a E i - Commodore Is Awesome

1
S ome (if not most) of you will have
heard by now of that favourite of all
computer industry subjects. the RS232
communications standard. People
have been known to go weak at the
knees and hide when it is mentioned.
Every day, it causes problems for
computer engineers connecting up
equipment such as printers and
modems. It has even given rise to a
lucrative business of building the socalled
"break-out boxes" for
monitoring and "fixing" RS232 lines.
If it is supposed to be an industry
standard, why does it cause so much
hassle? Every computer manufacturer
has different ideas on how to build a
computer; which devices to use, how
big the screen should be, etc. It would
seem that this train of thought also includes
the RS232 standard.
Now, before I go on. let me say that
the idea behind RS232 is great. A
standard interface for connecting
different types of hardware (including
computers) together. RS232 is really
useful in the field of telecommunications.
Modems are usually connected
to a computer via an RS232 link. You
can connect two computers together
and transfer programs between them
(a technique often called "porting").
Unfortunately, things are not this
simple. With different manufacturers
having different ideas on how to implement
the standard (Commodore is
no exception) a great deal of confusion
can arise when two pieces of hardware
are to be connected together. Most of
this confusion surrounds the way in
RS232
Interfacing with
the RS232
Many people switch off when they hear the mention of
the RS232 interface and related subjects. This article is
aimed at clearing up some con fusion and doubt.
By Steve Carrie
which the control lines of an RS232
should be used. Thankfully, this does
not concern us in this article since we
will be using only a basic RS232
interface.
Basic RS232
As you may know, RS232 uses the
serial method of data transmission.
Information is sent bit by bit along a
single wire to a receiving machine.
Since data may flow in both directions,
two wires plus a common return are
required to make a basic RS232
communication line. The lines are
usually connected to equipment by a
25-way D-type connector. Data
leaving a computer exits via pin two
and incoming data enters via pin three.
The Ov return is connected to pin
seven. This gives the "3-line" RS232
interface. There is no control over the
flow of the data in either direction
unless handled by the software (more
on this later), An RS232 interface
using more than these three lines is
known as an "X-line" interface.
It is best to use a three or four core
cable with a shield (we will see why in a
moment) rather than separate wires.
For the purposes of this article, this
is all we require. "Ah yes" you say,
3-LINE RS2 INTERFACE
pin
3
7
YOUR COMMODORE january 1987
82
"that's all very well but my
Commodore doesn't have a 25-way dtype
connector let alone any RS232
interface". True it doesn't have a 25way
connector but it does have an
RS232 interface. You mean you didn't
know'?
Commodore RS232
Since the time of the Vic 20, Commodore
gas "programmed" in a limited
form of RS232 port. I say programmed
because the hardware device normally
associated with RS232 communications.
the Universal Asynchronous
Receiver/Transmitter (UART), is not
present in the circuitry of the Vic 20,
C64. 64C, C128 and C 128D. The
omission of the CI 6 and Plus/4 is
intended. The CI6 cannot handle
RS232 comms (we shall see why in a
moment). The Plus/4 on the other
hand is omitted for a different
reason. This machine DOES have a
UART.
RS232 signals appear at the user
port. This is exactly the reason why the
C16 cannot handle RS232; it does not
have a user port (a strange omission by
Commodore. Anyone know why?). All
the conversion is handled by the
Operating System (Kernel). Thus
CONNECTIONS (25-way D-connector)
function
Transmitted data Sout
Received data Sin
Common (0v) Gnd