It seems that the VFO/data separator work in both case.
Regarding the data width difference , this probably because the HxC use a larger data pulse than the floppy disk drive.
Where have you took these measurement ? Right at the 1771 pins ? Can you take the VFO output clock too ?
what is the width of the floppy disk drive data pulses ?
Well, after a quick look at the 1771 / FD1165 datasheet and Ferguson Big Board schematic, i think that i have found the cause of the issue :
In external VFO mode the 1771 wait data and clock pulses from the VFO with a width under 350ns (150ns<>350ns).
The FD1165 generate data pulses with a width under 250ns (200ns +/- 50ns).
all ok... but:
The problem is that the Ferguson Big Board don't have the monostable circuit generaly used at the data input. So if the the data width pulses are wider than 350ns, the 1771 is not supposed to work correctly.
So the possibilites to get this work are :
- add a monostable circuit to reduce the data pulses under 350ns.
( this maybe done by a simple resistor/capacitor circuit : http://www.premiumorange.com/daniel.rob ... stable.gif
- try to bypass Ferguson Big Board VFO and turn 1771 in internal VFO/data separator mode (XTDS = 1). It can support pulses up to 1000ns in this mode.