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Wanted: TRS-80 Model III or 4

tempest

tempest

Veteran Member
Joined
Sep 16, 2014
Messages
930
Location
SE Michigan
I'm looking for a working TRS-80 Model III or 4 with at least one disk drive. I would prefer to trade for it, but I suppose cash is always an option. Here's what I have for trade at the moment:

Atari 5200
Nearly complete Atari 5200 boxed collection (only missing some of the rarest ones like BBSB, DSB, and Zaxxon although I have the last two loose)
Atari 5200 REI Starcon joystick (less than 50 were made I believe)
Atari 5200 Mastplay Interface (NIB)
Various other 5200 items (2600 adapter, various 3rd party controllers, etc.)


Spectravideo SV-328
Spectravideo SV-328 system (NTSC) with F18a for VGA output (can still do composite and RF)
SV-903 Cassette Drive
SV-601 Super Expander with SV-801 Disk Drive card
Lots of programs on cassette (with liner inserts)
Just Write jr cartridge boxed


Astrocade
Working Astrocade system with Ward Shrake Multicart and 4 joysticks (with numbers 1-4 stickers)



Intellivision
Boxed US Intellivision ECS computer (missing power supply)
Boxed US Music Synthesizer
Atari 2600 adapter


IBM PCjr
Demon Attack cartridge (loose)
Pitfall II cartridge (CIB)
River Raid cartridge (CIB)
various other boxed PCjr games, ask for list.


C128D
128DCR (NTSC) in very nice condition with keyboard
Jiffy Dos installed (with on/off switch in back)
Drive select switches on back (select device 8, 9, 10, or 11)
SID chip resistor mod so voice samples play at the correct volume
 
oldcomp said:
Interested in Model 4P ???.
Click to expand...

I'm not sure. I'd be using it for gaming. How is the compatibility compared to a regular model 4? I know they changed some stuff around like removing the cassette port (are most games available on disk now?).

Is it in good shape?
 
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The 4P is essentially a Model 4 in a portable case, although it does omit
the Model III BASIC mode ROM and Cassette PORT.
The Model 4P screen is smaller (9 inches versus 12 inches).
128k MEMORY 2 Double sided double density floppy drives.
There was no BASIC in ROM. Instead of having a ROM containing the Model III BASIC,
like the Model 4, the Model 4P had only a small boot ROM.
This boot ROM had the ability to load an image of Model III BASIC (normally named MODELA/III) from disk.
This meant that the Model 4P could run all Model III programs, although starting some self-booting disks
(such as games) required a two-step process of loading the BASIC image and then the disk.
I think you would like better model 4 over 4P (my opinion).
 
Other than the smaller screen and missing cassette port there doesn't seem to be too much of a down side. Maximum compatibility with games is what I'm mostly aiming for and it sounds like the 4P is actually MORE compatible than the 4 because you can load the Model III BASIC. I'm not sure what not having the cassette port would do as far as games. Are most cassette based games ported to disk by now? I would assume so.

What were you looking to get for it? Is it in good shape? You can shoot me a PM if you want.
 
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tempest said:
it sounds like the 4P is actually MORE compatible than the 4 because you can load the Model III BASIC
Click to expand...

Just to clear up a bit of confusion, the Model 4 has the Model III BASIC *in ROM*; the 4P omits it but it can load a copy of it into a protected section of RAM before switching to Model III emulation mode.

For most purposes a 4P is fine; it lacks the cassette port but it does take the cassette port output to an internal speaker so you don't need to use an accessory amplifier to get sound out of games, which is certainly convenient. Again, really the only limitation is you can't run cassette-only software but there's not that much out there you're likely to care about.

(however, re the previous post... so far as I'm aware a stock 4P does *not* have double sided drives? Mine doesn't seem to, at least.)
 
I thought there was some sort of minor compatibility issue with the BASIC that came with the 4. I might be wrong on that.

The 4P's internal speaker plays sounds from games that support it without any extra hardware? That's all kinds of handy. Any limitations to that?

So basically (no pun intended) I'd have to load the Model III BASIC into memory THEN insert another disk with the game on it and run that? Doesn't sound too bad. I assume auto boot disks work the same way (load BASIC then somehow reset the system and run the auto boot program)?
 
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tempest said:
I thought there was some sort of minor compatibility issue with the BASIC that came with the 4. I might be wrong on that.
Click to expand...

The version of BASIC that runs in Model 4 mode is pretty massively incompatible with Model III BASIC (it has more in common with the BASIC on the Model II/12) but the in-ROM copy of Model III BASIC is almost entirely identical to that in a real III. There might have been a few (auto-booting?) programs that were tripped up by the changes to the boot code (to recognize the difference between Model III and 4 DOSes and switch modes accordingly) but so far as I'm aware they're almost 100% compatible. The in-RAM version of the III BASIC has the cassette-related commands disabled (I seem to recall that commands like CLOAD just immediately return) so the *potential* for something to be tripped up by missing code would seem to be higher on the 4P. But... I think it practice it's likewise nearly 100% compatible with a Model III *disk system*.

