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	Datacommunication project.
				  By John "Boil" van Dijk.
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INTRODUCTION

Awhile ago I had some problems because I wanted to port over (convert to
another computer format) some programs and sources. I found a solution
using a modem program and cable. Based on the multi-link idea used in
some games. These techniques are also used with terminals and mainframes,
not to forget MIDI.
REMEMBER : MISTAKES CAN DAMAGE YOUR AMIGA, WE TAKE NO RESPONSIBILITY !


THE PROBLEM

To exchange information with other computers you could use more ways than one.
You could emulate the DOS of that other computer like DOS2DOS does but it is
very slow and you can come across a bigger problem : what about diskformats,
3½" and 5¼" ? and the final task is wether YOU can program it ? possibly not.

Another way is to use modems, a very nice thing but those incredible high
phone bills are terrible. The solution to all of this is a null-modem
cable over the RS232 port, ofcourse must the other computer also have such
a port, or have an interface that can emulate one.
If you have a modem program then the whole thing becomes very easy.
By the way, the amiga has such a port but calls it serial port (on the back
of your amiga)


HOW TO MAKE SUCH A CABLE ?

Making such a cable is very simple and cheap. You need 2 female 25 pins
connectors and a dual line cable with a coating that can reflect the
interference. Connect the wires as shown in the figure. (also on this disk)
The other connections make sure that the computer always sends or recieves
and to tell to the computer that the connection exists.
REMEMBER : the Amiga 1000 has different pin-configuration .....
The length of the cable could be over 10 metres but remember to use a 
good quality cable !

Many computershops sell these cables for reasonable prices, so if you
are not so technical or just plain lazy just buy one.
In the shop the cable will cost about $15 (depending on the length ),
not anything to go broke on.


HOW DOES IT WORK ?

The computers exchange datas over a serial RS232 port the same way a
modem sends its datas over the phone line.
Only now we don't use a phone line, just a few small wires.
The advantage is ofcourse that we don't have bad connections and can
use a high baud-rate (speed).


GETTING STARTED

So, let's try it out.
In the PREFERENCES is a separate menu called CHANGE SERIAL.
Here you can find all parameters used for serial data exchange.

BAUD-RATE is the speed of the information exchange, because the possibility
of a bad phone connection is excluded you could use 9600 Baud as standard.
Still this speed is very slow so after you got some experience higher
Baud-rates can be reached, after all if you know that MIDI works with
31250 Baud you could go higher. 
READ-BITS represents the size of a character. To recieve textfiles with
ASCII you only have to use 7 bits but take notice that whenever you
use for example German or Danish characters that these are not ASCII so
8 bits is necessary, if you are not sure always use 8 bits.
Also use 8 bits if you send something else as text.
WRITE-BITS is exact the same story as the READ-BITS only this time for
the send files.
STOP-BITS are syncro-bits that make sure the data is read well. Normal
1 stop bit is used but you can encounter 1.5 or 2 stop bits, also ok
when the other computer can handle it.
HANDSHAKING is used to give starting signals, with this cable it is not
needed but when you use it, use it on both computers.
BUFFER-SIZE is simply the memory buffer used to store data concerning the
communication. You could say : the bigger the buffer, the faster 
the routines.
Last is PARITY, normally NONE.

With a modem program the whole thing is quite easy, if you follow the
golden rule, use the same parameters on both computers.
Normally the manual of the modem program would have a capital for these
kinds of data exchange

Just try to send some text files over.


USING A NULL-MODEM CABLE IN GAMES

Some games use a null-modem cable to make a two player option such as
Populous and Flight Simulator 2. It is fantastic, I tell you.
Please refer to the manual of the specific game for more information.
Normally the gamecode itself will handle all settings and errorhandling
provided that a second computer is connected.


MAKING THE AMIGA TO A MAINFRAME

The advanced users can use the cable in the CLI over the
SER: device. It works like an ordinairy device provided that you have
the PORT-HANDLER in the l-directory. Now you can do things like :
'type df2:test to ser:' or 'copy df2:test to ser:'.

Similar to the SER: device we got the AUX: device (on workbench 1.3)
which makes it possible to use the other computer as a terminal.
Once again provided that the AUX-HANDLER is present in the l-directory
you can 'mount aux:' and make a newcli active on the terminal with
'newcli aux:'. Not that you actually see a window but only the text
that should appear in such a window will be shown on the terminal.
Programs like DPAINT will startup on the mainframe because it doesn't
use any text for operation in the CLI-window.
When it works you must see a prompt with the CLI-number on the terminal.

Now it is possible to have two users at the same time as long as the
rules of multitasking are followed (the perfect solution when 
Commodore can't deliver enough Amigas !) 
Back to the terminal, 'copy * to con:x/y/width/height/name' will
open a window on the mainframe where you can type your messages and
quit with CTRL-C. The other way around with 'copy * to aux' from the
mainframe.


FINALLY

I found very nice to play with the AUX: device, really you should try it,
connecting an ATARI to the AMIGA and make the Atari to a slave.
The new DevPac works with the cross-assemble technique, that means that
you can program on one Amiga (or Atari) and have TOTAL control over a
second one. For example you can use ALL memory on the slave machine without
crashing the master machine and when the slave computer crashes you can
examine everything, every single register and the cause on your master
machine. 
Not really optimal ofcourse because it will encourage the developing of
software on both Amiga and Atari at the same time.
In the end the Amiga version always is worse than the Atari version 
because the Atari is faster (higher clockfrequency) but if the Blitter
and all special chips are used, the Atari version must bite the dust.

Have fun with the cable.				CU, John.