[Coco] FDD 26 pin

[Mike Pepe]

Chester A Patterson wrote:
> Retaking the 26 pin Floppy Disk Drives adaptation. The two 720k FDD26
> are set to 0 and 1 and are on a single cable. I made a nice little board
> with a 26 pin connector to a 34 pin connector, with jumpers at the DSO
> 0/1/2 lines on the 34 connector to easily set the two FDD26 as either
> 0/1 or 1/2. It works great. BUT there is a strange behavior. The two
> FDDs have a READY line, which I have left NC. If there is no disk in the
> drive, then no amount of access will make the drive light come on, BUT
> the motors will turn. I suspect it has something to do with the READY
> line, but I am ignorant. The clincher though is that if I have disks in
> both drives, and access D0, D0 light comes on AND, half a second later,
> D1 light comes on as well! However, if I access D1 (with both drives
> with disk) it works normally (only D1 light comes on) I have checked and
> re-checked my wiring and all is well. What could it be? Thanks.
> 

I don't think it has anything at all to do with the ready line. That's 
an output the drive sends to the PC, which the CoCo ignores.

Not all drives will work if no disk is inserted.

Many of these PC drives have two motor select lines, one for A and one 
for B. I guess you should just hook them together. Something may be 
shorted out somewhere.

Two other possibilities I can think of.

1- your drive selects are not going to the right place or aren't going 
where you think they are.

2- the drives have a high impedance bus and lack sufficient "pull-up" on 
their end. (improper termination) If that's the case, all the lines that 
are inputs to the drive need a 150 ohm pullup resistor to +5v. Or, the 
opposite could be true- each drive has pullups and with both of them 
connected, the bus is overloaded. Remove one drive and see if the 
problem goes away. Try acessing drive 0 with 1 physically removed

-Mike