DIY ("Do-It-Yourself") 6502 computer
by Douglas Beattie Jr.
This is a hypertext version of my article which appeared in issue
#78 of TCJ (The Computer Journal)
-- you can order the hardcopy as a back issue from them.
There are many eight-bit processors, which have been around for a
long time. One of these is the 6502, which has been used in a
number of home computers. The 6502 is an ideal processor for a
prototype. It is also quite affordable.
The 6502 prototype
A prototype is a basic model from which to build and expand upon.
It allows us to evaluate and analyze a minimum system. With a
prototype, we can study the operation of components functioning
as a system.
This allows us to gain experience with the components, and better
understand how they work. It would be difficult to study a
component in detail without using it as part of a system.
As an example, perhaps there is a particular RS232 communications
chip you wish to learn more about. It is not practical to use the
chip all by itself. We connect it to other chips (components) and
utilize its ability to communicate. If it is a chip we haven't
used before, then we must learn to "speak its language," learn
how it operates. One practical way to do this is with other
computer chips, on a little circuit board, working together as a
system. This type of system is often referred to as a prototype.
The 6502 prototype board is surprisingly easy to build, and if
you build it on a bigger board than is needed, it can be expanded
and enhanced later.
After you understand the minimum system (your prototype) you can
add other things like a printer port, floppy drives, and even a
hard disk, CD-ROM, plotter, or whatever.
At first, we want a real feeling for the capabilities of the 6502
prototype board, so at least let's have a keyboard, and a video
screen to communicate with it. A computer terminal does both.
We can use the 6502 CPU to operate the communications chip. The
communications chip already knows how to communicate with the
RS232 modem port on our PC.
If you have a terminal, fine. Use that. If you have a TRS-80, you
could use that as well. I also remember somebody mentioning once
that a dumb terminal was a perfect application for a PC. All we
do is run QMODEM or TELIX or PROCOMM or whatever... and instead
of initializing a modem and dialing it, we fire up the little
computer board and compute on it.
This is the computer board; you should build it. It's just a
prototype, but the potential is here to expand and modify into
many useful things. It consists of the following circuit chips.
-- the heart or "engine" of the system.
2K or 8K EEPROM (jumper selectable)
-- a special type of Read-Only Memory, also known as "E-2-prom,"
this is like EPROM but no dangerous ultraviolet rays are required
to erase the chip. It is erased electronically.
32K Static RAM
-- usually called "S-ram," static RAM is simple to interface.
6522 Versatile Interface Adapter
-- combination of two parallel I/O ports (each with eight
synchronized lines, like for a printer device), and timers.
Timers are useful for one-shots or counting duration, or alarms
for overtime, etc.
something happens for a precise timed duration. It might be
a length of a tone, squirt of fuel or a douse of water, or a shot
how long something happens .. perhaps how long after dawn
your street light finally decides to go off. How long it takes
your little heater to heat the backyard pool up to comfy.
been in the freezer room for too long? an alarm sounds.
-- the Asynchronous Communications Interface Adapter. Don't let
the long technical name fool you. It's fairly simple. Basically,
it provides the interface to a modem or to another computer.
The ACIA connects your terminal to the computer board, so you
can interact with your computer board, update information, set
parameters of operation, duration of timing, error correction
Basically, that's it... CPU, RAM, ROM, VIA and ACIA
The 7404 (chip U2) is an inverter. It produces a high level from
a low input, and a low level from a high input.
The 74LS139 (chip U3) is similar to the popular 74LS138, but
instead of producing eight signals from three lines, it produces
four signals from two lines. The 74LS139 also includes two
identical decoders within one package.
The MAX233 (chip U7) is a single-chip solution for RS232 line
drivers/receivers, producing the proper voltage transitions
between 5-volt TTL level and 12-volt RS232 level. It is an ideal
chip for serial communication when only 2 input and 2 output
lines at most are needed. No external capacitors are required,
and it draws very little power.
Theory of operation
System ROM is jumper-selectable. The jumper option allows either
2K ROM or 8K ROM, addressing the 2K option at $F800, or the 8K
option at $E000. The schematic references an 8K (28-pin) socket,
but no distinction is made between the two -- either ROM will
work effectively if the jumper is set correctly.
Address decoding for upper memory is accomplished using 1/2 of a
74LS139 (chip U3A).
The 6522 VIA is decoded at location $C000..C00F. Chip selects
come from chip U3A and address line A12 (inverted).
The 6551 ACIA is decoded at location $D000..D003. Chip selects
come from chip U3A and address line A12 (true).
The CPU clock comes from a 1_MHz crystal oscillator. VIA and ACIA
clocks are driven from the buffered CPU clock PH2 (for greek
letter "phi", and 2). The ACIA also utilizes a separate
1.8432_MHz crystal oscillator for baud rate generation.
A full 32K of RAM is located at $0000..7FFF. Chip select for this
RAM is provided by CPU address line A15; when A15 is low, the 32K
RAM is selected. Read and write signals are provided by U3B,
another 1/2 of a 74LS139.
It seems that wire-wrap is the way to go. Now, if you have tried
to wire wrap before, and were not successful, this time you
1. because there is a wirewrap list.
