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TRS80 Model 4 (and probably III) floppy disk assignment and memory queries

1980s_john

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Apr 29, 2005
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322
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UK
Hi,

I've had a Model 4 for a couple of years, but couldn't get the floppy drives to work (it ran Basic in ROM fine). The machine always tries to boot a disk from drive 0 (thanks to Lorne for help with sourcing a boot disk) but it would always fail with 'ERROR' displayed.

I took the machine apart to get to the drives, and took them out of the machine. Both looked fine, I see they are single sided (so the double sided Tandon drive I had as spare wasn't an ideal replacement). I resolved the problem by swapping over drive 0 and drive 1, and the machine boots fine on the new drive 0. What I don't understand is how the drive numbering is set-up, as I didn't have to change any jumpers and I couldn't see a twist in the cable. Please can someone clarify how the machine determines which drive is 0 and which is 1.

Also it looks like there is space for a terminating resistor on the the last drive on the cable but none is fitted - is this correct?

Sadly after about 15 minutes the computer gave off a puff of magic smoke so I powered it off without managing to run much more than booting into NewDOS and TRSDos. Luckily it looks like just an X capacitor on the PSU, and also one of the smoothing capacitors is starting to bulge so I will replace that too, and hopefully carry out some more cleaning on the faulty drive (which is now drive 1).

The machine has 64K of RAM, I see there are 8 empty RAM sockets plus some jumpers and I assume 128k is an easy upgrade. Question though is why, what is the benefit?

Regards,
John
 

vwestlife

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TRS-80s (and even the later Tandy 1000 series PCs) do not use the twist in the cable between the two floppy drives. The cable is straight all the way through. Set the jumper on the :0 drive to Drive Select 0 (DS0) and set the :1 drive to DS1. You can swap in a double-sided drive from a PC as long as you set up this jumper accordingly. My Model 4D does not have the terminating resistor installed on either drive.

The extra 64K of RAM can be used by some programs or you can configure it as a RAM disk. Depending on the revision of your Model 4 system board, to enable it you'll need to either move a jumper or unsolder a wire and solder it to a pin on a nearby IC. Here are instructions:
http://www.classiccmp.org/dunfield/trs80/m4memory.pdf

I recently posted a YouTube video showing the inside of my Model 4D, where I show and discuss the floppy drives and system board, so it may be of interest to you:
http://www.youtube.com/watch?v=DtZjO2dy0lk
 

tezza

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Also, I've seen some Model III drives where the drive select was achieved by missing pins in the cable (as opposed to setting jumpers). Just check the cable to see if there are any missing pins in the two card edge sockets. If so, this is probably how drives are selected.

I can't see much advantage in boosting the memory in the Model 4, apart from wanting a RAM disk. Not a lot of software took advantage of the extra memory, although I believe there was a patch for CP/M which loaded the whole OS into the spare memory on boot.

Tez
 

1980s_john

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UK
Thanks for the link and Youtube video. I have a Model 4 non-gate array, reading the memory upgrade notes I think this means I need a PAL so I'll give this a miss.

Your 4D has half-height drives, mine has full height - as I wrote they don't have jumpers, and when swapped on the cable they swapped drive numbers. Our machines share the same Astec PSU, this has a jumper for either 115V (in US) or 230V (Europe/Australia). My Model 4 has a different video board, I guess these were sub-contracted out to various suppliers to the same spec.

Regards,
John
 

1980s_john

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Also, I've seen some Model III drives where the drive select was achieved by missing pins in the cable (as opposed to setting jumpers). Just check the cable to see if there are any missing pins in the two card edge sockets.

Aha! I never thought of looking, thanks, I assume this must be the same as my Model 4, I will check next time I can (machine is 100 miles from me currently, I have the PSU to work on first).

Regards,
John
 

vwestlife

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1980s_john said:
My Model 4 has a different video board, I guess these were sub-contracted out to various suppliers to the same spec.

The Model 4D used video boards designed for the 4P portable, which Tandy had a surplus of after the 4P ended production due to low demand. Because it was designed for the portable machine with the smaller 9-inch CRT, the board is smaller and is a higher quality design, to give a sharper text display on the 4P's smaller CRT -- which also gives it superior sharpness on the 4D's 12-inch CRT as well.
 

