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More Ports for Your Amiga

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The following is an excerpt from  an  article  scheduled  to  appear  in  the
February  1991  issue of Amazing Computing.  Please refer to that article for
more information about the I/O Board as well as ordering information. -JL

Happy Holidays!

12 December 1990

                         More Ports for Your Amiga

                          An I/O Expansion board

by Jeff Lavin
Copyright )1990 The Puzzle Factory

Many of you in the Amiga community have built Brad Fowles' excellent  "LUCAS"
accelerator board, which introduced the idea of "Public Domain Hardware".  In
this  article  I  will present another public domain hardware project for the
Amiga, which will enable you to add two parallel ports and two  serial  ports
to  your  Amiga  500,  1000 or 2000 for $70.  Furthermore, you can easily and
inexpensively upgrade to four parallel ports and/or four serial ports at  any

The hardware consists of a small printed circuit board with a  40  pin  cable
and  DIP jumper that plugs into the socket occupied by CIA B, and a small pcb
that contains the serial interface.  CIA B is physically moved onto  the  I/O
Expansion  board.

This hardware supports 15 standard baud rates, from 50 to 38,400  baud,  plus
MIDI  (31,250 baud).  It also supports full hardware handshaking.  Up to four
units may be open at one time, although the cpu may not be able  to  keep  up
with all four units running above 2400 baud.

Raison d'etre
As a hardware hacker of long standing, I have a  number  of  small  computers
with  all sorts of hardware attached to them, from extra ports to EPROM prog-
rammers, and longed to do the same with the Amiga.  Since the  Amiga  uses  a
pair  of  8520's (actually 6526's) for its I/O, I figured it would be a piece
of cake to add more 65/68XX family peripheral chips and be  up  and  running.
The  only problem was that, because there is no obvious chip select decoding,
I could never figure out how the 8520's were addressed.   One  day  a  friend
came  by,  and  we  were  able  to  figure  out  that  the  I/O   chips   are
"automatically" selected when certain addresses are generated by logic hidden
in the PALS.  Now that the final piece of the puzzle was in place,  I  wasted
no time and had a prototype in my Amiga in two weeks.

How it works
This hardware hack is possible because of two things  the  designers  of  the
Amiga did for us:

  1. The address space where the CIAs live is incompletely decoded.  This
     means the 16 CIA registers are echoed repeatedly over a large range.

  2. The locations where software is supposed to address the CIA registers
     is completely specified over a much smaller range.

These two facts make it possible for us to take the chip select from one CIA,
and divide it into four parts.  The addresses in the upper part are routed to
the CIA normally, and we "steal" the addresses  from  the  remainder  of  the
space for our own use.  Because the "hard" part, most of the address decoding
and  the  bus timing, has been done for us, we can get away with nothing more
complicated than an additional address decoder to split off our address space.

Unfortunately, this hack is not possible on the A3000  for  the  same  reason
that  it is possible on earlier Amigas.  The address decoding on the A3000 is
complete; there are no "extra" incompletely decoded addresses to "steal".

The VIA and ACIA registers are still echoed over a pretty wide address range.
We have specified where to address them for the same  reason  that  Commodore
has  specified addresses for the CIAs:  to ensure software compatibility.  We
would very much like to see enough people build these  boards  to  create  an
installed software base.  So programmers, please use these addresses when you
are  writing  all  those  neat  multi-line  BBS   programs   and   multi-user
applications, as well as process control programs, robotics demos, etc.

Of course, hardware is next to useless without software to  drive  it.   This
section describes the software available for the I/O Expansion Board.

The Serial Driver
Almost all programs written for the Amiga that use the serial port (with  the
notable  exception of some MIDI software) access it indirectly via a standard
software module called "serial.device".  As a result, most existing  software
will work fine with the I/O Board given a suitable driver, and we supply one,
named  "newser.device".   Simply copy it to your "DEVS:" directory and you're
in business.  Most programs will permit you to change the device  name  (from
serial.device  to newser.device) as well as the unit number (indicating which
port is to be used), or you may use the supplied IOpatch  utility,  described
below  (see  "Support Programs").  It is worth noting that, as with all other
programs relating to the I/O Board,  we  supply  complete  assembly  language
source code of the driver.  If you encounter a problem - and all else fails -
it's possible to fix it yourself.

The Parallel Driver
The four parallel ports on the I/O Expansion  Board  are  controlled  by  the
eightbit.device.   There are no known differences between this device and the
V1.3  parallel.device.   Applications  should  not  experience  any  problems
communicating  with  the  eightbit.device on the device level.  Full assembly
language source code of the driver is supplied with the I/O Expansion board.

