TRS-80 4P and 720K disk/image compatibility?

[Smokeless Joe]

I'm working on a Model 4P gate array version.

 

I've added a GoTek and can boot LS-DOS 6.2.1 and LDOS 5.3.1 (in Model III mode) from it.  I can also create, access, and boot from 360K disk images.

 

My experiments with adding a 3.5" floppy drive show promise; a random 1.44 MB drive seems happy to format and access 720K disks as either 180K or 360K.  

 

I can NOT seem to use disks or images at 720K capacity. I can create and use them on an emulator (trs80gp 2.5.2) just fine.  On the GoTek, a "FREE" command will show an image has 720K, but it can't take a "DIR" or run anything from it.  On the 3.5" floppy, it will act like formatting 720K (double density, two sides, 80 cylinders) is okay, but then chokes when verifying the format.

 

Can a Model 4P really use disks and disk images at 720K, or am I stuck with 360K?

 

 

[Smokeless Joe]

It turns out the cable is the problem.  
 

As per Tim Mann’s excellent FAQ, “Generally, Radio Shack handled drive select by pulling pins in the cable connectors”.  This initially included pin 32, “which later was officially defined as side select for two-sided drives.”

 

Sure enough, my cable has pin 32 removed, probably because it came with single sided drives. Trying to use a 720K double sided disk image on such a cable confuses it.  
 

Replacing the cable with one that has NO pins removed allows my GoTek to boot from and otherwise fully access 720K disk images.  Hooray!

 

Of course, I’ll need to pull some pins or adjust some jumpers to get drive :1 recognized (the GoTek is drive :0), but I’m optimistic.