Serial Port Data and Handshaking
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 tires 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 the user is send data into the modem at 9600 baud, the modem
