Raveon RV-M7, Technical Manual

Page: 58 / 67

RV-M7 Technical Manual
58
15. Appendix A. Serial Port Hardware
15.1 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 M7 modem. The M7
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.
The M7 modem supports to ability to invert the functionality of the RTS line. If ATCI (Com-Invert) is
set to 1, the RTS line will operate as above, with the logic levels reversed (e.g., send on .
CTS
This signal is an output from the M7 , and it indicates the M7’s ability to send more data. If the user
tries to send more data into the M7 than the M7 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 will negate the CTS signal once the
M7’s internal
data buffers become full.