Mini-Disks

Are the TRS-80 Mini-Disks different from the CoCo ones? What's actually required to hook a Mini-Disk up to a Model I?

"Mini-disk" is just the name RS used to refer to a 5.25" disk, as opposed to the normal "Disk" which is 8". To add a disk drive to your model 1 you need an expansion interface, a disk drive, cable, and DOS disk. Any RS 5.25" disk drive should work and they come up regularly on ebay, CL, this place, etc.

Hope this helps,

Ian.

Thanks. I picked up a pair of these at Goodwill and noticed how closely they resembled the CoCo disk drives I have. Good to know they're interchangable. I will probably use them with my Model IV, as my Model I has no expansion interface.

BTW, if anyone in the Omaha area reads this, there are two Model IIIs at the Goodbytes store on 72nd. One powers on, and they want $30 for it, the other does not and they $15 for it. I would have picked them up myself, but I already have a Model IV and III. Still tempting to get spares, but I don't have the space for them.

One thing to remember: at any given time (over the product lifespan of the full-height 5 1/4 inch disk drives) Radio Shack actually sold two different catalog SKUs, and they *are* different. In the catalog they would be "Drive 1", and "Drive 2 through 4" or "Drive 3" and "Drive 4", depending on whether you were talking about Model I, Model III/4, or CoCo drives. (Looking in the 1981 Catalog the part numbers were 26-1160 and 26-1161 for "1" and "2 through 4" respectively in the Model I section. The SKUs would differ for the other machines, the difference basically being the stuff bundled with the Drive 1/3 package. The "first drive", which in the case of the III/4 would be the *3rd* drive because the first two were internal, included a cable, a DOS disk, and/or a controller, depending on the target computer.)

The important point: The "first drive" always needs to go at the end of the cable furthest from the computer. Additional drives populate the other connectors on the cable. The "first drive" units are fitted with terminating resistors, the others lack them. If you mix them up, omit the "first drive", or put two "first drives" on the cable the system will be less electrically reliable. It's an obscure point, but speaking from experience the Model I's single-density controller in particular wouldn't give good results if you got the termination wrong.

(Now, if you *do*, for instance, in the course of collecting end up with multiple "first drives" and want to use them on one chain it's not a big deal to pop the lid off one and remove the resistor pack. It looks like a fat IC and is in an obvious socket on the drive itself.)

And... sigh. I'd totally pay $30 for a Model III that wasn't a thousand miles away.

That will be important to know. Since there were two drives, and two model 3s in the shop, I'm guessing that both drives were intended as third drives, which means I'll probably have to convert one if I want 4 drives on my model 4. But who wouldn't want that?!

*snicker*

I always found three drives to be about the ideal configuration for a TRS-80. Since the "dynamic overlays" design of the TRS-80 DOS-es was so dependent on having a DOS disk in drive 0 all the time it was handy to be able to just leave a DOS disk with a few extra utilities sealed in drive 0 and use 1 and 2 for program and data disks. (Also made it easy to copy disks, of course.) Having a fourth was just showing off.

(Actually, on a Model I, which is what I used the most, you could only have three drives if any of them were double-sided. Which is what I had. At the time government surplus IBM PC/XTs were so cheap I just bought a few and used the drives from them to upgrade all my original cases. Obviously that would probably be considered blasphemy these days.)

yup...i have tandons in many enclosures from IBM's. more reliable.

Eudimorphodon said:

The important point: The "first drive" always needs to go at the end of the cable furthest from the computer. Additional drives populate the other connectors on the cable. The "first drive" units are fitted with terminating resistors, the others lack them. If you mix them up, omit the "first drive", or put two "first drives" on the cable the system will be less electrically reliable. It's an obscure point, but speaking from experience the Model I's single-density controller in particular wouldn't give good results if you got the termination wrong.

Click to expand...

