TRS-80 Model 4 help requested

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Wed Feb 26 18:11:01 2003

> I recently picked up a TRS-80 model 4, that seems to have problems with
> its floppy drives.
>
> The machine is a base Model 4 with 64KB of ram (I think - haven't yet
> taken the EMF shield off the mainboard), and no peripherals attached.
> When I powered it up the first time, with or without a disk in the
> (bottom) drive, it displayed "Cass?" on the screen, and then I could press
> enter to that and the "Memory size?" prompt, and get a basic prompt.

I beleive that means it can't find a disk controller, so it assumes you
have a cassette-only system.

Try re-seating the 'ribbon cable' between the CPU board and the disk
controller PCB _at both ends_. I would estimate that over 50% or disk
problems on M3s and M4s come from this cable!

> Also, the disk I used was supposed to be a TRS-DOS (bootable) disk, but
> it's possible that they've gone bad after so many years. Are the disks on
> the Model 4 recorded so that I can read them on a PC (IE 48/96tpi MFM,
> compatible with the NEC D765)? I'd like to know if I can make backup
> images and/or see if the disks work on another machine.

Mostly. It is standard MFM encoding (well, the TRS-80 can do FM (single
density) too, but the M3 and M4 at least expect the boot sector to be
double desnisty, and the standard OSes use double density (MFM) encoding
for the entire disk.

The original drives are 40 cylinder SS units. The controller will support
DS and 80 cylinder drives, and many people fitted at least one of those
as an upgrade.

There is one possible problem with reading M3/M4 disks on a PC. The
TRS-80s used the Western Digital controller which can read a sector
closer to the index pulse than the NEC controller used in PCs (this is a
simplification, but it should do for the moment). Some PC disk
controllers can't read the first physical sector after the index hole on
a TRS-80 disk. When I was transferring LSDOS 6.3.1 from disk images on
the PC to real disks I found I had to format the disks on the PC, not on
the TRS-80 to avoid this problem.

-tony
Received on Wed Feb 26 2003 - 18:11:01 GMT