I was mentioned in a Robb Report article about the collectible value of vintage computers. It’s always an honor to be mentioned in the same article as Dag Spicer of the Computer History Museum.
I am now offering 8B PCB set to early adopters a special price of $275 plus shipping. Though my prototype system has been brought up and basically checked, it hasn’t been fully checked with a full complement of memory and PROM cards. Also I/O port testing has been minimal. There could be additional issues beyond those already documented in my blog that will require rework. Once checkout is considered fully complete, the price of a board set (minus PROM card) will rise.
The introductory set contains one each of the following PCBs. Extra SRAM cards are available now at $50 each. This set does not include the PROM card, which is still under development. Estimated time before PROM card will be available is 3 months, give or take 2 or 3 months.
1104 Front panel
1106 memory expansion
1107 4k SRAM (1)
1108 8B Backplane
Send an email with your address and and questions to:firstname.lastname@example.org for a shipping quote and ordering information.
I’m not sure when chassis sheet metal will be available, as I’m looking for bargain on a used Box and Pan Brake, so I can fabricate them at substantial savings over contractors prices. Note that not just any brake will do, it has to handle a minimum of 2′ of 16 gauge stock with at least a 4″ depth. for now, you can fabricate your own temporary chassis or use an off the shelf Bud AC-413. The only problem with the Bud chassis is that the original chassis were 3.5″ high and the AC-413 is only 3″ high.
During my debug of the 8B, over the last couple of days, I improved my understanding of the data display on the SCELBI front panel. Though it is clear from the block diagram, some things just don’t sink into my brain without some additional lessons, usually learned the hard way.
The data display simply shows the contents of the memory location that is currently being addressed. I already knew that when you stop or step the SCELBI, it stops after setting up the address bus, but before executing the next cycle. In 8008 terms, this is at CPU state T1. The two status bits on the front panel are simply the two high bits of the high byte of the address byte. These two bits are not actually address bits, but known as the cycle control decodes, which is why the 8008 only has 16K of address space and not 64K. Another fact to consider is that both 1101 and 2102 memory chips have different input and output data lines.
The interesting thing that is a result of these design decisions, is that you can do simple memory tests from the front panel by writing a memory location, without even reading it. The data you write should show up on the front panel, without the need to even execute a read instruction. If you have a write failure, you will immediately see that the data you thought that you wrote is being displayed incorrectly and therefore has been written to memory incorrectly.
One other thing, without memory in the system, the data display will always show up as all ones. You can still jam instructions into the CPU, but you will not be able to use the data display as an aide to troubleshooting.
I need to build another memory board before I will be able to run the floating point version of SCELBAL. I think I have most all of the parts on hand for at least 1 more board, so this shouldn’t take too long.
I had to debug two problems. First was a missing trace on the memory expansion board. This ran under the 7442 located at Z9 from pin 12 to ground. During design checks I check every pin on every chip for a connection to the correct net, but I missed the ground connection on this pin. The trace is completely obscured by the IC, so it’s invisible in images. It is documented in schematics and I probably should have caught it during design checking. Anyway, it’s not serious enough to qualify for a respin of the board, since it can be fixed by running a ground wire under the chip from pin 12 to pin 8 before soldering it (or the socket for it if you decide to use sockets) to the board. This fix will be practucally invisible to anyone inspecting the board.
The other issue was a pin bent under a memory chip. This took longer than the missing trace, as I was assuming that something was wrong with the soldering or layout on one of the boards.
I have done some rudimentary checkout of all the memory slots (with my 1 board) and so far they all look functional. The last slot was only half checked out, due to the lack of another 7442, which I haven’t gotten yet. I will not feel like that the system will be fully checked out until I get 3 memory plus prom board all working together. However, I’m confident enough that it will be ok, that for those of you that want to start building a SCELBI 8B, I’ll start a discounted early adopters program in the next couple of days.
Finally powered on. Still need to add and test memory expansion and memory card, but I can jamb instructions with the front panel. I’ll start checking out the memory as soon as I finish this post.
