Company Confidential
45
Raveon Technologies Corp.
12. Appendix A. Serial Port Hardware
12.1. Serial Port Data and Handshaking Signals
In computer terminology, the RF modem is considered a “Data Communications
Equipment” device, or DCE. The user’s hardware that the modem is connected
to is considered “Data Terminal Equipment”, or DTE.
Following is a description of how data and control is communicated over the
various serial port signals between the modem (DCE) and another device (DTE)
that the modem’s I/O port is connected to.
TxD (INPUT)
This line is used to transmit data from the DTE to the DCE. It is maintained at a
logical 1 state when nothing is transmitted. If Hardware Flow Control is enabled
in the modem, the terminal will start to transmit data to the modem when a logical
1 is present on all of the following lines:
•
Clear To Send
(output from modem)
•
Request to Send
(output from the DTE)
RxD
(OUTPUT)
This circuit is used to receive data from the DCE to the DTE. The modem will
start to transmit data to the DTE when a logical 1 is present on all of the following
lines:
•
Data Terminal Ready
(reply that it is OK to send)
•
Data Set Ready
(asserted by the modem)
RTS
On this line, the DTE will send a signal when it wants to receive data from the
FireLine
modem. The
FireLine
modem will not output data unless this pin is low,
or “Flow Control” has been disabled in the modem set-up. When a DTE (such as
a PC or microcontroller) wants to stop the flow into it, it negates RTS. Negated
"Request To Send" (digital 1) means "request NOT to send to me" (stop
sending). When the DTE is ready for more bytes it asserts RTS (digital 0) and the
flow of bytes to it resumes. DCE equipment works the same way but sends the
stop signal out the CTS pin. Thus it's RTS/CTS flow control using 2 lines. The
user may turn this feature off or on, using the
ATCH
command. The default is
OFF. To turn it on, issue an
ATCH 1
command.
CTS
This signal is an output from the
FireLine
, and it indicates the
FireLine’s
ability to
send more data. If the user tries to send more data into the
FireLine
than the
FireLine
can transmit over the air, it will negate the signal on this pin. The CTS
may be asserted when the serial-port data rate is greater to or equal to the over-
the-air rate. For example, if the over-the-air data rate is set to 2400 baud, and
