Last year, Paul Robson sent me this document, describing how he thought the SCELBI Oscilloscope Interface worked.

SCELBI Oscilloscope Interface Description

Paul’s reverse engineering efforts turned out to be right on the money. Having this document has helped me substantially with software design and bring up of real hardware.

There are some aspects to the design that I will elaborate on in future posts. Some parts of Paul’s document will not be obvious to some novices, even though they are described well enough in Paul’s document for many people to understand. Also, I’ve discovered some practical limitations to the implementation, as I go through physical hardware bring up that need further elaboration.

Astronomer use the phrase “First Light” when they use their telescopes for the first time. I decided that turning on my SCELBI Oscilliscope interface on for the first time is, in a sense the same kind of event, so I’m borrowing the astronomer’s phrase. I’ll bet some followers of my blog figured that I’d never get around to hooking this interface up, but they would be wrong.

Here is the test setup. I’m using my reprodution SCELBI 8H, and the “scope”, (not pictured here) is my Tektronics 465.

SCELBI Oscope Test Setup

I wrote a simple test driver that should have repeatedly printed two lines containing “ABC”. This is what I saw when I ran it.

SCELBI O-Scope Interface First Light

The bad news is that this is unintellegible. The good news is that I’m getting signals on both X and Y outputs. The Z output has no signal. Next step will be to figure out if this is a hardware, software or adjustment issue or combination of these.

The 1N5355 helped with the -18voltage, but the positive voltage still was too low. I resolved the positive voltage by changing the dropping resistor down to a value of 20 ohms. I also connected the center tap of the transformer secondary to the ground. With these three changes, the positive and negative 18 volts seem pretty stable.

However, my next problem is that the second channel on my Tektronix’s 465 oscilloscope has become intermittent. Looks like something is wrong on the vertical preamp board. A few hours of debugging hasn’t revealed any obvious component fault. Seems like an intermittent connection somewhere close the input jack is the culprit. I can’t very well test the SCELBI oscope interface until I get both channels working again, so I’ll have to keep working on it.

I’ve now created a few posts about the little split power supply that is used to power the SCELBI Oscilloscope Analog board. Here is another, and most likely, not the last.

After wiring the chassis and powering it up I found the positive side of this supply was 4 or 5 volts low with both digital and analog boards installed. After spending a few evenings fooling around with it, I decided that the main part of the problem probably is the relatively high zener impedance of 45 ohms of the BZX79C18 zener that I had installed in the circuit. I believe that the zener’s impediance is dominating the voltage regulation, not the zener effect. I’ve decided to try an alternate part, the 1N5355, with an impredance of 2.5 ohms. Hopefully it will work better for this application.

Edge Connectors Wired

I have the edge connectors wired and ready to go into the chassis. It should be apparent why I wire these connectors before mounting them in the chassis.

The original unit used wire wrap, but I had these connectors on hand, and decided to solder the wires on, just like the cassette and tty interfaces.

If you follow the schematics, most of the connections are pretty apparent. However sync (BP) and the four strobe inputs (BB, BC, BD, BE) are not. Turns out that sync will go to pin 10 on P1. Out 0 on the main chassis needs to be connected to sync, just like needed to be done with the cassette interface. From studying the images I have, I think that the strobe inputs are spread out between pins 9 and 11 of both P1 and P2. Pin 11 is ground and pin 9 is the strobe output of each port. In this case, the ground inputs of the P1 and P2 connectors are not used as a system ground connection, but only as a low logic input connection. There is some chance that I will have the P1 and P2 outputs swapped compared to the original unit, but without original software or access to the original hardware, I can’t tell for sure which port was connected to which input latch.

I think when they built the chassis for the oscilloscope interface, they should have swapped the ports and the oscilloscope connectors. This would have reduced the length of a number of wire runs. Who knows, maybe they did make that change on later units, but unfortunately no others are known to exist.

Power Supply Installed

This took a bit more time than expected, but at least the result is a little cleaner than I really hoped for.

I’ll wire the digital to analog board connections next. After that I’ll attach the power supply and external interface wiring to the edge connectors. I’ll mount the edge connectors in the chassis and connect the dangling power and interface wires to the appropriate connectors. I expect this to be pretty straight forward and go faster than all the previous steps to this project.

I’m not sure if I mentioned it before in this blog, but building the SCELBI Oscilloscope Interface is more complicated than building some basic computer systems.

Inside Repro Oscope Chassis

I bought several versions of terminal strips only to use a 4 postion version from Antique Electric Supply. I ended up cutting them down to make the three position set.

I’m going to wire up the power supply first, then wire in the edge connectors. Note that one side of the transformer shares a mounting screw with a terminal strip. Also, on the original unit, the edge connectors are mounted on a panel and secured to the mounting brackets. At the moment, I don’t have enough stock to make a full panel, so I’ve made a mini panel, and I’ll retrofit a full panel later on.

Here is a view of my version of the SCELBI O-scope Opamp Power Supply test setup. AC power is not connected in this photo, but would be connected to the transformer during the actual test.

Opamp Power Supply

I’m using a Triad VPL28-180 transformer and 1N4002 diodes. I found the diodes in my spare parts stash. The first smoothing capacitor is rated at 100uf/50 Volts. The +18 and -18 supplies have 470uF/25 volt smoothing capacitors. The latter are probably overkill, but it’s what I could find in my spare parts stash. The resistors are rated at 120 ohm, 1/4 watts. The zener diodes are the BZX79C18 that I mentioned in a previous post.

After hooking up to the analog board, the voltages are within a volt of +/- 18, with no measurable ripple. Transformer output is a little higher than with the original transformer that I tested with, but not enough to be of concern. Zener shunt current is OK, so I’ll just move this set up into the enclosure, without changing any components.

Well there is a reason why I changed majors from mechanical engineering to computer science.

SCELBI Oscope Front Panel

A few imperfections in execution, but it will look fine when the connectors and switches and such are added. This is a Bud AC413 chassis, the original chassis was constructed a bit differently, and 1/2 inch higher. I didn’t notice the Z and G when making out the rub-on artwork, but fortunately I included some extra text including an extra “GND”. The N turned sideways makes a decent Z.

I have a few more coats of clear lacquer to spray before moving onto the next stage.

For Comparison, the following is an image of an original SCELBI O-scope Chassis taken by Jack Rubin. It has been slightly photo-shopped to fix the perspective.

SCELBI Oscope Original Front Panel

SCELBI Keyboard Interface All Hooked Up and Running

For now, I’m using one of my PS/2 adapters (on top of the enclosure) to connect to a PS/2 keyboard. I was going to cobble together a cable for my reproduction Datanetics keyboard, but it requires -12 volts, which would have to be generated separately just for this keyboard.

Now I can focus on the Oscilloscope Interface.