New MOnSter6502 updates, with video!

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It’s been a while since we’ve had an update to the MOnSter6502 project–we’ve been very busy getting the second revision ready. At the same time, I’ve been designing a simple yet powerful 6502-based computer that can operate at the slow clock speeds required by the MOnSter6502.

But before I go into detail about that, take a look at this video update. It’s one thing to see photos of the MOnSter6502, but the video really brings out just how awesome this thing is in person! (Shameless plug for Maker Faire Bay Area 2017 where you should come visit us.)

The MOnSter6502 runs up to about 60KHz clock, which is quite a bit slower than the original. The computer I’ve designed for it uses another microcontroller to simulate hardware peripherals, inspired by capabilities of various ’80s computers and gaming consoles. The idea is to offload CPU-intensive video and sound tasks to the microcontroller, freeing up the 6502 so that it can be used in real time despite the slow clock.

Right now, I’ve implemented several software-defined peripherals

  • VGA video output with 256 color graphics, tiles, and sprites
  • Multichannel stereo sound synthesizer
  • PS/2 keyboard interface
  • KIM-1 style front-panel debugging keypad and LED display
  • USB-CDC interface with a 6502-accessible UART for communications with a host PC

The computer can also run a full validation suite on the connected 6502, which has been quite useful troubleshooting the highly complex MOnSter6502 boards.

The computer is still a prototype, but you can see some shots of it in the video above.

You can find more updates and information at the project site.

Maker Faire 2017

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The MOnSter6502 will be at the Bay Area Maker Faire this year! If you’re around, come by and say hi.

The Battle of Fives: How the NE555 and LM555 are Different

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A customer recently asked us some questions about the Three Fives discrete 555 timer kit. One in particular really got my attention.

What is the difference between the National Semiconductor LM555 and the Signetics NE555 timer ICs? Well, the Signetics part certainly came first and the National part was a second source, but the customer noted that The 555 Timer Applications Sourcebook, on page 5-31, states

…Table S-2 points out that the threshold overrides the trigger for the type LM555H (National), but the threshold is overridden by the trigger for the type NE555V (Signetics).

Let’s compare the two. First, here’s the NE555 schematic (click for larger versions).
And here’s the LM555 schematic. I’ve kept the component numbering consistent with the NE555 datasheet rather than National’s datasheet to make comparisons easy.
The LM555 makes three minor changes to the timer design:

  • The trigger comparator now has a current mirror active load (Q26 and Q27) instead of resistor load R6.
  • The threshold comparator gets a current mirror active load (Q28 and Q29) and an emitter follower buffer Q30.
  • R10 is now 7.5K instead of 15K, but I suspect there is a typo. Imagine several generations of photocopies. The 1 starts to look like a 7, and a decimal point appears.

The most interesting changes are the first two. How do these changes reverse the priority of the two comparator inputs?

The original NE555 gives priority to the trigger signal because transistor Q15 can always overpower the current coming from Q19A and Q6.

For the LM555 in the normal case where the trigger signal is active, Q15 is on, Q16 is off, and Q17 is on hard since its base is pulled to VCC through Q18, R10, and the current mirror Q19.
However, if both the trigger and threshold inputs are active, then both Q15 and Q30 are on. This leads to an interesting situation where the collector of Q18 is pulled to ground through Q15 and Q30. At that point, there’s nothing to provide current to the base of Q17, and any residual charge will probably drain away through the reverse leakage current of Q18. Q17 then turns off, and the output does the opposite of the NE555! Leaving the gate of Q17 hanging like that seems really odd, so I bet this was unintended behavior. I ran some LTSpice simulations so you can see what is going on. First up is the NE555:
NE555 Flop
And here is the LM555:
LM555 Flop
If you look carefully at the V(comp) trace right before 12ms, it actually goes negative due to Q18 behaving like a diode clamp.
This behavior doesn’t seem to get in the way of normal operation, but it is something a circuit designer would need to take into account. This is why designers and purchasing people should always be wary of “drop in” replacements, especially when the manufacturer claims “improved performance!”
National Semiconductor made the changes to improve the performance of the comparators, specifically their performance over temperature. I ran some more simulations so you can see the difference. Here is the NE555 set up in a simple astable circuit, with superimposed waveforms at 0C, 35C, and 70C:
And the LM555 at the same temperature ranges.
Note that I put the temperature coefficients only on resistors inside the 555 timers, not on any of the external oscillator components.
If you want to play with the LTSpice circuits, click the links below to download them.
A quick side note about the names: The LM in the part number stands for Linear Monolithic, which National Semiconductor used to describe many of their analog ICs. The NE probably stands for Network Electronics (the sources are anecdotal). Apparently the Signetics name came from Signal Network Electronics.

