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● APPENDIX
APPENDIX: COM CONNECTOR PROTOCOL
HARDWARE DESCRIPTION
This transceiver uses a full-duplex, asynchronous, serial
interface for communicating through the male 9-PIN
RS-232C COM connector. Bytes are constructed with 1
start bit, 8 data bits, and 1 stop bit (4800 bps can be
configured for 1 or 2 stop bits). No parity is used. The
pinout and the pin functions of the COM connector are
as shown below:
COM
Rear panel view
RXD: Transmit data is serial data transferred from the
transceiver to the computer.
TXD: Receive data is serial data transferred from the
computer to the transceiver.
GND: Signal ground pin
RTS: This signal is applied to the transceiver. It is used
to inhibit transmit data from the transceiver when the
computer is not ready to receive the data. Transmit
data is inhibited when the level is low.
CTS: This signal is applied from the transceiver. It is
used to inhibit transmit data from the computer when
the transceiver is not ready to receive the data.
Transmit data is stopped when the level is low.
CONTROL OPERATION
Most computers handle data in the form of “bits” and
“bytes”. A bit is the smallest piece of information that
the computer can handle. A byte is composed of eight
bits. This is the most convenient form for most
computer data. This data may be sent in the form of
either serial or parallel data strings. The parallel method
is faster but more complicated, while the serial method
is slower and requires less complicated equipment. The
serial form is, therefore, a less expensive alternative.
Serial data transmission uses time-division methods
over a single line. Using a single line also offers the
advantage of reducing the number of errors due to line
noise.
Only 3 lines are required theoretically for control of the
transceiver via the computer:
• Transmit data
• Receive data
• Ground
From a practical standpoint, it is also necessary to
incorporate some means of controlling when this data
transfer will occur. The computer and transceiver
cannot be allowed to send data at the same time! The
required control is achieved by using the RTS and CTS
lines.
For example, the transceiver is placed into the transmit
mode whenever the character string “TX;” is sent from
the computer. The character string “TX;” is called a
computer control command. It tells the transceiver what
to do. There are numerous commands available for
control of the transceiver. These commands may be
incorporated into a computer program written in any
high level language. Programming methods vary from
computer to computer; therefore, refer to the instruction
manuals provided with the terminal program and
computer.
12345
9876
1
2
3
4
5
6
7
8
9
—
Transmit data
Receive data
—
Signal ground
—
Receive enable
Transmit enable
—
—
Output
Input
—
—
Input
Output
—
COM
Pin No.
COM Pin Name
(Ref.: Computer)
Function
(Ref.: Transceiver)
I/O
NC
RXD
TXD
NC
GND
NC
RTS
CTS
NC