Pin 53 OF ASIC.
bit 4 DIR pin 52 of ASIC.
bit 5 HEAD_SEL pin 50 of ASIC.
bit 6 HEAD_STEP pin 51 of ASIC.
bit 7 unknown.
TC.EXE does not seem to use bits 3-7, they are always 0.
In many places TC.EXE turns GO on and off before other operations,
a reset of some type, perhaps?

Port4 26Ch and Port5 26Dh
read:
Not used by TC.EXE, probably write only.
Write:
Clock period registers. Both ports are written to the same
value during reading, value determined by disk and drive type.
During reading, Port4 determines the period of the clock given
to the UM8326B data separator. During writing, only Port4 is used, value determined by disk and
drive type and track length requirements. Reading clock period in nanoseconds
is calculated as:
(257-N)*20.833
when N (the number written to the port) is 251 (hex FB) or less.
Clock values above N=251:
FCh 208.33nS 4.80Mhz
FDh 166.66nS 6.00Mhz
FEh 125.0nS  8.00Mhz
FFh off, output low.

examples-
         Type                   Read          Write (adjusted up & down slightly if keep track length=Y)
360K disk in 360K drive     F5h (4.00 Mhz)    A1h 
360K disk in 1.2M drive     F7h (4.80 Mhz)    B0h
720K disk in 1.44M drive    F5h (4.00 Mhz)    A1h

TE.EXE uses the same values for MFM disks.
MCP.EXE uses a lot of different and very low frequencies - 250 to 775 Khz.

Port6 26Eh
Read:
Not used by TC.EXE, probably write only.
Write:
Not used by TC.EXE. TE.EXE Writes 01h to this port. Unknown purpose.

Port7 26Fh
Read:
Not used by TC.EXE, probably write only.
Write:
bits 0-3
Unknown purpose. I have only observed TC.EXE writing 0h and 0Ah to this
port. If output to this port is disabled, TC.EXE fails to identify
the type of disk and suggests using TCM.
bit 4 unused pin 49 of ASIC.
bit 5 unused pin 48 of ASIC.
bit 6 unused pin 46 of ASIC.
bit 7 unused pin 45 of ASIC.
