Please note that Pages 226 and 227 contain errors relating to the ports for Sector Number and the SDH Register. They are both listed as port 'CD' in the manual, when they should be 'CB' and 'CE' respectively. The correct ports are shown on page 220.
Strangely, although the ports for the Z-80 CTC (U25) are shown on page 220, their function is not listed on pages 225-7. I've not found the CTC ports [C4-C7] used in the HD driver source for Dosplus II, LS-DOS 6.3.1, Racet HSDS (on TRSDOS 2.0a), ATON, FMG, Lifeboat, P&T (2.2mH or Corvus versions) CP/Ms, Snappware SplitHD or TRSDOS 4.2.x. From the circuit diagram the CTC appears as through only CT/0 has an outside trigger (HDINTRQ) which I'm currently trying to trace back to Pin 11 of the Model II Bus: INTRQ* to confirm that this is indeed set up and able to be of use with the FreHD.