CRT Phosphor Video

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Inspired by commenter Katemonster, I’ve put together a short clip with a couple of CRTs from my collection, demonstrating various types of phosphors. There are charts out there that talk about persistence using vague terms like “medium” (compared to what?), so it’s nice to see a real video showing what such a CRT actually looks like.

For the video I’ve used my “orbiter” demo that uses Newton’s law of gravity and Newton’s 2nd law of motion (F=MA) to generate simulated planets that orbit around a sun. It’s a nice way to demonstrate persistence (the way the phosphor fades as the electron beam moves away).

P1 Phosphor
This is the basic green phosphor. At 525nm primary color wavelength, it looks slightly more blue than common super-bright green LEDs. The chart linked above lists the persistence time as 20ms which seems reasonable. The formulation for this phosphor varies between manufacturers so some tubes might be slower than others. It’s very common in early oscilloscopes and oscillographs, and apparently some radar systems as well.

P2 Phosphor
The P2 phosphor color has even more blue in it than the P1–it’s very close to “stoplight green”. The persistence is much longer as you can see in the video (30 seconds or more, depending on the ambient light levels). The charts and reference documents I have list the primary applications as oscillography and radar.

P7 Phosphor
P7 is a very interesting phosphor. It is a cascade phosphor, meaning that it has two layers of material. The electron beam strikes the first (outer) layer which emits a bright blue light with some light near ultraviolet. This high energy light excites the second layer (inner, in contact with the glass) which is a much slower material that emits a yellowish-green light with a very long persistence (around a minute). In the video I move the “orbit” trace off to the side so you can see that original afterimage persists.

It was used mostly for radar and sometimes in oscilloscopes to capture one-time events before storage tubes were invented.

So why use a cascade phosphor? One source states that it was originally designed to be used in intensity-modulated displays (varying brightness levels), but it turns out it also helped prevent radar jamming. Since the jamming signal was not synced to the radar pulses, a long persistence phosphor could average out the jamming signal and allow the operator to see the true signal as viewed on an A-scope (time-based pulse waveform monitor). [Cathode Ray Tube Displays, MIT Radiation Laboratory Series, pg. 626]

P12 Phosphor
This one is my favorite. It’s an orange medium-persistence (a few seconds) phosphor that was apparently used for radar indicators. I don’t know of any that were used in oscilloscopes.

P31 Phosphor
The P31 phosphor was invented as an improved P1 phosphor. It’s much brighter (P1 is 32% as bright) and has short persistence (<1ms). The color has a bit more blue in it–in fact, very close to the P2 phosphor’s color. I would say most analog oscilloscopes from the 70s to today use CRTs with the P31 phosphor.

In many cases these CRTs would be installed behind a colored piece of plastic acting as a color filter. For example, P7 CRTs were often installed with an orange plastic filter in front to make the blue/white phosphor look more similar to the secondary yellow phosphor. P31 CRTs usually have a blue or green plastic filter.

For further reading:

CRT Driver Boards, Now With Altium Sources

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Take a look at my crt-driver GitHub repository. I tidied things up a bit and more importantly, released the Altium project files, schematics, boards, and even the output job file. It’s all licensed under the Creative Commons Attribution-ShareAlike 3.0 license. Read the Creative Commons page for the full terms, but basically you can share or adapt any of it as long as you give me credit (a link to this blog would be appreciated) and make sure that you keep the same license so that others can do the same.

If you don’t have Altium (expensive, closed source), you can at least open and edit the schematics with CircuitMaker (free, closed source, limited). Sadly, CircuitMaker will not let you edit the Altium PCB layout.

Deuterium Arc Lamp

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On Saturday I found a deuterium arc lamp at a local surplus store. It was used, and most likely pulled from an ultraviolet spectroscopy machine. I could not find data on the specific lamp model, but I found a similar lamp. On the chance any of you might know what it is, the lamp is marked

D 805 K


West Germany


Before running any tests with the lamp, I wiped it down with isopropyl alchohol to remove any fingerprint oils. When heated, they can cause the glass envelope to bubble and even melt, destroying it.

To run this lamp, which is a gas-discharge type, you first have to heat up the cathode. There is a very thick double-spiral tungsten filament inside that uses 2V at 4.5A (or 9 watts!). Once it’s warmed up for a minute or two, you apply the high voltage to the anode. I connected it to a current-limited electrophoresis power supply set to 50mA. The lamp started at 350V and settled to an operating voltage of about 84V. Incidentally, the heat generated by this helps keep the cathode hot, and the filament current can be reduced to improve its lifetime.

Here’s a quick video showing what it looks like.

