The Gemini 80-Bus Saga - Part 2
HELP NEEDED!..... I am desperately seeking to archive Gemini 80-Bus software before it is lost forever. I am particularly interested in GemPen, CP/M bios versions, SYS and any disk images that may be around. I am also keen to preserve as much hardware as I can (finances allowing). Please get in touch if you can help or have a similar interest.
Thanks to the generosity of Charles Obrien of Hunmanby (formally Blunsdon Swindon), I am now the proud owner of a Gemini GM916 Galaxy MFB. The unit includes 5.25", 8" and 3.5" disk drives and a Winchester disk. The machine had been used by Charles when he ran a consultancy business called Obsys.
The Multi Format Bios allows CP/M to be configured for hundreds of floppy disk formats from many manufacturers and is ideally suited to converting from one format to another. The machine was purchased from a company called Timeclaim Ltd located in Woodbridge, Suffolk who also provided service, support and updates. The documentation that came with the machine was mostly produced by Timeclaim Ltd.
The Multi Format BIOS (MFB) is an extension of the normal Gemini bios and supports all the usual features such as disk error trapping, screen edit, screen dump etc. Howeve the MFB will support over 600 (yes 600) disk formats.
A Look Inside
Before switching the thing on, I thought it would be wise to open her up, take stock and make a few basic checks.
The machine consists of the following;
- GM813 CPU and Memory board
- GM832 SVC Video Controller
- GM849 Floppy/SASI Controller (Modified for MFB)
- An 8 inch double sided drive
- Teac FD55 48tpi 5.25 inch Drive
- Teac FD55 96tpi 5.25 inch Drive
- 3.5 inch drive
- Rodime 201 5Mb Winchester Disk
- GM827 Keyboard
- Gemini Monitor
The external condition of the computer is typical of a metal cased unit that has been stored in a loft for 30 years or so, however, looking at the inside you could be forgiven for thinking it had just been supplied from the factory.
Turning on suggested that a few sanity checks might have been wise as there was a distinct smell of burning. Time to strip, clean and test.
Dead On Arrival
With the PSU checked over and a minor repair to the video cabling it was time to ignore the smell and power the beast up. Alas, nothing. I decided to create a minimalist system i.e. CPU card and display card only, when I took out the GM849 disk controller, a small capacitor fell off the board, not a good sign. I added this to my snagging list and moved on.
Alas my minimal system was still not working. Thankfully I was able to test the SVC display card in the rack system that was the basis of the article The_Gemini_80-Bus_Saga_-_Part_1 this left the GM813 CPU card as a likely culprit. However, before getting serious I tried my rack based cards in the Galaxy just to check the bus and peripherals etc. the system booted as expected. So, time to get serious with the GM813 CPU.
Further inspection showed that the CPU was not doing too much. There was no activity on the address and data busses and no output on the screen. I am no expert at this stuff so it seems reasonable that there would be some false starts and retracing of steps. After several false starts, I was able to determine that the reset button when pressed was not reseting the Z80 CPU. This was proven when I manually applied a reset to the CPU with a jumper wire. The following was shown.
***** Gemini M-F-B 2 System *****
... not much else happened but progress is progress!
After discussing the issue over on the Gemini thread at https://www.vintage-radio.net/forum/showthread.php?p=1207265 John Hanson and Nick Coleman provided me with enough details for me to trace the reset circuit and identify that the issue was an old electrolytic capacitor. I replaced the capacitor and hey presto a working CPU card. While I was on a roll, I replaced the missing capacitor on the GM849 card, as I mentioned earlier, it just fell off the card when I removed it from the frame.
With the system re-assembled, albeit loosely, I was able to boot CP/M (Bios 3.2) from the first 5.25 inch disk.
Gemini Multiboard Computer System 64K CP/M-80 vers 2.2 (Bios 3.4) A>
This did not give me an MFB system though as an attempt to run ANALYSE.COM reported the following.
**** Not a Gemini MFB System ****
Accessing drive B: wasn't too successful either, the Winchester made some noises and the following was reported...
A> B: Drive B T=0002 S=0000 Xebec Error, code = 04
The Xebec Controller documentation suggests... Sector Not Found. The controller found the correct cylinder and head, but not the target sector.. So whilst not yet a fully functioning system, progress is steady.
... more to come soon...
A Note From the Author
Considering how many of these machines must have been sold, there seems to be very little left out in the wild, as a result, I am very keen to try and preserve the Gemini system.
I still have a lot of work to do and a lot to learn so if anyone owns or has a connection with the Gemini, please get in touch, I would love to talk things through with others and find out more about the company and its history. I can be contacted via twitter @johnnewcombeuk or via the Vintage Computer section of the vintage-radio.net forum (https://www.vintage-radio.net/forum/showthread.php?t=156106)
I would also like to take this opportunity to thank to John B. Hanson and Neal Crook for helping me in so many ways, and others that have added their support via forums and social media.
Appendix A - Converting Binary to Intel HEX
To convert a binary file to intel hex using the srec_cat utility (Mac/Linux/Windows) the following command can be used, the example shown here is for the MacOS version.
$ srec_cat mbasic.com -Binary -o mbasic.hex -Intel