This board provides a floppy disk controller for the RC2014 system. Unlike the existing controller it fits the form factor and also provides support for dual crystals for non speed changing drives or handling weird formats.
A pair of 74HCT138's decode the address bus. U2 selects the 8 byte range, whilst U3 then generates the various signals for the 37C65 controller (U1). Three signals directly drive DACK, LDOR, LDCR and provide the control registers and via DACK the termination count signal. The other two that are used are anded together via half of U4 and provide the CS line. A0 also feeds U1 to select status or data register.
U4C inverts the reset line as needed for the 37C65, and is also wired so that an address decoded by U2 resets the controller. This can be useful if the controller gets confused.
X1 provides the usual 16MHz clock for the floppy interface. X2 can provide an optional 9.6MHz clock for other data rates. With the 37C65C it is possible to use 32MHz crystal and EHD drives but this requires changes to ROMWBW and a very fast CPU.
J4 on the end of the card provides the standard 34pin interface, J3 provides the Amstrad style 26pin interface for a 3" disk.
The resistors can be about 1K or so with no problems when using later drives. Old drives may need them to be as low as 150Ohms as the floppy interface is a high current signal not simple TTL.
Thanks to the retrocomp group for all their help in debugging this.