2. I drew the schematic to represent the component layout.
You don't have to build it this way, but I tried to make it
easy for you. With a few sockets and a perfboard, you can build
There is a drawing of where the sockets should go.
- try to position the sockets on the perfboard according to the
There is a schematic which closely resembles the actual layout.
- follow the schematic as you build it; refer to it throughout.
There is a systematic wire-wrap list.
- not much more difficult than a dot-to-dot.
There is also a parts list.
- I picked all parts that are available from Jameco
Electronics. Jameco has been around for many years and has
Be sure to use Wrap ID tags. These are worth the extra cost. The
task of wire wrapping becomes a smoother process.
The toughest part of this whole project will probably be filling
out an order.
Filling in an order form, and punching in all those part numbers
is not exactly my idea of fun. Any spreadsheet program makes
light of the job. I used a really old version of Lotus 1-2-3 (the
one that runs on a PC-XT) -- it worked just fine. And evidently
Jameco didn't mind. Their service was fast.
Call Jameco for their latest catalog at (800) 831-4242, or e-mail
them at firstname.lastname@example.org, or request a catalog through their
computer bulletin board at (415) 637-9025.
((( see parts list )))
If you do decide to build this prototype, and then expect it to
do something, it will need two things:
An application -- what the computer will be applied to do.
Applications can be simple or complex, depending...
A program in ROM. That would be software, but we call it
firmware when it is programmed into an EPROM or EEPROM. Program
development usually requires a cross-assembler. You can also
"hand-assemble" without an assembler program.
I wrote a short test program to verify that the board is working.
BB0010A.AS transmits a sequence of "01234567" repeatedly to the
serial channel, while at the same time receiving keys through
interrupt. Whenever a key is received, it is also re-transmitted
to the serial port. While all this is happening, parallel port B
is turning LEDS on and off. This assumes that you have put eight
LEDS on the parallel port.
You might wire a 20-pin socket to port B of the VIA (as I have
done). By using a ribbon cable with IDC socket plugs on each
end, you can do all further experimenting on a breadboard. Use
port B, because it can sink more current (handle a bigger load).
The test program BB0010A.AS is only to prove that the board is
working. I might provide a follow-up article which will give
some more software examples.
((( see BB0010A.AS source code )))
If you already have a 6502 assembler, then cool. Otherwise, scour
the internet; I have found 6502 cross assemblers in any one of
six different locations. I also wrote one for IBM-compatibles.
This is MAS65.EXE and can be found at my web site, URL
If you don't have access to the internet, my advice is GET IT.
You have no excuse not to. It is affordable, easy to learn, and
readily available. It can be accessed through an x86 PC or a CP/M
machine or even a dumb terminal. You probably would use the x86
PC since more cross assemblers are available for the PC. I also
saw a CP/M cross-assembler for the 6502 somewhere.
You tell me! What would you use a small computer board for?
(Note one example above for the deep freezer time-out alarm.)
Feel free to send some suggestions via E-mail or Mail. The 6502
prototype is rather powerful; plenty can be done with it.
More To Come
In the future, we will look at ways to enhance the 6502 CPU,
expanding its ability. I will provide functionally equivalent
TTL and PLD examples, along with a diagram of the state machine.
David L. Wagner, Digital Logic, Harcourt Brace Jovanovich Inc.,
pp 265..275, 1988
Ronald A. Reis, Electronic Project Design and Fabrication,
Merrill Publishing Company, pp80..83, 1989
Rodney Zaks, 6502 Applications, Sybex Inc., 1979
Rodney Zaks, Programming the 6502, Sybex Inc., 1980
References and Data
Rockwell International, R650X and R651X Microprocessors (CPU),
Document No. 29000D39; Rev 8; June 1987
Rockwell International, R6522 Versatile Interface Adapter (VIA),
Document No. 29000D47; Rev 9; June 1987
Rockwell International, R6551 Asynchronous Communications
Interface Adapter (ACIA), Document No. 29651N90; Rev 4; June 1987
Rockwell International, R6500 Microcomputer System Programming
Manual, Document No. 29650N30A; Rev 3; April 1984
NEC Electronics Inc., Memory Products Data Book, Volume 2 of 2,
Document No. 60105-1-V2, 1993
Xicor Inc., X28C64 8K x 8 bit 5-Volt Byte-Alterable E2PROM,
document 3583-2.4; 7/24/95
National Semiconductor, LS/S/TTL Logic Databook, 1989
Maxim Integrated Products, +5V-Powered, Multi-Channel RS-232
Drivers/Receivers, document 19-4323; Rev 3; 5/94
(contact information was accurate in 1995,
but phone numbers and URLs may be outdated or gone.)
NEC Electronics Inc.
Maxim Integrated Products
About the Author
Douglas Beattie Jr. is an independant consultant for electronic
design and development. He left school early, as he was much too
busy studying the minutiae of state machines and combinatorial
logic to be sitting in a classroom, learning at a standardized rate.
He enjoys reading databooks and scrutinizing waveforms; designing
prototypes; preparing technical reports, and documenting his work
for future reference. His personal library consists of over 450 books
and technical manuals, with not one work of fiction.
Write to Doug at his office:
This address is current.
Douglas Beattie Jr.
5875 S. Adams Ave., Ste. 222
Ogden UT, 84405-6961