🇺🇸 Osborne 1: Part 6 (OZROM Memory Tester) [TCE #0521]
23The 3rd party OZROM BIOS ROM has a built-in memory tester, something that the original Osborne ROM but was removed. Thus why it's handy to have it available.
It has indicated that one of my 16Kbit RAM chips (4116-equivalent) is faulty.
There are four banks of 16KBytes with each DRAM chip handling one bit. The OZROM memory tester consistently indicates that the chip at location B24 is faulty ... ram bank B (16KB-32KB) and bit 4.
I have found, also, that running certain programs simply fail ... the CP/M Help disk doesn't work and SuperCalc similarly fails. This would make sense for anything that needs more than 16KBytes of RAM.
I removed the suspect chip and installed a new one ... and the memory tester now passes! Similarly, the CP/M Help and SuperCalc now also work!
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From Part 4:
Thanks to user "nelgin" on the VCFED Forums, we now have a binary image of the 3rd party OZROM BIOS for the Osborne 1. One interesting feature of this ROM is its built-in RAM tester. And, apologies for the flaky video capture - will try to sort it out for future.
To enable easy switching between the offical Osborne BIOS and the OZROM I created a simple 2732-to-2764 daughterboard:
https://github.com/0ddjob/Osborne_1
To get the full use out of the OZROM BIOS there is an accompanying setup disk (currently lost?), but we can try the RAM tester out at least ... just press [TAB] at the boot screen ...
RAM TEST
4000 00000000 (what the tester wrote)
00001000 (what the RAM returned)
Oh dear ... it seems I have a faulty RAM chip in bank B (4000-7FFF) and in location B24 (4th bit).
Good to know though!
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From Part 1:
Let's have a quick look at this Osborne OCC1 computer from 1981, a luggable Z80-based machine with 64KBytes of RAM, dual 5¼" floppy drives and a built-in 5" monochrome screen.
It only runs on mains power - as this one has come from the US but its power supply is very easily switchable from 120V to 240V.
It has an interesting array of ports on the front - full featured really: an external battery (DE9 socket), video output (edge connector), IEEE488 port, RS232 serial (DB25) and a modem port ... with modem.
That "EXT VIDEO" video output edge connector is very important - you either need a shunt to connect the top PCB signals to bottom, or a video output connector (with RCA socket) to allow video output on a larger screen. Without either, you will not get any video displayed on the internal screen!
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From Part 2:
None of the three disks I have will boot - I suspect that the CP/M system files aren't included on them so I need an actual CP/M 2.2 boot disk for the OCC1. You can switch to booting from Drive B: by holding [SHIFT]["].
The disks are labelled as "DD" ... double density ... but this requires an additional daughterboard inside the machine ... mine has none. So it appears to be "SD" or single density only.
I pulled the machine apart - wasn't too difficult, although you do need some space. It's definitely a lot easier than the IBM Portable PC to pull apart!
The 5¼" floppy drives are Siemens model "FDD 100-5" and use belts.
It came as no surprise that the power supply had a few RIFA capacitors used as class X safety filters ... and one had blown up. So I'll replace those whilst I have it apart: the main 0.1µF (100nF) that had blown and two smaller 0.01µF (10nF) that were starting to look a little iffy.
I know it's possible to buy a set of disks, but I should also be able to make my own images with the tools I have. I might try to get it running off a Gotek/Flash Floppy in the interim.
For that, this small PCB looks useful:
https://www.pcbway.com/project/sharep...
... or ...
https://github.com/WayneVisser/Osborn...
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