RV-M7 Data Radio Modem Technical Manual
Appendix A. Serial Port Hardware
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 ATCHcommand. The default is OFF. To turn it on, issue an ATCH 1 command.
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.
CD
On this line the modem indicates to the DTE that it has received a carrier from a remote device. It will assert this signal any time there is a carrier detected. The modem may be configured to assert this when an RF carrier is detected (any on-channel RF, voice or data), assert it only when another RF modem signal is detected, or always assert it. The operation of this line is configured with the ATR1 command. The default is 1 (asserts when M7 data is detected on the radio receiver).
DTR
This line is an input to the M7. It is normally ignored because the default configuration of the M7 modem is to ignore this signal. If the DTR line is enabled for use, using the ATK5 1 command, the DTR input line can be used to put the M7 modem into its SLEEP mode. With DTR enabled, anytime the DTR line is a logic 0, the M7 will be operating normally. With DTR enabled, anytime the DTR line is a logic 1, the M7 will go into a SLEEP mode, drawing much less DC power.
Sometimes, a “Null Modem” cable may be required to connect the M7 modem to another device. The specific connections are very dependent upon the type of hardware and handshaking used, but the following sections should help in configuring a null-modem cable.
How to use the handshaking lines in a null modem configuration? The simplest way is to not use them at all. In that situation, only the data lines and signal ground are cross connected in the null modem communication cable. All other pins have no connection. An example of such a null modem cable without handshaking can be seen in the figure below.
Simple null modem without handshaking
(DB-9 Female shown. Same wiring for male-to-male cable)
There is a problem, if either of the two devices checks the DSR or CD inputs. These signals normally define the ability of the other side to communicate. As they are not connected, their signal level will never go high. This might cause a problem.
The same holds for the RTS/CTS handshaking sequence. If the software on both sides is well structured, the RTS output is set high and then a waiting cycle is started until a ready signal is received on the CTS line. This causes the software to hang because no physical connection is present to either CTS line to make this possible. The only type of communication which is allowed on such a null modem line is data-only traffic on the cross connected Rx/Tx lines.
In this null modem cable, seven wires are present. The cable is shown in the following figure.
Null modem with full handshaking
(DB-9 Female shown. Same wiring for male-to-male cable)
The null modem cable with full handshaking does not permit the older way of flow control to take place. The main incompatibility is the cross connection of the RTS andCTS pins. Originally, these pins are used for a question/answer type of flow control. When the full handshaking null modem cable is used, there is no request anymore. The lines are purely used for telling the other side if communication is possible.
The main advantage of this cable is, that there are two signaling lines in each direction. Both the RTS and DTR outputs can be used to send flow control information to the other device. This makes it possible to achieve high communication speeds with this type of null modem cable, provided that the system has been designed for it.
