It had been a while since I used this system. I took it out because it's capable of formatting a few 8 inch IBM 3740 format disks (SDSS) needed for testing Digital RX01 drives. DEC did not provide a way to format disks.
The system did not boot. Great. By probing components I discovered I had a blown cap near the 7812 and a bad 1488 driver on the CPU I/O card, possibly blown because of the bad voltage regulator. I made the necessary repairs to return the system to operational status.
IBM 3740 format uses 128 bytes 26 sectors per track, ~241K capacity. There were a lot of disk formats back then, I had to double check I was building the right thing as the old systems are flexible yet cryptic.
The XOR S-4 system I am using has Mitubishi M2896-63-02U drives, they're 8" DS/DD and the disk controller uses a 1795 FDC chip.
NOTE TO FUTURE SELF: The bottom of the two serial ports by default is to be used by the terminal at 9600 baud. Use a NULL Modem cable. When you turn on the system it should put you to the monitor prompt (*). To boot from the left disk drive (drive A) hit CTRL+C on the terminal keyboard.