The August 1976 issue of Popular Electronics magazine had a breakthrough project on the cover. "The COSMAC ELF: A microcomputer trainer that's powerful, expandable, and costs as little as $80." (Click on the magazine cover to read the original article.) To put this in perspective, the Apple I was selling for $666.66 at the time; and the Apple II didn't even exist yet!

The ELF truly was a simple inexpensive computer. Thousands of people built it (including us)! In the process, we learned about computer hardware and software right from the ground up. This experience is largely lost today, when everyone simply buys everything already built and programmed for them.

The Plan

2016 was the 40th anniversary of the ELF. In honor of the occasion, we decided to recreate the original ELF at the Vintage Computer Festival Midwest in Chicago IL on September 10-11, 2016. To give people a "taste" of what it was like, visitors to our booth had all the parts and tools needed to build a real COSMAC ELF. This was a wire-wrap project -- no soldering at all! We showed folks how to wire-wrap, so by the end of the show, participants had almost finished a complete vintage computer from scratch.

This computer was then raffled off. Everyone who added at least one wire got to put his name on a raffle ticket. At the end of the show, we drew the winning ticket out of a hat. One lucky builder won a FREE genuine hand-crafted COSMAC ELF computer!

The photo is our 40th anniversary recreation of the original Popular Electronics Elf. Click here or on the photo for a movie of the VCF-ELF in operation.

Links

• Popular Electronics Aug 1976. The magazine article that introduced the COSMAC ELF.
• Entire Popular Electronics Aug 1976 issue. The article starts on page 33 (it's big; 8.8 megabytes).
• Josh Bensadon's Elf is a faithful build of the original Elf shown in the PC articles.
• Josh's Elf Schematic. A beautifully redrawn exact copy of the original.
• Lee' VCF-Elf schematic. Not as pretty, but it has all the fixes described below.
• Lee's VCF-Elf layout. to match the schematic above. This is the FRONT view of the board showing where the parts get placed.
• Josh's Wiring Templates provide the black label for the switches and lights, and identify the pin numbers for wire-wrapping the BACK side of the board. Template with TI TIL311 hex displays. Template with HP 5082-7340 hex displays. To build your own VCF-ELF, be sure to print these templates ACTUAL SIZE.
• Wire-Wrapping primer from Jonard Tools (formerly OK Tool).
• Wire-wrap list is a to-from checklist for all the wires, written by Lee, Norm, Josh, and Chuck.
• The 1802 Membership Card and Lee's other 1802 stuff.
• VCF Midwest Vintage Computer Festival Midwest official website.

Parts

By pooling our resources, we had enough parts to make four kits; so that was the "production run". It's ONE for the money (auctioned off at the show with the proceeds donated to VCFMW-11); TWO for the show (the one built at the show and raffled off); THREE to get ready (Chuck built one to test and demonstrate that it all works); and FOUR, I don't know (we gave it to Dave Ruske, who sold it on eBay to provide funds to support the www.cosmacelf.com website). :-)

Here's the VCF-ELF parts list, in case you want to build one for yourself. We used premium vintage parts wherever possible to keep the "look and feel" of the original; but there are cheaper alternatives (like ebay.com) if you're on a budget.

Changes from the original

The first illustration shows the parts placement on the front side of the board. The second is a paper template, to label the controls and provide pin numbers on the back to make wire-wrapping easier. The last picture is the finished prototype, all wire-wrapped and working. How's it look? :-)

It could have been built EXACTLY like the original. An exact copy would appeal to historians; but not to the average person today. Some parts are rare and expensive, and no clear expansion path was provided. Therefore, a few changes were made to be cheaper, easier to build, and more useful in the modern world. Also, because Lee has engineer disease he couldn't resist "improving" things. :-)

• Added individual data LEDs.
  • Located just above data switches.
  • More logical to use with individual input switches.
  • Avoids rare/expensive/high-power hex display.
    - The hex displays take 10x more power than everything else put together!
  • Implementation: Replace buffers IC4-IC5 with a single 74C374 octal latch IC4.
    - Latch needed anyway for individual LEDs.
    - High drive: Can drive individual LEDs, hex displays, or both.
    - Can also be used as a general-purpose 8-bit output port.

• Added bypass capacitors (an obvious oversight in the original article).
  • Replaced electrolytics C1-C2 with ceramic capacitors C1-C6.
  • Extended 14-pin IC sockets to 16 pins, and put bypass capacitors in the extra pins.

• Replaced diode-resistor OR gates with a 4071 quad 2-in OR gate IC5.
  • Two sections replaced D1-D4 and R8-R9.
  • Third section replaced IC5 pins 14-15 (buffer for RUN switch).
  • Fourth section replaced IC11 14-15 (buffer for Q LED).
  • IC count remains the same, and we've eliminated 6 discrete parts.
  
  
• Replaced individual pullup resistors with SIP network R1-R2.
  • Replaced R1-R10 with R1-R2.
  • Easy to mount in IC wire-wrap socket.
  • Included series resistors for individual data LEDs.