DOS-Level Support
"DOS-level support" refers to the ability to get and send data via the serial
and parallel ports with standard AmigaDOS commands, such as TYPE or LIST,  or
with  any  program that does serial or parallel I/O via AmigaDOS, rather than
directly via the Exec-level "newser.device"  or  "eightbit.device".  Although
this  sort  of  capability  is not frequently used, it is useful from time to

In a perfect world,  DOS-level  support  would  mean  nothing  more  than  an
appropriate  MountList  entry, specifying a driver name of "newser.device" or
"eightbit.device", and some unit number of your choosing, corresponding to  a
DOS  name  such  as  "SER1"  or  "PAR2".  Unfortunately, Commodore supplied a
version of the Port-Handler and Aux-Handler with Workbench V1.3 that  doesn't
permit  this;  rather,  they're  hard-coded  to use either "serial.device" or
"parallel.device".  The printer.device suffers from a similar limitation.  At
the time of this writing, we don't have a solution, other than using IOpatch.
However, we expect to have replacement handlers ready by the  time  you  read
this.  In addition, the handlers in Workbench V2.0 have the capability to use
any device and unit, so this whole problem is non-existent if you have V2.0.

Support Programs
Several programs  are  available  for  use  with  the  I/O  Expansion  Board.

SERprefs  functions  much  the same as the serial section of Preferences, but
allows  you   to   set  and  save  parameters  for  all  four  units  of  the
newser.device.  These are saved in "S:Serial-Preferences".

Many programs allow you to specify the device name and unit number,  so  that
using  an alternate device driver is no problem.  For those applications that
insist on using a particular device, we  have  written  a  nice  little  hack
called  IOpatch.   This program SetFunction()s the exec OpenDevice call.  The
user puts this program in his  startup-sequence,  or  otherwise  invokes  it,
before  he runs his application program.  This patch will make a small window
appear, whenever OpenDevice() is called, with a choice of units; 0-4.  Unit 0
will select the internal serial or parallel port, and units 1-4  will  select
one  of  the  newser.device  or  eightbit.device units.  Please note that the
names of both drivers have been selected to be the same length as  the  names
of  the original devices.  This has been done to facilitate file-zapping as a
last resort.  Of course, software may be written  for  the  newser.device  or
eightbit.device initially.

A suite of simple test programs to check the I/O Expansion Board Hardware can
save you hours of hardware debugging time.  Chip selects, as well as read and
write signals, are generated for all chips.   One program  simulates  a  very
simple character-oriented terminal program for checking an ACIA.

A nice little program to drive a real-time clock-calendar is also  available.
The  clock  hardware,  based  on  the  OKI  MSM5832, is capable of generating
interrupts at 1024 hz, once per second, once per minute, or hourly.  Software
to take advantage of this feature is left as an  exercize  for  the  student.

I would like to thank Dan Babcock for the many hours he put  in  writing  and
debugging  the  serial  device  driver.   This  was  surely  one  of the most
difficult parts of this project.  Paul Coward, of DigiSoft, provided us  with
the  parallel device driver, no small achievement either.  Jim Cooper, of The
Software  Distillery,  made  many  helpful  suggestions  concerning  software
issues,  and  especially  DOS  compatibility,  and helped us get up to speed.
Bill Seymour provided invaluable help in layout  and  pre-production  of  the
PCBs,  and  also provided design help.  Finally, this task was made easier by
the help and encouragment of Doug Sears and Grace Lavin.

I think this is a pretty neat little hack.  I also feel  that  it  is  simple
enough  that  if  I  hadn't  come  up  with  it, someone else would have.  It
provides some much-needed additional I/O for the Amiga  1000  or  2000  at  a
rock-bottom  price.   If  enough  software becomes available to warrant it, I
will try to set up some sort of software clearing house  for  use  with  this
board.   Keep  an  eye  on  BIX  or  my  BBS,  The  Symposium,  for any news.
Meanwhile, I hope you enjoy using this board.  And don't let the  blue  smoke

If you need to get in touch with me, here are some possibilities:

    BIX                 jblavin
    USENET              [email protected]
    The Symposium       My own BBS, (503) 935-7883, 24hrs, F8N1

    The Puzzle Factory
    P.O. Box 986
    Veneta, OR 97487
    (503) 935-3709
The Puzzle Factory, Inc.  | Jeff Lavin -- [email protected]
Veneta, Oregon            |-------------------------------------
Voice : (503) 935-3709    | Remainder of signature line
Data  : (503) 935-7883    | under construction.