communications
The
OL
has two serial ports (called
SER1
and SER2) for connecting
it
to
equipment
RS
232
C
which uses serial communications obeying EIA standard RS-232-C or a compatible • •
standard.
The RS-232-C 'standard'
was
originally designed
to
enable computers
to
send and receive
data via telephone lines using a modem. However,
it
is
now frequently used
to
connect
computers directly with each other and
to
various items of peripheral equipment,
e.g.
printers, plotters,
etc.
As
the RS-232-C 'standard' manifests Itself
in
many different forms on different pieces
of equipment,
It
can be an extremely difficult
Job,
even for an expert,
to
connect together
for the
first
time
two
pieces of supposedly standard RS-232-C equipment.
ThiS
section
Will
attempt
to
cover most of the basic problems that you may encounter
The RS-232-C 'standard' refers
to
two types of equipment.
1 Data Terminal Equipment
(DTE)
2 Data Communication EqUipment (DCE)
The standard envisaged that the terminal (usually the
DTE)
and the modem (usually
the DCE) would both have the same type of connector.
•
•
2 _ TxD (output)
___
~
2
3
...
RxD (input)
•••••••
3
DTE
7
...
GND (ground)
._-_.~
7
DCE
•
The diagram above illustrates how the
DTE
transmits data on pin 2 whilst the DCE must
receive data on
its
pin 2 (which
is
still
called transmit datal). Likewise, the
DTE
receives
data on pin 3 whilst the DCE must transmit data on
its
pin 3 (which
is
still
called receive
datal). Although this
is
confusing
in
itself,
it
can lead
to
far greater problems when there
is
disagreement as
to
whether a certain device should be configured
as
DCE or
DTE.
Unfortunately, some people decide that their computers should be configured
as
DCE
devices
whilst others configure equivalent computers as
DTE
devices. This obviously
leads
to
difficulties
in
the configuration
of
the serial ports on each piece of equipment.
SER1
on the
OL
is
configured as DCE, while SER2
is
configured
as
DTE.
Therefore
it
should
be
possible
to
connect at least one of the serial ports
to
a given deVice simply
by using whichever port
is
wired the correct'
way.
The pin-out for the serial ports
is
given
below. A cable for connecting the OL
to
a standard 25-way '0' type connector
is
available
from Sinclair Research Limited.
SER1 SER2
pin name function
pin name function
1
GND
signal ground
1
GND
signal ground
2
TxD
input
2
TxD
output
3 RxD
output
3
RxD
input
4
DTR
ready input
4
DTR
ready output
5
CTS
ready output 5
CTS
ready input
6
+12V
6 +12V
TxD
Transmit Data
DTR
Data Terminal Ready
RxD Receive Data
CTS
Clear
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