Model 3 Ram question
Jonathan Herr
ram up to? (I've got a 16k machine, and a 48k machine.
Also the 48k machine seems to have (i think) alignment issues. What sort of
disk drives could work with the model 3 controller? I've got a whole lot of
old 5.25 floppy drives for pcs. Could/would any of them forseeably work?
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marmotking
360K PC floppy drive will work, as long as it has a drive select
jumper. PC's relied on a cable twist to select between drive 0 and 1,
but that trick will not work on a TRS-80...hence you must have a drive
that lets you set the drive select. But even that is a little more
complicated. Tandy drives would come with all 4 drive selects enabled
and then the cable would be missing pins at each card edge connector.
Thus you didn't have to change the drive jumpers to move it between
drive 0, 1, etc. The drive select in essence came from the position
on the cable. Look at your 360K drives and see if they have a drive
select switch/jumper. Sometimes even when they don't you can modify
the board on the drive to add one. I've done this a few times.
There's another more obscure issue, which has to do with the disk
change/ready signal. If I'm not mistaking, you also need to have a
jumper on the drive that lets you change the behaviour of the ready
signal to be a true ready signal. In a PC it doubled as a disk change
signal. I know I have drives with this jumper on them, so I know that
it's an issue on at least some 360K drives. I think some 360K drives
just had a true ready signal and it's not an issue.
Now, if you want to use 1.2MB drives, that's a little trickier. Not
only do you need the drive select jumper and potentially the disk
change/ready jumper, but you also need to adjust the drive motor
speed. 1.2MB drives run at 360 rpm (I think) where as the TRS-80
controller is expecting 300 rpm. Some TEAC drives have a jumper that
lets you make the drive run at 300 rpm when in Double Density mode.
If that's the case, you're in business (just remember that 1.2MB
drives don't always write Double Density very reliably even in a PC
but they do read double density fine).
If you want to use 720K or 1.44MB 3.5" drives, you can do that too.
Again, you'll need a drive select jumper (unless you just want to use
it as drive 1, in which case PC drives are hard wired for drive select
1). You also might need the disk change/ready jumper. But 720K and
1.44MB drives always run at 300 rpm, so speed isn't an issue. The
TRS-80 will only do 720K on a 3.5" drive (that's double density with
80 tracks) so you need to find 720K media. 720K and 1.44MB 3.5" media
is different and therefore are not interchangable and the difference
is more than just covering the media type hole on the 3.5"
disk...there are differences in the magnetic coating.
There are some good web pages describing all this out there. I think
there's a good one on Tim Mann's site, if I remember right.
BTW, I have used 3.5" drives which don't have the disk change/ready
jumper and they seem to still work fine. I realize it may just be
which operating system I'm using on the TRS-80, but this particular
setting doesn't seem to be a necessity with all OS's.
Richard VanHouten
> What sort of ram does a Model 3 computer take, and how big can i expand the
> ram up to? (I've got a 16k machine, and a 48k machine.
>
> Also the 48k machine seems to have (i think) alignment issues. What sort of
> disk drives could work with the model 3 controller? I've got a whole lot of
> old 5.25 floppy drives for pcs. Could/would any of them forseeably work?
>
Just about any 5.25" double density drive will work (addressing may be a
problem, though). 1.2M high density drives won't.