• Used 1.8 MHz ceramic resonator in place of crystal.
  • Has internal capacitors (eliminates old C3-C4).
  • Easy to mount (plugs into normal IC wirewrap socket).
  • Frequency is correct for 1861 video, and serial baud rates up to 9600.
  • Replaced R11 with 5 meg trimpot R3. It makes the clock frequency adjustable from 10KHz-2MHz.
    - Adjust it near 1.8MHz and it "locks onto" the resonator frequency.

• Wired unused section of flip-flop IC12 to latch address bit A15 and /A15.
  • This allows simpler memory expansion. A15=0 selects 0-32k, /A15=0 selects 32-64k.
    - Example 1: Two 2114s in place of two 5101s provides 512 bytes.
    - Example 2: Expansion board can have one RAM at 0-32k, one EPROM at 32-64k.
  • Added /DIS input, with pullup resistor. Expansion boards can pull /DIS low to disable on-board memory and replace it with external memory.

• Added serial I/O (using Q and EF3).
  • 6-pin header (P1 pins 2-4-6-8-10-12) mates with Sparkfun.com #DEV-9718 5v TTL FTDI USB-serial cable to provide both serial I/O and power.
  • Need to load "bit banger" serial loader program to make it work (~40 bytes).
  • Resistor R4 in series with Q output prevents damage if VCF-ELF is powered and USB adapter is not.
  • Diode D5 in series with EF3 input prevents damage if USB adapter is powered and VCF-ELF is not.
  • Jumper P1 pins 1-3 to enable LOAD switch to control DMAIN (saves a diode).
  • Jumper P1 pins 5-7 to enable IN switches to control EF4 (saves a diode ).

• Power Supply
  • Removed 5v regulator. Only the hex displays need it; everything else is CMOS and works on 3-6v. Provided two alternatives:
    a. External regulated 5v supply:
    - Connect USB 5v supply + to P1 pin 8, - to P1 pin 12.
    - A Sparkfun FTDI serial adapter has the right pinouts to provide both power and serial I/O.
    b. Battery power.
    - Connect 3-cell AA battery holder + to P1 pin 9, - to P1 pin 11.
    - Battery holder fits underneath switches.
  • Added ON/OFF/STANDBY Power switch.
    - With battery connected, STANDBY powers RAM to maintain its contents.
    - Diodes D1-D4 prevent back-feeding power between battery and external supply.
  
  
• Added Expansion connector P2.
  • 40-pin (2x20) header, with standard 0.025" square pins on 0.100" centers. Expensive, but:
    - Wire-wrap pins (no soldering, easy to change).
    - Mates with standard female IDC ribbon cable sockets.
    - Mounting screws securely hold it to the board.
    - Ejectors avoid pulling on cables to unplug.
  • Expansion board can lay horizontally, with edge supports just like the Elf board (so expansions extend the Elf rearward).
  • Has all 1802 pins for expansion, except:
    - VCC pin 16, replaced by /DIS input with pullup resistor.
    - CLOCK pin 1 replaced by /A15 output.
  • Pins are in the same physical locations as the 1802.
    - Pin numbers changed, because ICs and headers number their pins differently.
  • Minimal parts needed for expansion.
    - Memory expansion to 64K is only 3 ICs (one 74HC373 high address latch, one 32k RAM, one 32k EPROM).
    - Video expansion is only one IC (1861).

Credits:

The VCF Elf project would never have been possible without generous donations of time and materials by Lee Hart, Josh Bensadon, Ed Keefe, Dave Ruske, Walter Miraglia, Chuck Yakym, Bill Rowe, and Norm Nelson; and to our many advisors and volunteer builders. A hearty THANK YOU to everyone that helped to bring this delightful project to completion.

Lee Hart

Wire Rappers

(sung to the tune of the original "Ghostbusters" theme song)
(You know; it goes Boom, Boom, cha-ka-cha Boom... Boom...)

If your circuit's strange, and it don't work good.
Who ya gonna call? Wire Rappers!
If it's got to change, if you only could.
Who ya gonna call? Wire Rappers!

(I ain't 'fraid a no bugs)

Don't pack no heat, no boards fulla lead.
What 'cha gonna use? Wire Rappers!
No burns or fumes, gonna wrap instead.
What 'cha gonna use? Wire Rappers!

(I ain't 'fraid a no bugs)

Just place that wire, on the right pin.
What is it called? Wire Rappers!
Now wrap it on, with a clockwise spin.
That's what we call, Wire Rappers!

(I ain't 'fraid a no bugs)

Ain't no big sweat, if you get it wrong.
What ya gonna use? Wire Rappers!
'Cause it un-wraps too; move it where it belongs.
What ya gonna use? Wire Rappers!

(I ain't 'fraid a no bugs... bugs... b-b-b-b-bugs)
Wire Rappers!

(By Lee Hart. Cartoon used with permission of the artist, Piero Tonin)