On 13 Feb 2012 at 22:33, Eric Smith wrote:
Later versions of the FDC card also allow the WD1791
FDC chip to be
reset by writing to port E8. The data is ignored.
Aside from some of my own comments that have been added, this
information is summarized from the Radio Shack "Technical Reference
Manual TRS-80 Model II Catalog Number 26-4921 Revised Floppy DIsk
Controller Supplement". Any errors above are almost certainly mine.
26-491, according to what I have, is the standard Model II tech ref.
Where does one find the supplement? Was the schematic of the late
model FDC ever published? I've seen 2 versions of the late model
unit--one with the "disable head load timer" jumpers (Rev. A) and the
other without (original late model). Were there any subsequent
revisions?
I've had good results with the late model FDC supporting standard PC
1.44MB/1.2MB/1.23MB drives without making up adapter cables. Just
populate the 34-pin header pads below J0, jumper pins 21-22 on J0
with a simple shunt and connect pins 10-14 and 12-16 on the new
header. You can use a standard PC floppy cable (with a twist on the
A: drive) and drives (preset to DS1). Seems to work fine.
If you needed a real READY signal, many 1.2M 5.25" drives can be
jumpered or option pads connected on some 3.5" drives to provide a
READY instead of a "disk changed" signal. Many 1.44M 3.5" drives can
be modified to spin at 360 RPM, so they're basically
indistinguishable from their 8" cousins.
--Chuck