The Vintage Computer Federation hosted the Vintage Computer Festival East 2026. Other pictures of show
One of the themes for the show was Dawn of the PC. The Ohio Scientific Instruments (OSI) is an example of the pre PC computers it displaced. The TI Professional was a failed attempt to compete with the IBM PC.
The OSI I brought is similar to the one I used in High School. I got it from a friend who decided I would be a better home for it.
The TI Professional was our first computer that my mom bought when I was in college.
On the back wall I had pages about OSI computers and my TI Professional and OSI history
It was the 50th anniversary of OSI so a number of people exhibited OSI computers. Start of slides of the OSI exhibit
This display is a combination of my 2021 VCF East display and my 2016 display. The new things were the OSI 560Z board which added to the 6502 a Z80 and Intersil IM-6100 PDP-8 chip. I also added 430B board which was driving the XY monitor. The new boards and trying to modify the 560Z board to add IM-6102 memory expansion chip took too much time so didn't leave enough time to get demos better organized. Wasn't able to get the 6102 operating reliably so removed it. Without the memory expansion the 6100 couldn't talk to the 430B board so wasn't able to run the PDP-8 graphics games I planned. Did some quick demos using the Z80 instead.
The 560Z seems to be intended as a development board. It uses 4 Programmable Interface Adapter (PIA) chips so it can read/write 64 signals around the processor. It allows you to single step the clock and see and control the state of many of the signals around the processor. The software provided was quite primitive showing the signals at the PIA pins and not formatted to make sense understanding the processor operation. The 560Z has a local bus which I had 8kx12 memory and the 430B board. If the processors were only using local resources they can run in parallel with the main 6502. If they need to do I/O they halt and the 6502 uses the PIO to simulate the I/O device then restarts the processor.
The 430B has 2 digital to analog converters (DAC), 1 analog to digital converter (ADC), unblank for XY monitor, 8 bit digital output, 8 bit digital input, and serial interface. The serial interface could be configured for RS-232 or Cassette tape I/O. The ADC is a tracking converter which uses a counter driving a DAC which is compared to the input analog voltage. The comparison selects on each clock if the counter counts up or down. If the signal changes slow enough the converter will track the signal changes. If the signal changes too fast the counter can't follow it and the value read will not be accurate.
The unblank signal timing was wrong to generate a clean display. More info is in pages about OSI computers above.
Only fix the TI needed was the 4 pin Molex connector powering the hard drive was making poor contact so the drive speed was varying. Using contact cleaner and a jewlers screwdrive to squeze the barrel contacts closed fixed the problem.
I was just planning to bring the disks I used for the last OSI demo but coundn't find where I safely stored them. Did find a couple disks with useable stuff for demo. Was going to write new disks at the show but got errors when writing the disks. One of the blanks I brought the coating was failing so I had to clean the head after. Other disks I brought didn't have that problem but still wouldn't write without errors. The disks I did bring some of the BASIC programs wouldn't run properly so was limited in what I could demonstrate. See schematic for my mod
Photo by Christian Liendo
The following picture links also have descriptions of what is shown in the pictures.
Exhibit ( 59K)
Discussing exhibit (103K)
OSI 1 ( 78K)
OSI 2 ( 94K)
OSI 3 ( 76K)
OSI 495 (101K)
TI Professional ( 87K)
Feel free to contact me, David Gesswein djg@pdp8online.com with any questions, comments on the web site, or if you have related equipment, documentation, software etc. you are willing to part with. I am interested in anything PDP-8 related, computers, peripherals used with them, DEC or third party, or documentation.
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