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Ohio Scientific Challenger 2P


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  Ohio Scientific Challenger 2P by Bill Degnan - 09/25/2010 22:52
The 1977-1979 Ohio Scientific (OSI) Challenger 2P (C2-4P) is made up of two boards: OSI 500 (rev B) CPU and 540 I/O card. The keyboard controller model is 542. There is also a controller card backplane model 582. Click image for more pictures.

Initiated project, photos taken for closeup view of chips and board.

The OSI C2-4P upon power-on, before hitting the BREAK key to reset the system.

LINKS and comments from Bill Dromg.:

65VB7.6 ROM This photo shows 1702 Eproms with two missing labels. (Datasheet attached.)
As you know they require negative 9 volts along with the positive 5 volts to operate.

The six pads to the left of the lower 1702 are for a toggle switch to select eproms but also needs some other board mods.

The 6850 ACIA below the 1702's is for serial I/O.

(see page 84 for memory map)
(see page 131 for machine monitor)

  OSI Model 540 1977 by Bill Degnan - 09/29/2010 20:55
Closer inspection of the OSI Model 540 in this system reveals a number of missing 2102 RAM chips. I learned that these are not necessary, but some may be bad, as the screen does not fully clear. Note that the prompt is present near the bottom, despite the two columns of garbled characters.

OSI Model 540 1977 Pic 1
OSI Model 540 1977 Pic 2

From Mark's Lab:

From Bill Dromgoo

Bill, I notice on your display the monitor responds with C/G/M after reset.
It should be C/W/M.
Cold start /Warm start and Monitor.
C is 01000011
W is 01010111
M is 01000111
in binary.
G is 01000111 in binary
This implies that the forth bit from the left in the display memory is stuck at

I would try switching two of the display memory chips to see if there is any
change in the display.

Info on 540 video board.

The 540 occupies the address range from $D000 to $DFFF.


From Mark

From your screen shot, the C/W/M prompt is C/G/M and is offset 16 columns form where it should be, so it looks like A4 is tied/floating high and D4 is tied/floating low somewhere between the 500 board and the 540 board. Since the 540 board is indexing all the columns it's probably the bus buffers/bad connections.

If you go into the ROM Monitor by pressing M then . can you type in all the numbers between 1 and 7? And does the CAPS lock not being down prevent the C/W/M prompt from responding? If so the keyboard is scanning correctly and the CPU can read/write all the data lines correctly so it's probably a bad 8T26 "B1" on the 540board. If not maybe it's a problem on the 500 board.

  OSI Monitor OS-65V by Bill Degnan - 10/01/2010 20:52
From Bill Dromg.

The OS-65V Monitor responds to the following keys:

0-9,A-F Hex digits

. Change to Address Mode

/ Change to Data Mode

G Go to address shown on screen and execute code there

RETURN Increment address (only in Data Mode)

L Transfer control to audio cassette/serial line. This command enters
the Data Mode, ignores the keyboard and listens only to the
cassette port (if present). To transfer control back to
the keyboard, press reset or load $00FD with a $00 via

If you enter values in the address range of the 540 board they will be displayed
on the screen as the ascii value of the hex number in memory.


  OSI 2P Update - RAM OK by Bill Degnan - 10/02/2010 09:09
I was able to resolve the problem with the prompt, it now correctly says C/W/M ?

BASIC loads to the point of asking for the memory size, but I cannot go any further.

The 16-character wide columns or random characters are still present, the prompt is pushed over 16 characters.

I believe all of the RAM is OK. Voltages in and out seem ok.
I believe the keyboard is OK (except for the 7 key?)

I swapped around the 8T26 chips on both boards, I traded chips' positions (there are 4 on both boards), there was no change. This tells me circumstantially that all 8 chips are OK, but the bus itself may be bad. At least that's what I plan to check next.

I also checked various points for voltages, I need to spend more time and compare with the schematic and chip data sheets.



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