The FDC communicated with the 8080 CPU via DMA on the bus. I/O ports F0-F7H
were utilized. Port F0H was the status port, F1H the command port.
Ports F3H & F4H set the length of data in bytes, ports F5H and F6H set
the transfer address. Port F7H commanded the drives themselves. 8"
Persci drives were utilized along with a hard sectored disk.
Processor Technology very early on saw the need for a floppy disk system.
Initially their disk system was popular. The Disk operating system PTDOS was
quite encompassing at the time with their own BASIC and a storage capacity
of 386K/disk. However as the reliability of the Persci drives came
into question, the appearance of the LSI 1791 FDC chip and the general trend
towards soft sectored disks (particularly 5" disks) the system fell out of
favor. Nevertheless properly "tuned" the Helios-II system was a joy to
work with.
The large but very detailed manual explaining the whole system can be
obtained
here.
