Ohio Scientific

Challenger 4P

Model: Ohio Scientific Challenger 4P
Year: 1979
Processor: 6502 @ 2MHz
ROM: 8KB BASIC + OSI Monitor
Video: 64 characters, 32 lines monochrome ASCII w/ extended character set (OSI-540B)
Sound: Tone generator
OS: OSI BASIC V2 Rev 3.2?

My OSI Challenger 4P has had a few things replaced on it since I got it. A few of the 2114 SRAM chips were dead and needed to be swapped out to give it back a full 8K. The power supply was considerably hotter than it should have been, rendering the top of the machine untouchable in about 30 minutes of operation. The internal wiring harnesses were also incredibly messy and sketchy, relying on electrical tape for insulation. The keyboard also had a number of dead switches, which also needed replacement. I took care of the keyboard and RAM problem first, rendering the machine atleast usable.

After a chance encounter at VCF West in 2019, I scored a box of OSI stuff, including an A15 backplane PCB. I removed the original wiring, along with the old backplane cover, and stand-alone dsub connectors, in favor of the A15 board and a chassis fan. Oh, and the power supply got replaced with a modern switchmode supply using an adapter plate to allow use of the original screw mounts. The result is much cleaner internally, runs at a reasonable temperature with a light breeze on your fingers as you type, and you don't risk your house catching on fire.

Click here to view the entire Challenger 4P gallery

overview 502 top oldpsu on screen screen garbage workbench
runningbasic side view improved softwarehaul top view improved psu improved backside improved

OSI-400 Superboard Replica

Model: Ohio Scientific 400 "Superboard" Replica
Year: 1976
Processor: 6512 @ 1MHz
RAM: 4KB (OSI-420C)
ROM: 256B Monitor ROM
Video: 32x32 monochrome uppercase ASCII (OSI-440)

This machine is a kit build, not unlike many OSI machines, but this one can't really be considered a Challenger. I'm starting with a glitchworks replica 400 board for CPU, which was tested with 1K of RAM, serial, and a simple ROM monitor (presumably OS65A). I've adapted the CPU socket to use a 6512 in lieu of a 6502, and added a discrete level shifter for RS232.

I acquired an authentic 420C RAM board and 440 video board at VCF East XVIII, and added them all to a backplane. The 440 video board has a parallel ASCII keyboard input, which can be adapted for a number of keyboard models. One of the three suggested keyboards in the manual is the Sanders 720 terminal keyboard, a common surplus item in many 1970s hobbyist magazines like Byte. I own a 720 keyboard and decided this was the perfect use for it, despite its limitations and non-standard ASCII layout. However, I have elected to modify the keyboard slightly to my needs, while ensuring that every change is reversible (should a rare Sanders terminal turn up without a keyboard). I've fabricated a DC-37 to DB-25 adapter board to make interfacing with the native connector easier. I've also made an internal interposer board to add in features and places to connect additional key switches which include:
∘ Control (not present on the 720)
∘ Delete (present, but generates a non-standard ASCII code)
∘ Graphics mode toggle
∘ Reset
I'll probably post the design files for both once I'm satisfied that they work properly.

I've added an EEPROM socket and decode logic onto my 495 prototype board to provide eWOZMON support over serial -- it's easier to use than OS65A for debugging for the moment. Tests of writing to the 440 video board from serial, reading from the Sanders 720 from serial, and finally writing from the keyboard directly to the screen (albeit with a crude method). I've tried some tests with OS65V but have not had much success booting it consistently. Some of the keycodes respond too fast, others not at all. I'm also testing a new self-contained power supply to replace using my bench equipment.

I would like to run eWOZMON from video, OS65V, and possibly BASIC too, but really getting any sort of general functionality from the project will be nice. I also want to add parallel output from either a PIA or a RIOT, and some variety of sound output, even just something simple and appropriately 1970s. At present, the 440 video board is only equipped with RAM for storing text data, but I plan to add an additional 2KB of graphics RAM. The authentic method would be to use another 420C RAM board with ribbon cables. I will instead be using a newer 2KB SRAM chip for convenience sake.

Ultimately, this will be similar in function to an original Challenger 65V-4K from OSI.

OSI400 OSI420C OSI440 OSI440testing OSI440video
sand720 sand720adapt sand720and440 sand720debug sand720interposer sand720keytesting
OSI495ROM OSI582corroded OSI440screwy OSI440good OSIboards

This page was last updated on 8-12-2023

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