SCELBI Oscope Progress

November 30th, 2016

Read the article in the previous post in order to get an overview of the Oscilloscope Interface design. Here is an expanded image of a single starburst character as my implementation currently stands.

SCELBI O-scope Drawing

SCELBI O-scope Drawing

This is clearly not what is desired in displaying text, but you can see how the scope is attempting to draws a starburst image. In order to make more sense of what was happening here, versus what I thought the design should do, I drew a series of lines over the top of actual image.

Expected Oscope Display

Expected Oscope Display

The software writes two bytes “A” and “B”, which controls which lines are “on” and which ones are “off”. The line starts at line “A0” and proceeds to draw through “A7”, then B0 through B7. A0 represents bit 0 of the first byte, A7, bit 7 of the first byte. B0 through B7 represent the second byte. NA represents lines that are never “on”, those lines are simply used to reposition the cursor in order to draw the next line. For purposes of debug, I had all lines turned on for this image.

Next I moved the arrows to match what is seen on the oscilloscope.

Oscope Display Trace

Oscope Display Trace

This technique clearly revealed two big issues, and a few smaller ones.

The first major problem is that the Y axis is inverted versus what was expected. I’ll have to investigate hardware connections to clearly understand what is going on with that. It could be as simple as incorrect hookup between the digital and analog board, or perhaps an incorrect analysis of the design. In any case, this can be handled by reversing line segment assignment in order to flip the characters.

The second issue is apparent without the process of analyzing the drawing sequence. That is the bright spot to the right of the character. That problem is actually due to the software algorythm used to draw a series of characters. An ASCII character is sent to a lookup routine that searches through a table of characters in order to find the matching bytes that must be sent to the “A” and “B” registers of the digital board. Once the lookup is complete, the 2 bytes are written to the digital board. The problem is that the scope is stuck on a single spot while the lookup is done. Even though the “Z” input should dim the scope, it isn’t turned all the way off. The solution to this problem will be to look up all the text and create a buffer of all the data to be sent to the “A” and “B” registers. That should avoid burning in one spot.

The other problems are uneven and tilted lines. I’m hoping that this can be eliminated though carefull adjustment of the variable resistors on the analog board. I’ll discuss this further in a future post as I work through those issues.

The good news with all this analysis, is that it is clear that the majority of the hardware must be working correctly in order to display this much of the starburst, as distorted as it is.

SCELBI Oscilloscope Board Hardware Description

November 29th, 2016

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.

SCELBI Oscope Interface – “First Light”

November 19th, 2016

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

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

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.

O-Scope Repair

November 12th, 2016

After a few evenings chasing this intermittent fault, I thought I finally had solved it when I sprayed some Detoxit gold on a couple of the channel 2 input preamp selector switches and it snapped back to life. The next day while moving back to bring up of the O-scope interface, channel two failed again, this time appearing to be completely dead. This was actually good news, because a solid failure can be easier to debug than an intermittent failure.

This morning I figured out that there was a short between an input capacitor case ground and chassis ground.

O-scope Intermittent Short

O-scope Intermittent Short

Scraping the debris from between the capacitor ground and nearby chassis ground seems to have rectified the issue, hopefully for good. Having a good working channel to compare the bad channel with, made this repair easier than it otherwise might have been.

Oscope – analog power supply working well enough (I think), however…

November 7th, 2016

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.

Oscope Power Supply Part ??

November 2nd, 2016

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.

Some Thoughts about Happiness

October 25th, 2016

When Thomas Jefferson wrote the first draft of the Declaration of Independence, he wrote of three inalienable rights. Among the three, was the “pursuit of happiness”. While some people think that the phrase, “pursuit of happiness” is about the acquiring of material wealth, as portrayed in the recent movie by the same name. In reality, that isn’t the case. Scholars know that Jefferson admired Epicurus, a Greek philosopher, who founded a system of philosophy. The following description of Epicurianism is from Wikipedia:

The philosophy originated by Epicurus propounded an ethic of individual pleasure as the sole or chief good in life. Hence, Epicurus advocated living in such a way as to derive the greatest amount of pleasure possible during one’s lifetime, yet doing so moderately in order to avoid the suffering incurred by overindulgence in such pleasure. The emphasis was placed on pleasures of the mind rather than on physical pleasures. Therefore, according to Epicurus, with whom a person eats is of greater importance than what is eaten.

So Jefferson was advocating a pursuit of happiness as an unalienable right, he was in essence advocating a pursuit of Epicureanism philosophy as a fundamental aspect of a free man’s life.

You might wonder why I am talking about Epicureanism. Why is it so important, that I would make an effort to share these ideas with you.

Well, I think that happiness is something that you should pursue, and it is absolutely worth pursuing. Happiness does not necessarily come automatically with other goals like attaining an education, advancement at work, raising a family, or striving for any other goal. Happiness should be a goal unto itself and you should pursue it, the same way that you might pursue your other goals. Those other goals may or may not bring you happiness, and, at best, attaining those other goals most likely will only bring you temporary happiness. Therefore, you should make happiness a goal unto itself.

Make sure that whatever you do in life, make sure that you set aside a little time each week to think about what gives you happiness and make finding a little happiness each week, one of your goals for that week. You may not succeed every week, but just thinking about it and working on a plan, should provide some hope for the next week. Furthermore, I believe you should encourage and help your partners, family and friends to find a little happiness each week.

This essay was written over the spring and summer of 2016 by Mike Willegal

SCELBI Oscilloscope Edge Connectors

October 15th, 2016
Edge Connectors Wired

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.

SCELBI Analog Board Power Supply Installed

October 14th, 2016
Power Supply Installed

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.

Scope Chassis ready for wiring

October 11th, 2016
Inside Repro Oscope Chassis

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.