a E i - Commodore Is Awesome

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Thus
Fig 1
(except in the case of the Plus/4) the
user port coupled with the Kernel
becomes almost the equivalent of the
6551 UART (the 6551 is the device that
the software is supposed to emulate).
In the Vic the user port is controlled
by a Versatile Interface
Adaptor (VIA). In the other machines
a Complex Interface Adaptor (CIA) is
used (not a lot of difference as far as
this article is concerned). Now before
some of you run and hide at the
mention of V I As and ClAs, let me say
that (thankfully) we do not have to
program these devices directly, The
Kernel handles all of the RS232 associated
programming.
Figure 1 shows the RS232
associated connections to this port
looking from the rear of the machine.
Note that the user port uses a 0.156"
edge connector (available from
Maplin: order number BK 74R). The
Lop row of terminals are identified with
numbers while the bottom row
terminals are identified with letters.
This means that it is very easy to
connect the socket the wrong way up
which could have disastrous con-
sequences for your computer. The
correct way is with the letters to the
bottom as shown.
RS232 LINES FROM THE USER PORT
M T
-
3LAJ
LBJ Lai ILI Lfi ILLI
-
1
Received data
Request to send
1
Data terminal rdy.
Received line sig.
Clear to send
-
Data set ready
6
Transmitted data Scut
Received data S i n
-
A Protective ground GND
Signal ground G N D
1
M
r
-
9
-
1
2 + 5 V supply
Pin ID Description Abreviation
Sin
RTS •
OTR
DCD •
CTS •
DSR *
•These lines are not used in 3-line mode
Signal ground is the Common return.
Protective ground for screening.
Maximum of 100mA may be drawn from
the +5V supply
Connecting Up
RS232
The terminals labelled Sout (transmitted
data) and Sin (received data)
and GND (0v common) are the three
lines we need. Note however that there
are TWO terminals labelled Sin. The
reason for this lies with the way in
wrhich the RS232 on these machines
works. One of the Sin lines is for data,
the other is a fl ag or trigger input.
Because the Commodore RS232 relies
on Non-Maskable Interrupts (NMI) or
interrupt request (IRQ) in the case of
the Plus/4, there must be some form of
detection to let the system know when
data is being received.
When data is being received over
the Sin line, the voltage level on this
pin changes rapidly, causing interrupts
to occur. The system software collects
the data on the Sin line, placing it in an
area of memory called the Receive
Buffer. There is also a Transmit buffer
for outgoing data. Thus the reception
and transmission of data is basically
transparent to the user. However, this
method has its problems as we will see.
The practical upshot of all this is that
BOTH Sin terminals must be
connected to the Sin line. Since they
are next to one another, this is no great
problem.
YOUR COMMODORE january 1987
83
Also note the connection called
PROTECTIVE GROUND. This
terminal should be connected to the
shield of your cable if you suspect any
outside electrical interference of
causing data errors.
While on the subject of making
connections, you will have to use a
soldering iron. Now don't run and hide
(again). If you are not too sure about
handling one, get an electronics
hobbyist friend to do it for you.
It should go without saying that
YOU SHOULD NEVER MAKE
CONNECTIONS TO THE SOCKET
WHILE IT IS CONNECTED TO
THE COMPUTER. Always
disconnect the socket BEFORE making
or changing any connections and
NEVER connect the socket to a line
machine. ALWAYS switch off the
power BEFORE plugging or
unplugging!
OK. So you've connected your
socket up and plugged it in the correct
way... what now? If you only want to
communicate with another
Commodore (Vic, C64, Plus/4 or
Cl28) then there is no great problem.
The only thing to watch is that you
must connect the Soul of one machine
to the Sin of the other as in Figure 2 in
order for them to exchange data (a bit
obvious really!).
Commodore connections
If you don't intend connecting your
machine to anything other than
another Commodore, you can skip the
next bit all together and get on with the
programming. If you have a friend
with one of the machines mentioned,
you will be able to type messages to
one another handy if you live next
door and you have a long piece of 3core
cable — your own mini-network!
(Not recommended if you live across
the street or several houses away!)
How far you will be able to keep the
machines apart depends very much on
the operating conditions. Electrical
interference may cause errors and
there will come a point where the line is
just too long. It's best to experiment
with what you've got.
Non-Commodore Connections
This is where things get tricky. Connecting
to anything other than a
similarly equipped Commodore has its