Updates:
I have fully assembled and tested both my "red" 18-slot Altair 8800c and my "blue" 9-slot system. Photos of each can be found earlier in this thread.
They both worked pretty much right away, but each had its own unique idiosyncrasy that I had to work through. None were design problems or anything of that sort, just little "hitches" that I thought might be worthwhile to share.
First was the disk drive interface cable problem I mentioned above. Easy enough to resolve once I discovered this:
- Damaged edge connector
Somehow I had mangled that cable end, which obviously made the drive non-functional. But after I replaced it, the system was good as gold. In the end, I had about five feet of cable run between the FDC+ card and the diskette drives. Works just fine at that distance.
Second was the issue I mentioned earlier in the thread with ROM BASIC on my Rev.H 88-2SOIJP board. It was intermittently loading BASIC, no matter how I set the sense switches. I could always boot from diskette, but I couldn't always start ROM BASIC from the front-panel.
I initially thought it was slot-sensitivity issue, that the SIO board worked right if it was placed close to the CPU. But that was just coincidence. It wasn't a slot sensitivity problem.
Both Martin Eberhard and Mike Douglas helped me out with this. They each have enormous patience, or if they didn't, they at least didn't let it show.
Martin gave me several troubleshooting suggestions, including even some small test programs to enter from the front panel. He even let me send the board back to verify. Worked for him, so we continued to look at my system. Martin quickly came to the conclusion that the problem was in the front-panel.
Mike also gave me lots of pointers, including a willingness to help me diagnose the interface board with a scope. I have two interface boards, so once we determined the problem was there, I knew it would be pretty easy to find. When I dove in a little closer, I realized that one of my interface boards worked properly but one didn't. Even the one that didn't work would boot diskette and do pretty much everything except STOP. After IPL, the stop toggle would no longer stop the system.
Once I began to focus on the STOP problem, I ended up swapping chips until I found the defective part. It was the 74LS02 NOR chip in location "L." Swapped it out and the STOP toggle was able to halt the system.
The last thing I encountered was the card guides in the 18-slot system. I had ordered 36 guides from JM Precision but they are actually a little too tall and the slots don't go deep enough. So they prevented the cards from penetrating the edge connectors enough for reliable operation. I ground the tops 1/4" to get the right dimension for the ejectors, but the slots were still wrong. They didn't go deep enough to allow the boards to seat properly. I ended up disposing of all of them and having new ones made from the STL file in the following archive:
The files in the ZIP archive include the left and right 9-slot card guides and the single card guide as is used on the 18-slot board. Of course, you'll need 36 of those to provide for all slots on an 18-slot motherboard. If you have a 9-slotter, you just need the left/right 9-slot pair. They're different setups.
I might add that the 18-slot motherboard from JM is excellent. It's thick enough to support an engine block. And it could be that some edge connectors are taller than others. I used the connectors sold at JM and advertised as being "manufactured by the original company to the original design for a true authentic result." If you get those, be sure to download the STL file above and have card guides made from it.
I already listed the hardware for mounting the 9-slot motherboard earlier in this thread. It's actually in a link to the
3D Printing Service thread. The hardware needed for the 18-slotter follows:
(72) #6 x 3/8" Truss head Phillips wood screw - These are used to mount 36 card guides, a pair for each of the 18 slots.
(36) M3 x 10mm female threaded stand-off - These mount the board to the chassis.
(36) M3 x 12mm screw - These mount the board to the standoffs, going through the edge connectors.
(36) M3 x 6mm screw - These mount the standoffs to the bottom plate of the chassis.
You'll need to drill the chassis to mount the 18-slot motherboard. You can use the motherboard to fabricate a drilling template out of cardboard.