With the base machine seemingly working well, it was time to try my luck with the expansion unit. I really should have inspected the power supply first because it let out quite a lot of smoke from a line filter capacitor self emolliating.
Accessing it involved removing the cover, removing the fan cowling, and then releasing the supply which, conveniently, has connectors at both the mains and DC ends. I replaced both line filter caps.
The hard disk drive did spin up, but that seemed to be all it did. There didn’t seem to be any calibration activity. On power-down it sounded like it ground to a halt. I think it’s unlikely to be viable. I removed it – a very chunky full height IMI 18MB unit.
The floppy disk drive did work, which was good because that meant that the link to the expansion box was working and, of course, the floppy disk controller was also working. The 5.25″ drive appears as drive C. The CMOS settings need to be changed to 3 drives for it to work.
The expansion unit also had a printer card to accommodate a centronics style printer. The printer port on the computer is serial and maybe specifically set up for the data general printer. Both the serial ports on the computer are incompatible with IBM ports.
I added a spare CGA card and connected a CGA monitor. With the computer settings set to MONO the external CGA monitor becomes the default screen. The mode can be set to BW40, BW80, CO40, and CO80. If the mode is set to MONO the LCD is reactivated. Without the expansion unit connected, all of these modes work with the LCD. The external monitor makes life much easier.
I thought that an XTIDE card might work with the One. I just took one from an XT and plugged it into the expansion unit. DOS 5.0 booted, but there were some issues during the autoexec. That was enough to show the potential, though.
The system will also boot from the 5.25″ drive in the expansion unit. It will not boot from the rear floppy disk drive, which unfortunately doesn’t seem capable of formatting, although i have been able to write with it. Perhaps a gotek candidate.
I have been able to boot later generic versions of MS-DOS but they may have incompatibilities, eg the real time clock does not work.
I added an XTIDE card and programmed it with the large XT BIOS. The behaviour was a little unexpected, probably because the floppy disk implementation is not 100% compatible with an IBM PC. The Universal BIOS has trouble associating the drive with drive letters often confusing the flash disk with the third floppy. Selecting floppy drives by letter at startup also resulted in boot failures.
The XTIDE Universal BIOS does seem to be a little befuddled by the machine. Selecting the boot drive by typing a letter sometimes worked, but at other times produced baffling results. Curiously, invoking the menu and then using the arrow keys to select the boot drive from that menu worked fine. Perhaps the relevant data is reread at that point; only a universal BIOS expert would know.
Also, fortunately, the Universal BIOS will allow the system to boot from any of the floppy drives – as long as it is selected from the Universal BIOS menu.
MS-DOS 3.2 is limited to 32MB partitions, which is a little tight. This is probably not a major issue for this machine which is likely to have only occasional use. I could not get the card to boot, but it worked fine otherwise.
I set up a second CF card with DOS 5.0. I was able to make this one bootable and i could set up a full 512kB partition. There is no specific version of MS-DOS 5.0 for the One, so there are a few places where IBM incompatibility rears its head. Firstly the battery backed real time clock does not work, and secondly MS-DOS 5 seems to politely ignore the incompatible serial ports.
With XTIDE and Universal BIOS there are a variety of oucomes depending on the operating system and the boot drive:
| OS | Boot Drive | Notes |
| MS-DOS 2.1 | A 3.5″ Front Internal | Boots. Can access floppy drives A(front), B(rear), C(5.25″). Appears to not have hard disk support. Was unsuccessful formatting C. |
| MS-DOS 2.1 | B 3.5″ Rear Internal | Boots. Can access floppy drives A(rear), B(front), C(5.25″). Appears to not have hard disk support. Was unsuccessful formatting C. |
| MS-DOS 2.1 | C 5.25″ External | Not attempted. |
| MS-DOS 2.1 | CF Card | Appears to not have hard disk support. |
| MS-DOS 3.2 | A 3.5″ Front Internal | Boots. Can access CF card with 32MB disk as D. Can access floppy drives A(front), B(rear), C(5.25″). Can write to rear drive but not format. Can format 5.25″ drive and write. |
| MS-DOS 3.2 | B 3.5″ Rear Internal | Boots. Can access CF card with 32MB disk as D. Can access floppy drives A(rear), B(front), C(5.25″). Can write to rear drive but not format. Can format 5.25″ drive and write. |
| MS-DOS 3.2 | C 5.25″ External | Boots. Can access CF card with 32MB disk as C. Can access floppy drives A (5.25″), B(front), D(rear). Can write to rear drive but not format. Can format 5.25″ drive and write. |
| MS-DOS 3.2 | CF Card | Does not boot. Have to boot from floppy. Then card can be accessed. See above. |
| MS-DOS 5 | A 3.5″ Front Internal | Boots. Can access CF card with 512MB disk as C. Can access floppy drives A(front), B(rear), D(5.25″). Can write to rear drive but not format. Can format 5.25″ drive and write. |
| MS-DOS 5 | B 3.5″ Rear Internal | Boots. Can access CF card with 512MB disk as C. Can access floppy drives A(rear), B(front), D(5.25″). Can format 5.25″ drive and write. |
| MS-DOS 5 | C 5.25″ External | Boots. Can access CF card with 512MB disk as C. Can access floppy drives A(5.25″), B(rear), C(front). Can format 5.25″ drive and write. |
| MS-DOS 5 | CF Card | Boots. Can access CF card with 32MB disk as C. Can access floppy drives A(front), B(rear), D(5.25″). Can format 5.25″ drive and write. |
The rear drive seems to have a strange fault that allows writes but prevents format. May just be in the detail of the format program. The drive system is not 100% IBM compatible.
When formatting DOS 5 seems to treat all drives as 360kB by default. It may be possible to use format options to override but i found it very reluctant. Use driver.sys in config.sys to create new drives E and F that are 720k. Required on any DOS 5 boot disk.I thought that an XTIDE card might work with the One. I just took one from an XT and plugged it into the expansion unit. DOS 5.0 booted, but there were some issues during the autoexec. That was enough to show the potential, though.
The system will also boot from the 5.25″ drive in the expansion unit. It will not boot from the rear floppy disk drive, which unfortunately doesn’t seem capable of formatting, although i have been able to write with it. Perhaps a gotek candidate.
I have been able to boot later generic versions of MS-DOS but they may have incompatibilities, eg the real time clock does not work.
All up, it’s a pretty complex picture!
I thught there may be a performance hit from using the expansion unit so i ran the performance test that is in checkit. The video speed is a little faster on the internal screen. Of course this may be due to the graphics implementation.
