Membership Card modifications

Ways to modify or upgrade the COSMAC 1802 Membership Card, see this Web page for details. Last updated Sept 15 2014. Edited by Herb Johnson, (c) Herb Johnson


Text on this page is by Lee Hart, from cosmacelf Yahoo discussions in 2012. As Lee Hart notes, the design of the Membership Card was discussed at length, in 2006 through 2009. See this Web page for chronological links to that discussion. Look into the mind of Lee Hart and others as they discussed a minimalist version of the original ELF 1802. The notes below, represent reactions to the actual Membership Card product in the first few revisions.

But by 2014, the Rev G revision supported additional RAM and ROM, which solved one set and problems. Also, serial to USB support was available with simple devices in the form of tiny microcontroller boards or cables with built-in microcontrollers. Look at the most current revision Web pages for the Membership Card, always linked from the "home page" for the Membership Card for recent information.

- Herb Johnson

Q and A's

Q: why run at 5 volts? Why not run at 3V, use less power, etc.?

A: The supply current is trivial even at 5v, so you're not really straining anything.

Basically, most chip manufacturers only test their parts at one supply voltage. On parts of the vintage used in the Membership Card, they tested them at 5v. They work at other voltages; they just weren't explicitly tested there.

The Membership Card works fine at lower voltages. You have to slow down the clock as the supply voltage goes down, but there's a trimpot to do that. Power consumption goes down roughly as the square of the voltage. If it draws 10ma at 5v, it draws 2.5ma (1/4th as much) at 2.5v. Since you're at 1/2 the voltage and 1/4 the current, total power consumption is 8 times less. :-)

The Membership Card still works at even 1.6v, however, the LEDs don't so you can't see anything happening. Two AA cells will work, but as they get old and their voltage drops to near 1.0v per cell, the LEDs get too dim to be of any use. Thus three AA cells is a more reasonable power source, as you can keep using them and still see the LEDs until they are almost totally dead.

[Also see the tech note on low power operation. - Herb]

See Rev C Membership card notes for power consumption notes.

Q: Can the MC be clocked higher for more speed?

Sure. I've run it as fast as 2 MHz. The limit is the speed of the clock oscillator U4 (4093). It would go even faster with a 74HC132 in place of the 4093; but the pinouts are a little bit different and would require some cuts and patches.

There's a trimpot to set the clock speed. It normally has a range of about 10 KHz to 500 KHz. It is set with a resistor and capacitor (R1 R2 C1), which can be changed for other speeds (up to 3 MHz at 5v). I picked low speeds because power is proportional to speed, and I was after low power. The various Elf computers typically had clock speeds in the 1-3 MHz range. 1.79 MHz is common because that's what the 1861 video chip needs.

Q: I want to put a plug on the side so I can plug additional cards together

The easiest way would be to replace the 30-pin header on the Membership Card with a right-angle header. You could then build the Front Panel board (or any other board) with a right-angle header as well, so the two could plug together side-by-side.

This would result in the component side of the Membership and Front Panel Cards facing in opposite directions. But if you're designing a new Front Panel Card (say, with a hex keypad and display), this could be fixed.

It would also be possible to make a board that goes *between* the two existing boards, to add some function (like an 1861 or equivalent video circuit). It would have the female 30-pin connector on the bottom, and header pins on the top.

Or, such a board could be connected by removing the 1802 from its socket, and plugging the new board in its place. This would provide access to *all* the 1802 pins. The 1802 would then be relocated to the new board.

Q: how about a card with 4 plugs?

This is an interesting concept. It would be possible to design a bus and a whole series of little "credit card" size boards that all plug together. In the olden days, this led to things like the 4.5"x7.5" RCA Microboards with their 44-pin edge connector.

Today, more powerful chips allow much smaller cards. For instance, a large memory card is reduced to a single chip. The smaller cards costs less, and can be easier to build. The Arduino boards sort of do this, but without a defined bus or board size.

Q: are there standard backplanes for ELF-type designs>

There were many "standard" 1802 buses. Each manufacturer invented his own, and pretty much ignored what the rest did. (The nice thing about standards is that there are so many to choose from.)

Personally, I think the idea of a many-pin parallel bus is a dead idea. The connectors are big, expensive, and create reliability problems. Today, I think a serial bus makes a lot more sense. Imagine a series of computer boards (CPU, memory, I/O) each with a 4-pin USB connector to connect them.

You need about 40+ pins for a "complete bus". That makes for a rather large and expensive connector. There are also problems in protecting it from static damage, or from intermittent connections.

Q: Cigar Box SteamPunk 1802...?

That's another idea I like! I have a bunch of ancient telephone lever switches and indicator lights that I was planning to use to make one. Your idea of a fancy cigar box was a great way to get the cabinet.

You can simply wire the switches and lights in place of the tiny ones on the existing Membership Card's front panel. For the lights, I was thinking of keeping the LEDs but put them in antique-looking pilot light holders.

Q: hex nixie tubes...?