1980s_john

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UK
Tezza yes you were 100% correct, upon inspection the plugs on the floppy lead on my Model 4 did indeed have contacts missing - very ingenious solution, makes spare drives a common part.

I fixed the PSU (3 new X2 filter capacitors and one new smoothing capacitor that had failed but wasn't critical) and just have the duff floppy to fix now (I forgot to pack my floppy disk cleaner for the trip - try again next time!).

I have some new queries so will start a new thread,

Thanks again,
John
 

1980s_john

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UK
Hi,

I tried cleaning the heads on my Tandy model 4 floppy drives, but I am still left with only one working drive. The second one spins OK but doesn't read any data. The drives are Texas Peripherals, full height single sided. Can anyone advise please how to service these drives, or offer a repair service?

Alternatively I would be willing to buy a matching working drive.

Thanks,
John
 

leeb

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Realize that the one that WAS drive ONE likely has a resistive TERMINATOR (or jumper block) on the drive!!!

You need to move that block to the MOST OUTBOUND drive (which SHOULD be the connector for DRIVE ONE) before doing anything else!

Make this change and then try again! :D

EDIT: just reread your 1st post....

Do EITHER of them have the terminator block/jumper?

I would try swapping in the DS drive... it wont hurt anything as it will still be accessed as SS, and will help confirm that you don't have either a cable or controller issue (which would be BAAADDDD...)

:D
 
Last edited:

1980s_john

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Apr 29, 2005
Messages
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Realize that the one that WAS drive ONE likely has a resistive TERMINATOR (or jumper block) on the drive!!!

You need to move that block to the MOST OUTBOUND drive (which SHOULD be the connector for DRIVE ONE) before doing anything else!

Make this change and then try again! :D

EDIT: just reread your 1st post....

Do EITHER of them have the terminator block/jumper?
:D

Umm... not sure. I took some photos a few months ago:

http://s127.photobucket.com/user/john-a-s/library/Tandy/Tandy Model 4

but only seemed to snap the PCB for one drive (with no terminator block), I guess the other didn't have a terminator either. I read somewhere that Tandy skimped on terminators :)

I really would like to find another working full height Texas Peripherals drive. My photos show the bases of my two drives, one clearly has 'Tandon' in the metal casting. The other has a sticky label 'Texas Peripherals 403662'. I found a TP drive for sale here:

http://www.recycledgoods.com/zoom/8508

Very clear photos, but a different controller board with ICs (shock!) so a bit newer than the ones I have. Can anyone provide some definitive info on TP drives, are they just a special Tandon re-badge for Tandy (based in Fort Worth)?

I think I have another make of single sided drive somewhere so that will be my fallback option.

Regards,
John
 

jltursan

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As a last resort, you can try replacing the faulty drive with a standard PC one. I've replaced drives in my M4 and M4P, although it's easy, you need some care selecting the 3.5" drive, seems that not all drives work reliably.
Basically you need to use a straight, fully pin populated, floppy cable; a PC drive capable of being configured as DS0 or DS1 (the RDY signal is not used by the TRS-80 so you don't need to care about it) and a way to set the Tandon drive as DS0 or DS1. I used a switch pack to replace the shunt installed in Tandon drives to accomplish the later.
The actual setup of my M4 is a 3.5" as drive 0 and the original Tandon as drive 1. As far as I've tested this setup, both drives are working fine (the 3.5" drive is working as a SSSD drive of course, I would love to get the full 720KB :))
I'll post some photos to give you an idea...;-)
 

1980s_john

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TRS-Ian

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On a standard TRS-80 Model III or 4 (I've opened many in my life that still had the factory seal over the screw) there is nothing plugged into R51. You'd think there should be, but apparently the internal cable on these computers is so short its not needed.

The same is not the case for an external drive. The first external drive for a Model III or 4 is a 26-1164, and if you open it up you will see the same Texas Peripherals (TP) disk drive that is inside the M3/4. ON THIS DRIVE look at R51 and there is a blue pullup resistor. Remove it and try to operate the disk drive? What happens? It works fine, but the drive motor spins continuously. What if you add a 2nd external drive to a M3/4? You buy a 26-1161, which again has nothing in R51.

