SCELBI Keyboard Interface Checkout

This setup is being used to checkout basic functionality of the SCELBI Keyboard Interface card. The card was designed to interface to a Don Lancaster Keyboard as described in a couple of articles published in Radio-Electronics in 1973. The first article was the February issue and contained instructions on building a basic non-encoded keyboard. The April issue contained a follow on article with a description on how to build a keyboard encoder. These links contain the original articles and are hosted on Michael Holley’s comprehensive SWTPC pages. This keyboard/encoder design does not latch keypresses, but simply outputs what it sees, as the keystrokes occur. The strobe is supposed to come after the keyboard data, and end prior to keyboard data, so interfacing hardware doesn’t have to do anything fancy to latch the data.

The SCELBI keyboard interface accepts 8 bits of input data from the keyboard, with the most significant bit (MSB) being a strobe. The interface to the SCELBI computer contains 8 bits of data, with the MSB being a data available indicator. The data available indicator is cleared by a single input (high to clear) from the computer. The data and strobe coming from the keyboard can be independently inverted or not, by correct selection of jumpers around Z1 and Z4 and inclusion of the Z1 and Z4 7404 inverters or not. Leaving the 7404s out and bypassing them with jumpers will result in inverted data. With my PS/2 keyboard adapter, I needed to use the 7404 to keep the interface from inverting data. At one time, I had a version of PS/2 adapter firmware that would invert data, which would have been handy in this case, but I removed that feature some time ago.

It appears that SCELBI engineers had trouble using the Radio-Electronics keyboard strobe to latch data correctly. The strobe is OR’d with the data bits in order to make up an internal strobe for use within the board. Many keyboard designs internally latched data, and this extra circuitry doesn’t work since the incoming data never goes away, and the strobe stays stuck on. For my testing, I used one of my PS/2 adapters. I considered two solutions to this problem with strobe. My first thought was to modify my PS/2 keyboard adapter firmware to make it emulate a non-latching keyboard. In the end, after playing around with the interface, I decided to simply delete the gates that OR’d keyboard data with strobe. This can be seen in the following photo as the deletion of IC Z7 and the jumper from the pad at pin 2 to the pad at pin 12.

Configured Keyboard Interface

With this simple modification, I have bench tested the keyboard interface to the extent possible. Next, I need to write a small 8008 keyboard driver and connect to my reproduction SCELBI 8H.

The SCELBI cassette and oscilloscope interfaces both use a rather unusual variation of the jelly bean 741 op-amp. This is the TI SN72741, which puts this op-amp in a 14 pin DIP package. Though they can be had on e-bay, I recently discovered that expediters.com has them for $5.50 each. They can be found at this link:http://www.expediters.com/index.jsp?path=product&part=845153&ds=dept&process=search&qdx=0&text=Sn72741

This SCELBI 8B power supply is built in a shadow box type chassis that is approximately 10″x5″x6″.

The internals are simply two Power-One linear Supplies, a switch, a fuse, 3 binding post type connectors, an amphenol 78S4 type connector and a fan. Here is an image of the interior. The fan is connected to the top, so isn’t visible in this view. The 5 volt supply is a C5-6 and the -9 is a modified B15-1.5. Be aware that the -9 supply is not powerful enough for a fully loaded 8H chassis.

SCELBI Power Supply Internals

SCELBI Keyboard Interface

Looks like I have enough parts on hand to build one of these babies up. It expects a decoded but unlatched keyboard. I don’t have one of these, so I’ll modify the firmware on one of my PS/2 adapters to act like one. With that I should be able to hack together a simple 8008 driver that will eventually work with MEA on the SCELBI.

SCELBI 8B Complete

Here is a complete, original, SCELBI 8B system that I was recently able to examine and operate. In the past, I already had examined the TTY and Cassette interfaces. This time, I was able to examine the power supply and will put up some posts in the near feature, including interior shots.

This document provides rework instructions that will bring the PCB up to the revision seen in the schematics. I now have a pretty good understanding of how this interfaces works and will provide more details in a future post.

Oscilloscope Rework Instructions

Back of SCELBI O-Scope Digital Board with rework highlighted

This is an image of the current state of the back sidelayout of the long awaited (by a few) digital board overlaid over an image of the actual board. Base image is curtesy of Jack Rubin, the only person I know, that has seen an SCELBI oscilloscope up close and personal in recent memory. This one is in the Computer History Museum’s off site storage facility.

I haven’t worked on any of the lettering, but I’ve gone through multiple passes of the rest of the board. Based on file size, this board is actually more complex than the SCELBI CPU board.

One other thing that I’m going to do is more study of the rework on the only known Oscilloscope digital board. The arrows in the image point to areas where rework was applied. Initial review seems to indicate that the schematics found at scelbi.com match the board after rework was applied. There is also a bit of rework on the front side.

The analog and keyboard layouts are ready to go, though I’ll probably give each of those one more quick design review pass before ordering a batch.

Link to SCELBI’s app on Apple’s App Store This is the same version (2.7) as available for download from my SCELBI web page. If you have used this version it would be great if you would review this Application on Apple’s App Store.