Deuterium is an isotope of hydrogen: hydrogen has one electron and one proton, and deuterium takes that and adds a neutron. It is not a radioactive isotope, unlike tritium, which has two additional neutrons. According to Wikipedia, Deuterium is used in these lamps because it emits more UV with a wavelength less than 400nm.

If you’ve got one of these lamps and you plan to light it up, you’ll need eye protection. I ran it at a very low beam current (most likely it was designed for 300mA!) and the light was not so intense, but you might want more than just a pair of sunglasses if you’re going to full power…

Inside a TTL Logic IC

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Since 2014 is generally considered the 50th anniversary of TTL logic, I thought I’d take a TTL logic chip apart and do a little analysis.

So I started with a DM7438N, lot code M:P9006Y. Looking at National Semiconductor’s device marking convention document, I take this to mean that it was manufactured in week 6 of 1990 at a subcontractor’s fab in the United States and assembled in Malaysia.

The 7438 is a quad 2-input NAND buffer with open-collector outputs. That means the die should look symmetrical to a degree.

To take it apart, I used a rotary tool to carve out the encapsulation material on the top and the bottom, and then picked at it with side cutters until the chip fell out. Sadly I cracked off a corner of the die including one bond pad, but it’s still possible to figure out how it works.

What does all this do? See the image below. I’ve cropped all but one gate and highlighted the various semiconducting regions in different colors. I’ve also given designators to all the components.

Red represents the N-type collector epitaxial diffusion. Cyan represents the P-type base diffusion, and purple represents the N+ emitter region.

The schematic looks like this:

That dual-emitter transistor (Q1) sure looks strange!

How does it work? Well, if both A and B inputs are a logic high, then Q1 is off, but some current flows from R1 (4K ohm) through the base collector junction (since it is, after all, a PN junction) and feeds the base of Q2. Q2 turns on, and its emitter current feeds R3 (1K ohm) and Q3. Q3 turns on as well, and the output Y gets driven low. The non-inverted version of the output signal is available at the collector of Q3 (biased through R2, a 1.6K ohm resistor), but this particular chip doesn’t use it.

If either A or B goes low, then Q1 gets turned on. Current flows through the base emitter junction and the base gets pulled to about 0.6V above ground. No current flows through the base of Q2 because the voltage on the collector of Q1 is just too low for any current to flow. Q3 therefore stays off, and the output Y goes high impedance. By the way, this is what open collector means–the collector of the output stage transistor is left “open” with no corresponding transistor above it to pull it high.

Diodes D1 and D2 are just for input protection.

There are a couple of unused components. There is a resistor right below R1, and another resistor below R2. There are two extra transistors with a shared collector to the left of Q3. A different top metal mask could connect these extra components into the circuit and change the function of the device.

Can you think of some other gates that could be built by changing the top metal mask? Remember that there is only one metal layer which limits where you can route the traces.

XL741 – Discrete Op-Amp

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We’ve done it again! My friends at Evil Mad Scientist Laboratories have a new kit for sale. Following on the success of our Three Fives discrete 555 timer kit, we’ve had a lot of requests for a discrete 741 op-amp.

The XL741 is based on the datasheet schematic of the original uA741 op-amp IC from 1968. You can wire it up in a classic op-amp circuit and probe nodes inside the IC so you can see how the chip works. Play with differential pairs, modify the compensation, and change bias currents to your heart’s content!

Flea Market Find–Dual Gun CRT

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At the electronics flea market on Saturday, I found a 3ABP7 dual gun CRT. This one was built by DuMont, most likely intended for the 3″ version of their 5″ Type 279 Dual Beam Oscilloscope.

So of course I had to fire it up. There are two sets of deflection coils, so I drove them with one deflection board and cross-wired the deflection coils to flip the image around on the second gun.

The guns themselves have a common cathode connection and separate grids, which forces me to drive them both in parallel since my deflection board video amplifier keeps the grid at a constant potential and drives the video onto the cathode.
Dual gun 3ABP2 CRT

Here’s a closer look at the guns. The filaments are connected in parallel so this tube uses twice the normal current.
Dual gun 3ABP2 CRT

CRT Board BOM Updates

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I have corrected some mistakes in the bills of materials for the CRT driver boards (the files in GitHub have already been updated):

  • ScopePower U2 part number from TS271CN (DIP package) to TS271IDT (SOIC).
  • ScopePower R3 part number from VR37000003305JR500 (33 meg) to VR37000001005JR500 (10 meg).

If you already ordered parts, I apologize. The TS271CN is a great little op amp and the DIP package version will work fine in a breadboard. That’s what I will do with the ones I accidentally ordered. The 33 meg resistor is useful as a bleeder resistor if you build the post deflection acceleration module (to be posted soon).

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