(A diskless desktop 4 *only* acts like a Model III.)

The 4P's internal speaker plays sounds from games that support it without any extra hardware? That's all kinds of handy. Any limitations to that?
Click to expand...

Not that I know of. There were a few games out there that would make buzzing noises by abusing the cassette relay, I imagine those don't make any noise as I'm pretty sure that's absent.

So basically (no pun intended) I'd have to load the Model III BASIC into memory THEN insert another disk with the game on it and run that? Doesn't sound too bad. I assume auto boot disks work the same way (load BASIC then somehow reset the system and run the auto boot program)?
Click to expand...

Once the III ROM image is loaded it stays in RAM through resets until/unless you boot a Model 4 DOS disk or power the machine off. So the procedure would simply be to boot the Model III DOS of your choice, insert your self-booter, and hit the reset button.

The one limitation I don't care much for about the 4P is *if* you want to use a floppy-based TRS-80 for "productivity" I personally consider three disk drives the optimal configuration while the 4P's disk controller board only supports two. (without hacking) But for just playing games that's obviously a non-issue.
 
Ah so that's how it works. Thanks for the clarification. Assuming there are no 'Model 4 only' games, I'd probably be sticking either to Model III Basic or CP/M anyway.

I assume the smaller monitor is still plenty big enough for gaming? If so, then the 4P might be what I'm looking for until I can get my Model III fixed.
 
oldcomp said:
My both floppy drives on Model 4P are DSDD -360K
Click to expand...

Is yours modified, or are you saying it came with DSDD drives stock? The Radio Shack catalogs listing the 4P state they're "180k Double Density" drives, IE, single sided. Granted I've never tried a DOS that supports double sided disks on mine so I can't say absolutely that they're not double-sided drives and Radio Shack just ignored the second head, but they did make a point of listing double-sided disks as a feature of the 4D so I'd be mildly skeptical of that.
 
How reliable are the disk drives on the 4P? They pop up on ebay from time to time but they never bother to test the drives (probably because they don't have a BASIC disk). Are they usually working or a 'buy at your own risk' kind of a thing?
 
Eudim.. This computer was given to me 20 years ago, and I don`t remember asking
any question about floppy drives.
Yah, the best thing would be to check yours.
If you have blank disk and LDOS 6.3, try format 2 sides
 
How hard is it to swap out the disk drives on a TRS-80 Model III? I have a broken Model III with a working monitor, keyboard, and two disk drives (the motherboard died due to a shorted power supply I believe). I've been considering buying a non-working TRS-80 and trying to fix it using my left over parts or getting one of those disk driveless ones and putting mine in it. But I'm not sure if you can just slap disk drives into a system that didn't originally have them or if it's more involved than that.
 
Swapping drives is easy. As long as they are of the same TPI or Tandem drives due to pins being pulled on the original drive cables and all drive selects in place. And pin 34 is pulled which is side select!

Now if you want to say add half high drives or full high double sided drives a little more tricky. Would need to make a fully pinned cable. Get an un-keyed 34 pin edge connector to connect to the controller end. Then 2 keyed 34 pin connectors for the drives themselves. And of course a 34 connector IDC cable.

Maybe could reuse old TRS-80 cable and remove the 2 drive connectors and replace them with fully pinned ones. And hope it's long enough! Then you get double sided read/write capability and it would act more like a normal drive in the sense that you would use drive select jumpers. And as always last drive in line needs a termination resistor.

In my Model 4 I use 3.5 floppy drives! And they are standard PC 1.44 drives. I use Older Teacs that still have drive select 0 or 1 jumper at back. I just mounted in an 3.5 to 5.25 adapter frame. And you can use 1.44 media as well just cover the density select hole with electrical tape. And whoever says you have to use 720K drives is ill informed. 1.44 drives work great and even more reliable and media is still available!

Hopefully not to confusing! And sorry long winded.
 
That helps. I'll see if I can find a driveless system cheap or one with a bad monitor. The problem is that when people list them as non-working they don't know what's wrong with them and there's a good chance that it's the motherboard or power supply. I'm probably better off getting a working system, but this may be an avenue I can pursue if I get desperate or the right system comes along.
 
I must have missed something. :)

I have a chance to buy a Model 4 with only 16K and no disk drives, but it's in very nice condition. I assume it's the early non-Gate Array model since it has the older style keyboard and non-green monitor. Can I use the disk drives and board from my dead model III to upgrade it? Is it an easy task? Do I just plug the new board and drives in or is it more complex than that? Also, how hard is it to upgrade the memory to 48K?
 
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