I believe these were Tandon drives, in which case they are configurable in the drive controller hardware. There are two DIL sockets on the controller board. One takes the terminating resistor pack, the other ships with a cut-once jumper set that determines the "number" of the drive by grounding a particular line on the connector cable. For flexibility, the jumper set can be replaced with a DIL switch pack. The resistor pack goes on the drive last on the cable. The jumper configurations are available in Tandon documentation, easily located. So with a little courage, I would think any drive could be configured to run as any drive number.

Different drive models may have different configuration methods, but the principle is always the same - a drive is identified by the grounding of an interface line.

Rick

Ok here is my external drive knowledge for those interested... there are a couple of limitations:

Early US 26-1160 has a Shugart SA 400 drive is an is only capable of 35 tracks and stepping at 30ms - it has a terminating resistor (TR) in it
All Aus 26-1160 has a Teac disk drive which is capable of 40 tracks but only slow stepping - it has a TR in it
Early US 26-1161 is the same as the US 26-1160 but with no TR in it
All Aus 26-1161 is same as Aus 26-1160 but with no TR in it
(all if these drives are slow stepping and will work with the M1 and CC, but not the M3 and M4 because they need faster stepping 40 track drives)

So to overcome the above problem, when the M3 was released
RS phased out the early US 26-1161 and put a Tandon disk drive (same at in the M3/M4) in the newly revised 26-1161 (call it the "late" 26-1161)
Since the M3 and M4 needs a TR in the last drive and the 26-1160 doesn't step fast enough a newcomer "26-1164" appeared on the market which has a TR in it.

If you wanted to add external disk drives to a M3 or M4, you first need to add a 26-1164 and then a "late" 26-1161.

On the Coco they released the drives in this order:

Early silver CC drive 26-3022 is a TEC brand drive capable only of 35 tracks and slow stepping - with TR in it - it works as drive 0 on a M1 in place of 26-1160
Early silver CC drive 26-3023 is the same as 26-3022 but without a TR, and it can be used on a m1 in place of 26-1161
(both these drives won't work right on a M3 or M4 for the same reason as the early early 26-1160/61 drives)
Late white CC drive 26-3029 is the same as 26-1164 - and works with the M3 / M4 - in fact a large number of the drives which came with the 26-3029 kit came with a disk drive just marked "Mini Disk".

After this for the CC they moved to the FD-500 thinline type disk drive which works on the M1/3/4 and is small and neat (I use one on my m1).

Hope this helps,

Ian.

RickNel said:

So with a little courage, I would think any drive could be configured to run as any drive number.

Click to expand...

Yeah. Radio Shack just assumed that their TRS-80 customers either lacked courage or couldn't be trusted (sort of ironic when you think about it) so they set up the different SKUs to make it all plug-and-play. When it comes to drive select the original Radio Shack units were jumpered in a bizarre way so they'd respond to *any* drive select signal, while the Radio Shack supplied cables had "teeth pulled" from the various sockets so a drive would automatically appear at the location dictated. (It's roughly analogous to the twist in IBM drive cables.) I imagine most "advanced users" dispensed with the notched cables and set up the drive select jumpers instead.

On that subject, I do remember having problems undoing the "any signal" jumpering on some of my single-sided units so they'd mix with the double-sided drives on a straight-through cable. (I was doing that until I'd acquired enough DS drives to replace them all.) One in particular I remember had some hackery done to the controller board with cut traces and jumper wires. I don't *think* that was a Radio Shack brand drive (some of my enclosures were RS, others third party), but your mileage may vary.

(Edit: Yeah, also forgot about the stepping speed issue. Because of it some of the old mechanisms indeed won't work with the Model III/4, unless you use a third-party DOS that lets you tell the controller to slow down. Note, however, that some newer half-height drives might also be too slow; in particular I recall some of the early TEAC half-heights used a lead screw positioner that was slower than the 12ms(?) step rate expected by TRS-DOS 1.3.)