RV-M7 Data Radio Modem Technical Manual
- General Information and Overview
- Specifications
- Electrical Inputs and Outputs
- Serial Port Commands
- Using the M7 Modem – Packet Mode
- Using the M7 Modem – Streaming Mode
- Installation
- FCC Licensing Information
- M7 Diagnostic Provisions
- Tuneup and Alignment
- Skyline Compatibility
- Antenna Information
- Appendix A. Serial Port Hardware
- Troubleshooting
- Mechanical Information
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RV-M7 Data Radio Modem Technical Manual
Using the M7 Modem – Streaming Mode
This section describes the operation of the Streaming Mode of operation. This mode is selected with the command ATMT 2.
In Streaming Mode, radio transmissions will begin whenever data enters the mode, and the transmission will continue as long as there is data to send. The transmitter will automatically key when data enters the modem, and there is no need to assert any control lines. It will automatically de-key when there is no more data to send.
Figure 3 illustrates the difference between the Packet Mode and the Streaming Mode of operation.
` Figure 4 (Streaming Mode of Operation)
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In Streaming Mode, the user data may still enter the modem at any baud rate, as set with the ATBD x command. The M7 modem will buffer the data and send it out over the air in the same order as it enters the modem. When the buffer is empty and there is no more data coming into the modem, it will automatically de-key the radio and go back into the receive mode. The M7 modem will send a hidden end-of-message signal to the receiving modem, thus avoiding any extra data bytes “dribble bytes” from coming out of the user serial port. When the modem is operating with very weak signals, the end-of-message signal may be obscured, and missed by the receiving M7. In this case, additional noise bytes may come out of the user serial port.
While receiving, the modem will also output the receive data out the serial port at the rate set by the ATBD command. If the serial port baud-rate is slower than the over-the-air rate, an internal buffer in the M7 modem will hold the data as it is sent out the serial port.
Unlike Packet Mode operation, there is no error-detection nor error-correction in the Streaming Mode, so user data may contain bit errors. The user’s application must be able to handle these errors or additional bytes of noise data.
If the application that is using the M7 cannot tolerate have erroneous data when the channel is noisy, the modem should be operated in the Packet Mode instead of Streaming mode ( In Packet Mode, data is always first checked for bit-errors, and never outputted if it detects any errors).
The end of a transmission is detected by the receiving modem by the presence of a special end-of-message signal. Transmitting M7s automatically put the end-of-message signal out after the user’s data has been transmitted. If the receiving modem does not receive this signal (due to noise or interference), the receiving modem may continue to output some more data, until it detects that the RF carrier is gone, or the received signal is actually noise. This may take a byte or two of time, and during this time period, the receiving modem may output random noise bytes.
To reduce, and virtually eliminate bit errors and additional noise bytes, the user may configure the M7 to require and RF carrier Detect before receiving any data. Because the RF carrier Detect Threshold is set above the noise-floor of the receiver, bit-errors will be rare if RF carrier is required to receive. By default, the M7 does not require RF carrier detect to receive. To enable it, use the ATRF 1 command. To disable the need for RF carrier detect, us the ATRF 0 command (Factory default).
When ATRF is 0, the modem will be more sensitive, and be able to receive weak signals, but there is more likely to be bit errors when the signals are weak.
If large amounts of data will be sent with the M7, and the serial port is operated at a faster data rate than the over-the-air rate, it may be possible to overflow the internal data buffer. To ensure the transmit buffer does not overflow, enable and use hardware flow control. Hardware flow control is enabled with the ATCH 1 command. Note that theM7 modem will always indicate the status of its internal buffer using the CTS signal on the DB-9 serial connector. When CTS is negated, the internal buffers are more than 80% full. When it is asserted and it is “Clear to Send”, the buffers are less than 80% full.