Short answer, leave R51 empty for the internal drives in a M3 or 4.

Ian.
 

Chromedome45

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jltursan: I use 2 3.5 inch floppy drives in my model 4 and both give me full 720k support. And you don't need to use 720k floppy drives either. Just cover the density hole with a piece of tape on a 1.44 floppy. And format away. When doing a format select: double density, 2 heads and 80 tracks. And that is all it takes. As long as you have a fully pinned cable. I use TEAC 235H drives which can be selected as drive DS0 or DS1 by a jumper on the back.
 

1980s_john

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On a standard TRS-80 Model III or 4 (I've opened many in my life that still had the factory seal over the screw) there is nothing plugged into R51. You'd think there should be, but apparently the internal cable on these computers is so short its not needed.

The same is not the case for an external drive. The first external drive for a Model III or 4 is a 26-1164, and if you open it up you will see the same Texas Peripherals (TP) disk drive that is inside the M3/4.

Thanks Ian for the info on Tandy part numbers. I had a look through the list here (great scans of the original catelogue pages): http://www.trs-80.com/sql-search-rs-catalogs-process.php

and the suitable Model III/4 drives are:

26-1127 - Internal (although this may be half-height??)
26-1162 - Internal
26-1163 - Internal
26-1164 - External

Looking on eBay these crop up from time to time, I assume they are all TP 40 track drives inside. There are plenty of Model I and CoCo drives too (such as 26-1160, 26-3022 etc) but these are all 35 track so are unsuitable for Model III/4.

I found my full height single sided drive, it is a Teac FD50A, cased up for a Tandy Model 1 (labelled as 'University of Bath', looks like the power supply and case were made in their workshop), sadly this is 35 track also.

Still looking for a full height TP made 40 track drive to match the 'worker'.

Regards,
John
 

jltursan

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I use 2 3.5 inch floppy drives in my model 4 and both give me full 720k support. And you don't need to use 720k floppy drives either. Just cover the density hole with a piece of tape on a 1.44 floppy. And format away. When doing a format select: double density, 2 heads and 80 tracks. And that is all it takes. As long as you have a fully pinned cable. I use TEAC 235H drives which can be selected as drive DS0 or DS1 by a jumper on the back.

Lol!, I've never tried to format with 720KB capacity!, I was convinced that some kind of parameter tweaking was needed. I'll try it for sure, thanks!. Btw, I suposse that you're talking about LS-DOS isn't it?, I'm usually working with this OS and also Montezuma CP/M, no idea if the later needs configuration using its SETUP tool.

@1980s_john: I've some spare full height Tandon drives taken from several M4. AFAIK they were fully functional; but of course, they'll need some testing.
 

1980s_john

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Apr 29, 2005
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UK
As a last resort, you can try replacing the faulty drive with a standard PC one. I've replaced drives in my M4 and M4P, although it's easy, you need some care selecting the 3.5" drive, seems that not all drives work reliably.
Basically you need to use a straight, fully pin populated, floppy cable; a PC drive capable of being configured as DS0 or DS1 (the RDY signal is not used by the TRS-80 so you don't need to care about it) and a way to set the Tandon drive as DS0 or DS1. I used a switch pack to replace the shunt installed in Tandon drives to accomplish the later.

Thanks! I found this guide:

http://fjkraan.home.xs4all.nl/comp/lnw/mom/mom_4.html

describes Tandy drive set up in appendix B. It includes how to use the existing Tandy pulled pin cable on a double sided drive by removing the keys in the cable connectors then turning the connector upside down - very ingenious!

Regards,
John
 

1980s_john

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Apr 29, 2005
Messages
322
Location
UK
Hi,

Update on my Tandy model 4, I did a trade with jltursan and received a working full-height Texas Peripherals drive. Once I (finally) plugged it in both drives worked fine (hurrah!), so I can now run disk programs:

game.jpg

I never did figure out how to load LDOS and then swap disks and load a programme with just one floppy drive.

Thanks for everyone's help,
Regards,
John
 
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