Doubletop
Member
I was in the loft getting down the Christmas decorations. I also bought down two computers I built some years ago. Long before IBM released the PC. This first one was built from a kit in 1979 around the time of the Apple II.
It’s a Compukit UK101 a Brit knock off of the Ohio Scientific Superboard. It came with 4Kb of RAM and the upgrade to 8Kb cost me £50, equivalent to £285 today. That would buy you a top end Intel I7 processor.
It got heavily modified over time, ending up with 64Kb RAM And floppy disk drives for 8” disks that held 128Kb of data! It became the development platform for #2 but more of that later.
Internals
Main Board
PSU 5V 10 Amps - no switch mode here.
Home made expansion card AtoD DtoA and other things I can't remember
Then came the full homebrew. This one is about 36 years old. It was never intended to compete with Apple. I don’t think there would have been a huge market for it.
It started life as an add on to the UK101 with a circuit board connected to the original 6502 microprocessor socket and provision for 6502, Z80 and 6809 microprocessors plus a floppy disk interface. See the three original circuit diagrams that I found in a box of papers today
It never saw a 6809 and more and more was added to it that eventually the two computers were separated and all the boards put in an old rack chassis.
Z80 Based it runs CP/M 2.2. It grew, at first with 8” floppy disks , then 5.25” disks and then a hard drive.
CPU card and serial card with parallel connector for EPROM programmer
Flip side of the cards hand wired with Verowire.
Video and Memory cards from Elector designs, again with mods
Two floppy controllers 1993 and 1771 for 8" and 5.25" drives
Two parallel cards one for keyboard the other for printer. There are two spare ports that have never been used
SASI/SCI1 for the hard drive on the left and the keyboard parallel card
It lives!! To be safe I’ve used a different power supply. I don’t have a monitor that can deal with the obsolete video signal (MDA) so it took a bit of juggling with the oscilloscope to confirm that something was actually happening when powering up. Then it took a bit of forensic investigation to check what baud rate the serial port was running at.
The keyboard worked, sort of, so flying blind the command was typed in to redirect output to the serial port and bingo! There it was on the PC screen!!
Things are a bit flaky after being in a box for upwards of 25years., Occasionally, the processor sometimes doesn’t run, the video card doesn’t fully initialise, the keyboard sends spurious characters and the SASI/SCSI1 had drive makes so much noise that it may not last.
At first, I thought that things I had expected on the hard drive were missing but CP/M never expected large drives and the file system only initially designed for floppy disks. In order to use all the capacity, I’d set up 4 logical drives and then segmented them by user number. It took an overnight read of the manuals to remind me the existence of user numbers.
I’ve got a video adaptor on its way from China ($20) in the meantime I’m trying to get something working so I can offload everything from the hard disk in case it dies. Not that easy as technology has moved on over the years.
Pete
It’s a Compukit UK101 a Brit knock off of the Ohio Scientific Superboard. It came with 4Kb of RAM and the upgrade to 8Kb cost me £50, equivalent to £285 today. That would buy you a top end Intel I7 processor.
It got heavily modified over time, ending up with 64Kb RAM And floppy disk drives for 8” disks that held 128Kb of data! It became the development platform for #2 but more of that later.
Internals
Main Board
PSU 5V 10 Amps - no switch mode here.
Home made expansion card AtoD DtoA and other things I can't remember
Then came the full homebrew. This one is about 36 years old. It was never intended to compete with Apple. I don’t think there would have been a huge market for it.
It started life as an add on to the UK101 with a circuit board connected to the original 6502 microprocessor socket and provision for 6502, Z80 and 6809 microprocessors plus a floppy disk interface. See the three original circuit diagrams that I found in a box of papers today
It never saw a 6809 and more and more was added to it that eventually the two computers were separated and all the boards put in an old rack chassis.
Z80 Based it runs CP/M 2.2. It grew, at first with 8” floppy disks , then 5.25” disks and then a hard drive.
CPU card and serial card with parallel connector for EPROM programmer
Flip side of the cards hand wired with Verowire.
Video and Memory cards from Elector designs, again with mods
Two floppy controllers 1993 and 1771 for 8" and 5.25" drives
Two parallel cards one for keyboard the other for printer. There are two spare ports that have never been used
SASI/SCI1 for the hard drive on the left and the keyboard parallel card
It lives!! To be safe I’ve used a different power supply. I don’t have a monitor that can deal with the obsolete video signal (MDA) so it took a bit of juggling with the oscilloscope to confirm that something was actually happening when powering up. Then it took a bit of forensic investigation to check what baud rate the serial port was running at.
The keyboard worked, sort of, so flying blind the command was typed in to redirect output to the serial port and bingo! There it was on the PC screen!!
Things are a bit flaky after being in a box for upwards of 25years., Occasionally, the processor sometimes doesn’t run, the video card doesn’t fully initialise, the keyboard sends spurious characters and the SASI/SCSI1 had drive makes so much noise that it may not last.
At first, I thought that things I had expected on the hard drive were missing but CP/M never expected large drives and the file system only initially designed for floppy disks. In order to use all the capacity, I’d set up 4 logical drives and then segmented them by user number. It took an overnight read of the manuals to remind me the existence of user numbers.
I’ve got a video adaptor on its way from China ($20) in the meantime I’m trying to get something working so I can offload everything from the hard disk in case it dies. Not that easy as technology has moved on over the years.
Pete