While wiring the backplane, I found that SCELBI made a change in the RDYN jumpering. Instead of jumping XA02 A-X to A-Y and A-Z, the 8-B instructions connect XA02 A-X to A-Z and connect XA02 A-Y to B-F. I’ll find out as the day goes on, if that makes stepping any more reliable. If so, it would make a good change for the 8-H, also.
The chassis was hacked together out of wood and sheet metal. I’m looking into finding a used Box and Pan brake so I can cut and bend my own chassis. The ten Bud chassis’s Corey had made for the 8H project cost $1000, and that left no room for profit, so he sold them at cost. Also, I want to make chassis for the expansion cards and power supplies. The whole thing just adds up to too much expense. Also if I had them made, when the batch is sold out, then future SCELBI builders are left without options, cause I can’t afford to buy ten and have nine sitting around for months.
For the prototype, I’m just wiring up 1 input and 1 output bit, so I can download programs and use a terminal. Here is the back panel.
Trying to take out Isis by bombing them from a few high flying aircraft is like trying to destroy a fire ant colony by stomping on it’s nest.
This approach has only really succeeded once – and that was when we resorted to using nuclear weapons at the end of World War II.
There is an old saying that might lead to an alternative approach that might have more promise – Keep your friends close and your enemies closer.
Memory Expansion Board is almost done – I’m short one 7442, which will prevent me from checking out the last slot, but otherwise it’s ready to go. This board went pretty quick, the 67 resistors took as long as anything else on this board.
For now, I’m hacking together a wooden chassis with sheet metal front and rear panels. Once I get the aluminum version figured out, It will be easy to move the backplane and front panel switches from this hack job to the final chassis.
As far as the possibility of the short on the 4K SRAM board, though it’s pretty remote, I decided to add some masking tape to the bottom of one end of the fuse holder to eliminate it entirely.
Given where I’m at at this moment, I expect I’ll be able to power up the 8B with 4K of SRAM within a week.
I learned this last year and figure I should share it before I forgot or worse.
At VCF southeast in 2013, I briefly met with Nat Wadsworth’s sister in law. She told us how Nat kept birds in his basement and developed the SCELBI on the second floor of his house.
She told us that Nat was studying bird diseases which accounts for the biology part of the SCELBI name.
I found a small issue in the layout of the 4K SRAM card. The trace leading from the ground past the fuse holders passes too close to the fuse holder for comfort. Since there is no solder mask on these boards, it would be best to mask this trace at this point with tape or something so it can’t inadvertently touch the fuse holder and short +5 to ground. Reviewing my layout, it looks like I missed aligning this trace correctly to the original layout. The layout in this area was obscured by the 10uF capacitor, and I didn’t notice the difference when comparing my layout to the original.
Next board to solder is the memory expansion board. It it will be fun putting in the 67 resistors that populate that board. Then I’ll have to cobble together a chassis for testing.
I just posted an update to the Apple 1 registry. One system was deleted and one added, so the total count holds at 63. The new one is John Anderson’s, which will be sold at auction, next month. I just was contacted by the owner of what is probably an unlisted system, so the count could grow to 64, very soon. Christopher’s system, which has been shown at K’Fest, VCF midwest and VCF east has been undergoing some restoration, so I added an updated image and additional information.
Note that the success of the Apple 1 registry is largely due to contributions of owners and former owners and other interested parties and I greatly appreciate all new information.
What’s really exiting to me is the new SCELBI registry. If you think Apple 1’s are rare, I could only find information on 13 SCELBIs. I recently received information on what could be a 14th, but it also possibly could be the missing Freeman Museum unit. The images on the SCELBI registry will be a little different than the Apple 1 registry, as the images will emphasize the quirks and differences between the units. Many original SCELBIs don’t exactly look like factory stock systems, so Iit should be a good resource for people building reproductions.
I expect that some of the information may be incorrect. Bear with me as I expect to refine this registry quite a bit in the future.