You could use nixies, or neon bulbs with some extra driver circuitry. Nixies were generally limited to 10 segments (no hex version), but there were 7-segment neon or fluorescent displays that could be used. If you're after the steampunk look, 7 neon lamps arranged like a 7-segment display might be good. Or maybe a pair of old analog meters with scales marked "0...F" :-)

Q: a (wind-up?) music box...?

Heck, the Membership Card power consumption is so low that you *could* power it with a wind-up key! Borrow the mechanism from one of the wind-up radios.

Q: USB support...?

USB requires a lot of computer "horsepower". The only practical way to do it is to add a second modern micro that has built-in USB capabilities. These can be pretty small; after all, they put them in mice.

The 1802 would basically send/receive simple ASCII serial data (perhaps with Q and EF4) to the USB chip, which translates it into the complex high-speed protocol required for the USB bus. A series to USB chip only needs to be an 8-pin micro.

2014 update: discussion of serial-to-USB adapters for the Membership Card can be found on the linked Web page. Revision G of the M/S card includes a serial interface. Or, one can build an external serial interface to support "RS-232" voltage levels. See notes and links in the Revision G support Web page.

[As for USB speed:] With a 2 MHz clock, the 1802 bus is only capable of handling data at 250Kbytes/sec. Even USB 2.0 runs at 1.5Mbytes/sec, and USB 3.0 can hit 625Mbytes/sec. It is *plenty* fast enough to send the entire data, address, and all control line signals in real time.

Another approach is to leave out the 1802, and just emulate it with the micro that's also doing the USB. It becomes a "virtual" computer, whose "CPU" might be an 1802, or 6502, or Z80, or whatever.

This has the advantage that the hardware for your CPU, memory, and I/O boards might all be identical. You're just programming each micro to pretend it's a CPU, or memory, or I/O chip. These boards plug together via USB cables, which provides both power and data.

And there lies the rub. Who would care enough to do [such an emulation]? Is there any *point* to doing it? Most people would say "just forget the whole thing and buy Arduinos".

Q: Only my very old Linux PC still has a 25-pin DB25 printer connector to connect to the M/S Card front panel board.

Understood. But the parallel port was the easiest to interface to. Anything else would have added chips, and required a ROM and software.

I think a USB port would be the "next step up". But it would require adding a micro just to handle the USB protocol (as I sincerely doubt the 1802 could do it). With luck, the extra micro could be some cheap little 8-pin chip.

Again, given the cost and scarcity of the 1802, it might be better to use a micro with more pins, and simply program it to emulate the 1802 *and* provide the USB interface. But that starts us down a slippery slope -- who knows where it would end?

Note: Chuck Bigham completed a Rev B kit in Jan 2011 and connected his Membership Card to a Windows PC via a Pixaxe conroller. Also: by 2013, Bill Rowe built an Arduino interface to the Memberhip Card. Check the software Web page for some details and links. Or check the Yahoo cosmacelf discussion group for other "mods". - Herb

[General expansion of bus or I/O]

[That's] possible, and even straightforward. But it takes time, andcosts money. Who is "up" for such an effort?

And, I fear that once you have it, it doesn't really do anything you can't already do with existing Elfs. The journey may be fun, but it just takes you to a place you've already been before. [And] it's tough to find a good, available, affordable connector that fits and has enough pins. That's why I think your best bet is to use the 1802's 40-pin socket as your "expansion connector".

[However,] for example, I can see replacing the top Front Panel board with a new board. It would have male pins on the bottom to plug into the 1802 socket on the Membership Card. It has a female 40-pin socket on top that the 1802 plugs into. It also has whatever extra circuitry you want; more memory, or an 1861 video output, etc. You could also plug a hex keypad into the 8-bit input connector that's already on the Membership Card, and have what amounts to a VIP in an Altoids case. :-)

[Note: two CPU boards can be stacked through the CPU socket to double the I/O and to have both ROM and RAM. See the Rev C to Rev D changes support page for details. - Herb]

[But the Membership Card] was designed as a minimalist project. That usually means leaving out expansion in favor of simplicity and low cost. Back when I first conceived of the project, we kicked the idea around on this list. I had several variations: Some smaller, some larger, some with more features, etc. In the end, the one I built was the version that seemed to have the most interest. There were already several "more expanded" designs available, and anything smaller than the Membership Card becomes more of a toy than something one could actually use.

Q: [Comments favoring the small and simple design of the Membership card.]

Thanks for your kind words. Though, I have found it fun to build little projects with these credit-card-size computers. I wind up using it like the Parallax BASIC Stamps, for dumb little projects. I'm currently working on one for my battery operated lawn mower. It will be a battery state-of-charge gauge, and battery charger controller (as what came with it was barely usable junk).

- all responses by Lee Hart

see the COSMAC 1802 Membership Cardhome page for details. Contact information:

Herb Johnson
New Jersey, USA
To email @ me, see
see my home Web page.

This page and edited content is copyright Herb Johnson (c) 2014. Contact Herb at, an email address is available on that page..