Tandy TRS-80 Model 4P
Author: Graham
System 80 Monitor
The System 80 came with a Thorn 7433 black & white TV that was used as a monitor. It had had a couple of homebrew additions.
The first was a composite video input, so modulation/demodulation is redundant.
The second was a green screen, which was appropriately agricultural. The mount has broken and in any case is too rough even for my low standards.
It seems to be working fine.
I have removed the mount, and rubbed back the glued areas. It’s currently awaiting paint and then i’ll attempt to mount some coloured perspex that looks a little more appealing.
IBM PC XT
SGI Indy
Tandy TRS-80 Model 4
Sirius One / Victor 9000
The Sirius One was a moderately successful attempt to produce a 16 bit business machine using MS-DOS and CP/M-86. It was targeting a similar market of that of the IBM PC and although it runs the same operating systems as an IBM PC it is not a PC-compatible. Both machines were released in 1981.
The machine was sold as the Sirius One in some markets and as the Victor 9000 in others.
It was designed by Chuck Peddle, who is famous for designing the Commodore PET. It sold well in the UK and Europe.
The Sirius One did have some significant advantages over the IBM PC, including 800 x 400 pixel graphics, more sophisticated sound, and 600k single-sided 80 track 5.25″ drives.
The drives used variable speed to more efficiently use the longer outside tracks. The Apple Macintosh 3.5″ drives used a similar approach.
Furthermore, it has some nice touches like prompting for disks with icons and automatically detecting disks. It is happy to boot from either drive.
Soft keys for contrast and audio volume also add to the feeling that this computer is quite highly evolved for 1981.
Like the IBM PC, it had expansion slots – but not the same as the IBM ISA slots.
This particular machine was spotted by a ARC colleague, Mike, at a former recycling depot. It had been a little mistreated during the “about to be recycled” process, but nothing too severe.
It has 128k of memory on the main board and another 128k on an expansion card. Printer and serial ports were built in.
I think the monitor should be sitting on a pivot base, but that must have been lost along the way.
The Monitor required a simple cable repair after some rough treatment. It’s powered from the main unit.
I inspected the power supply. Somebuddy had powered the unit on fairly recently, probably in preparation for sale, and one of the line filter capacitors had smoked. I replaced 3 X and 2 Y capacitors. I load tested the power supply, and it was good to go.
The drives were in very good condition and i just cleaned and lubricated the rails. One drive light is a little dull.
Overall, the machine gives an appearance of far greater quality than an IBM PC. It comes apart easily.
The keyboard is not great. The foam pads will need to be replaced (like a Compaq Portable or Kaypro II).
The machine did not come with any floppy disks. Fortunately, there are a lot of disk images available online.
Writing them is a bit of a challenge because of the unique format. They need to be written with an 80 track drive but with double density. I used a Panasonic JU-475-5. I tried another drive (Newtronics), but it seemed to only write reduced current for high density disk media.
Fluxengine supports the disk format and is happy to write it using a greaseweazle:
fluxengine write victor9k_ss --drive.rotational_period_ms=166.67 -i dbasemp.img -d drive:0I found this method a little prone to crashes. The alternative is to convert the images to scp format using fluxengine, and then write the scps using greaseweazle. This does not verify the writes, though.
I now have the fluxengine hardware so that’s a third option to try.
The screen is very sharp. The brightness is controlled with soft keys. The monitor has an antiglare mesh built in.
Kermit works well.
At the Sirius end:
- set parity none
- set baud 9600
- set default-disk b:
- set flow-control xon/xoff
At the pc end (Windows 10 with CKW)
- set file type binary
- set baud 9600
- set carrier-watch off
- set flow-control xon/xoff
- cwd <directory>
- send <file>
Using HD 5.25″ FDDs as 8″ Drive Substitutes
High density 5.25″ disks have much the same capacity as 8″ double density disks, and they operate at the same data rate. A 5.25″ FDD typically has a few more tracks than an 8″ FDD.
This means that a floppy disk controller expecting to see an 8″ disk drive can potentially be fooled into working with an HD 5.25″ drive. This can be very handy if you don’t have an 8″ drive or if you want a more compact setup. This trick seems to work ok with a real HD drive or with a gotek/flashfloppy. 8″ disk images can be written to HD 5.25″ media without alteration using Greaseweazle, for example.
