Mike's Hobby Blog

SCELBI 8b PCBs

The SCELBI 8B backplane, SRAM and memory expansion boards just arrived. They may take a couple of weeks or so to check out. If they work, I’ll be able to get floating point SCELBAL up and running on an 8B for the first time in a very long time, and so will you. There is a semi-reproduction that has run SCELBAL, so maybe not the first, but it all depends upon how you classify that system. To start with, I’m building a minimal, barebones, chassis mainly to allow check out of the SRAM and get SCELBAL running.

You will notice that there is a stack of cards involved. The big question is whether I finally screw up a layout and have to ditch one of these batches of cards. To date, I’ve never made a small prototype run of a few boards to check my work. There were prototype’s made of a few of my smaller cards, but they were either made on prototyping boards or etched here at home, where the cost is almost nil, compared to professionally made PCBs. The benefit of this rather risky approach is that I save a great amount of money by not making what would be rather expensive prototypes. This directly translates into a lower cost project for you and me. In addition to the risk of making a complete batch of dud cards, another downside is the great amount of time spent reviewing the layout before I eventually pull the trigger.

Some people have been asking for these for a couple of years – well I hope the wait will be worth it for those people.

I have also started serious work on the PROM card. Turns out that I will probably end up going over and tweaking the entire design that I started with, so it will take a little while. Not as long as doing the 3 other boards, but those that are interested will have to wait a little while longer for the PROM boards.

I’ll have the PCBs in a week or two, but I also need to order a few parts so that I can build them up and do some sanity testing.

I plan to reproduce all the SCELBI PCBs. There are 16 types of boards in all. The 1106-1108 8B boards are practically ready to be made. I’ll probably tell the PCB factory to start on them this week. This will mean that in a few weeks I’ll have 10 of the the 16 boards done. The really good news is that most of the remaining cards are only single width, so reproducing them should go much faster than the double width system cards.

Here is the complete list of the PCBs that SCELBI Computer Consulting made and my reproduction status. The peripheral cards are listed in my most likely order of reproducing them

Main System Cards
Reproduced
1100 CPU – 8H/8B
1101 Data bus buffer – 8H/8B
1102 Input – 8H/8B
1103 Backplane – 8H
1104 Front Panel – 8H/8B
1105 1K SRAM – 8H

In progress
1106 Memory Expansion – 8B
1107 4K SRAM – 8B
1108 Backplane – 8B
1109 PROM – 8B

Peripheral Cards –
Reproduced
2104 Teletype interface*

Not started
2105 Keyboard
2100 Oscilloscope digital**
2101 Oscilloscope analog
2102 Audio Tape output
2103 Audio Tape read

* not as high as normal vector boards
** double width vector board

Hi SCELBI fans,

Based on scarcity of reports, it may not seem like it, but I am still actively working on the SCELBI 8B card set. It is pretty tedious work, and reporting every new set of tweaks to the layout is not very exciting, so I haven’t been reporting ongoing progress.

To be honest, I prefer exploring new projects, rather than revisiting old ones. Though the SCELBI 8B is “new”, it leverages so much of the 8H design, that it really seems more like revisiting the 8H, than doing something new.

At this moment, I am going through final design checks of the SCELBI 8B backplane, memory expansion, and 4K SRAM PCB layouts. I already have quotes and have done DFM checks with my PCB supplier. I think I will probably build up an very basic 8B system without I/O in a temporary chassis in order to get these cards checked out. This is so I can offer the PCBs for sale for those of you who can’t wait for the whole package. Based on what I know at the moment, I might be ordering PCBs in as little as a week or two from now.

The chassis sheet metal and the EPROM card will come a little later as follow on efforts.

After that, my next efforts will probably be the SCELBI keyboard and oscilloscope interfaces. Those will be “new” to me, so will be a lot more fun to do than the 8B has been. The O-scope interface poses some interesting challenges, as we don’t have any original software for it. We will have to craft some drivers using only the hardware implementation as a reference.