Cromemco 4FDC

Later Cromemco disk controllers such as the 16FDC and 64FDC contained both single and double density data separators and the 64FDC also supplied write pre-compensation.

The FDCX4, in addition to using an analog phase-locked-loop data separator in all modes, also used write-precompensation.

When this switch is OFF the RDOS program resides in the computer memory space from address 0xC000 to 0xC3FF.

If this switch is ON the 1 KB ROM containing RDOS will automatically be disabled after CDOS is bootstrapped in from a disk thus clearing memory space from 0xC000 to 0xC3FF for system use.

(In this mode the ROM is actually disabled by an output to port 40H which is done automatically by CDOS).

Signals to the drives from the 1771 are TTL-buffered with separate buffers for mini and maxi connectors.

If the 4FDC is allowed to perform the interrupt acknowledge, it will pull down its Priority Out/ line to signal others in the chain not to respond.