Radio Shack TRS-80 Model III

APPENDIX
Pin
Designations
and
Signal
Descriptions
The
mechanical
specification
of the
RS-232-C
requires a
25-pin connector
(called a
DB-25).
The
following
table
specifies the
pin
assignments
and
signal
descriptions
as
they
apply to the
Radio
Shack
RS-232-C
Interface.
Pin Number
Abbreviation
Description
1
PGND
Protective
Ground
2
TD
Transmit
Data
3
RD
Receive
Data
4
RTS
Request-to-Send
5
CTS
Clear-to-Send
6
DSR
Data Set
Ready
7
SGND
Signal
Ground
8
CD
Carrier
Detect
14
STD
Secondary
Transmit
Data
18
SUN
Secondary
Unassigned
19
SRTS
Secondary
Request-to-Send
20
DTR
Data
Terminal
Ready
22
RI
Ring
Indicator
Table 2.
Pin
Designations and
Signal
Description
Protective
Ground
: This
must be
bonded to
the
chassis or
equipment
frame
.
It
may
also
be
connected to
Signal
Ground.
Transmit Data:
Direction-to
data
communication
equipment.
Signals on
this
circuit are
generated by
the data
terminal
equipment
for
transmission
of data to
remote
equipment. This
signal
should be
held in the
marking
condition during
intervals
between
characters and
at all
times
when
no data
is being
transmitted.
Received Data:
Direction-from
data
communication
equipment.
Signals
on this
circuit are
received from
remote
equipment
which
transmits
data to
the
terminal.
This signal
should be
held in the
marking
condition
during
intervals
between
characters and at
all times
when
no data
is being
received.
Request-
to-send:
Direction-to
data
communication
equipment.
This
signal is
required by
the
terminal
equipment to
control the
direction of data
transmission by
the
data
communication
equipment.
On
one-way
or
duplex
channels, the
"on'
'
condition
maintains the
data
communication
equipment
in the
transmit mode.
The
"off
'
condition
maintains the
data
communication
equipment in the
non-transmit
mode.
On
a half
duplex
channel, the
"on"
condition
maintains
the
data
communication
equipment
in the
transmit mode
and
inhibits the
receive
mode.
The "off
'
condition
maintains the
data
communication
equipment
in the
receive mode.
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