There are a few small issues. The first is the 8″ drive interface is usually 50 pins and the 5.25″ interface is usually 34 pins. The disk controller may have both interfaces (eg the Jade DD and the 16FDC) but not always (eg the Pulsar Little Big Board only has a 50 pin interface). If there is no 34 pin interface, then an adapter will be required. The 50 pin interfaces vary a bit, so a specific adapter may be required.
The second is that 8″ drives can detect whether a drive is single or double-sided and tell the host. The operating system driver may exploit this information (eg Jade DD CP/M) so there may need to be a way to fake this. For systems that use only single sided or only double-sided disks the signal can be tied appropriately otherwise a switch may be required – and if there are different drives in the system then it may be necessary to take the signal low through a diode from the drive select line.
Third is that 8″ drives typically produced a ready (RDY) signal. Without this signal, a host may just hang. This signal is available on many drives and can usually be setup on a gotek/flashfloppy but it may not be connected on the 34 pin interface. This can be overcome by connecting pin 34 on the 34 pin interface to the appropriate pin on the 50 pin interface.
Microbee 32
2031 Disk Drive & Running a Game on the 4016
The disk drive looked awful inside and out and once the loose debris was removed it still looked awful.
The cover was very corroded and the IEEE-488 connector was broken although the pins were intact.
After a wipe over, a blowout, and a first board wash started to look a little better with the board appearing to be less corroded than the computer boards.
There was no rush to get the drive going because I didn’t have a computer to test it with, and I didn’t have a connecting cable. I ordered a short GPIB cable from China and an edge connector locally with a view to modifying the standard cable.
PCB corrosion was treated with soapy vinegar and after two passes the board was quite acceptable. The IC legs were in much better condition than those on the computer boards.
The tin can was treated with deoxit over several days and was then cleaned and painted with zinc paint.
The drive was thoroughly cleaned and lubricated. It had no corrosion and the head carriage moved freely.
The case was processed in much the same way as the computer bases. See above. The corrosion was severe enough that filler was required. I’m at a loss to explain why the case was so corroded but the PCB was good – despite the ventilation slots on the top of the case.
Transformer voltages were checked before power on. I was surprised to find that the drive started with a spin and then gave a solid light as per the manual. This was a good sign.
Alas, smoke followed, which is when I realised that there was probably a Rifa hidden away inside the IEC inlet filter.
Although I had a spare filter, it was too large to fit under the wiring cover, so I have reverted to a simple inlet with no filter. I don’t like doing this, and I will revisit in the future.
I made up the cable by unpicking one of the GPIB connectors and replacing it with an edge connector. The supplier sent the wrong size, so I had to cut it down to suit (2×12 rather than 2×22).
I made up a 2031 demonstration disk using a greaseweazle and a 40 track drive. I had trouble doing this via SCP, but it worked fine writing direct. Initially the drive was unreliable, but after a more exacting cleaning of the head it ran and passed the drive performance test.
Most games were written for 40 columns, so for the 80 column machine to work it has to run in a 40 column compatible mode. This is done by running a program which effects the changes: CBM4032.
The program was easy enough to find, but I had to put it in a disk image. As is often the case, there’s an app for that. I used DirMaster. Then the image just has to be written with greaseweazle.
I found that sometimes the disk drive stopped working. When it did, it is because the head carriage was not moving reliably. My best guess was that there is an intermittent fault in the stepper motor drive circuit. It did not seem to be the connector.
The soldering on the board looks very good.
When the drive worked it worked very well, so the fundamentals seem good.
I used the scope to have a look at the stepper motor drive signals. One was not like the others. The stepper is driven by a quad resistor array FPQ3724. The transistor at pins 12,13,14 did not seem to be presenting a 0.7V drop in operation. On the meter it looked fine. Pin 14 was quite corroded, so perhaps some moisture had got in. It’s also possible that it had just blown – not unusual. An identical array is quite hard to get so in the short term I’ve patched a BC337 transistor in parallel. This seems to have resolved the problem for now.
NCR PC4i
The NCR PC4i was a rather ridiculous example of love at first sight, and i paid accordingly! I purchased it via gumtree and after calling the seller and convincing myself that he was legit, i had it shipped from Perth to Adelaide. i still think it is a beautifully crafted and technically excellent IBM PC compatible.
This machine has monochrome CGA graphics. The motherboard, together with the ISA cards, is in a cage that is removed from the rear. Considering the all-in-one package (like a compact mac) everything is remarkably accessible.
I maxed out the memory to 640k and added serial I/O, game port, an ethernet card, and an XTIDE board. It happily connects to my network and can be accessed via FTP.