Application Information
What can you do with an M48A ?
The M48A is a 4800 baud radio modem, compatible with narrowband radios.
Just a faster i1200, or a simpler RPM1 ? ....... not quite.
What does an M48A do ?
In the simplest possible sense, the M48A is just a radio modem baseband device. It provides an
interface between a user's asynchronous (“RS232 type”) serial port and the audio channel of a low
power radio module.
The M48 will transparently handle a half duplex serial stream at 4800 baud, buffering framing and coding
the data into a baseband signal that a narrowband wireless module can handle. At the receive end it
decodes the audio signal back into serial data. The timing requirements of the radio are allowed for with
internal FIFO buffers, while the noisy imperfect radio baseband path is hidden from the user behind S/N
efficient bit coding and data packetisation, with frame synchronisation and a CRC error detection
protocol.
The interface
An M48 can be used with just two serial data lines (TXD, RXD). In many applications that is sufficient. If
data is being sent in both directions (half-duplex) however, it is necessary to allow for the turnaround
time (transmit to receive or vice versa) as the M48 cannot handle transmit data input while it is decoding
receive data. (Also, in advanced modes, such as when user interface data rate is higher than 4800 baud
(see later), then transmit data hand-shaking is needed)
In these cases it is useful to have more information as to the status and timings of the link. The M48
provides additional 'handshaking' lines:
RX flow. Goes 'high' when the device has valid data to output
(“DTR” out)
TX flow. High when the transmit buffer is ¾ full, low when empty (“CTS” out)
RX hold. When pulled low, prevents device from outputting data (“RTS” in)
Buffers
As alluded to earlier, data into and out of the M48 is buffered through a pair of small FIFOs. On the
transmit path a 128 byte buffer is used, while on receive a 64 byte buffer is implemented. These buffers
actually occupy the same memory space on-chip, so the part cannot receive and output serial data .
Transmit operations will always over-ride receive. A byte sent to the device will clear the receive buffer,
even if it contains valid data. (An algorithm which reads the 'RX flow' signal can prevent this occurring)
In simple, transparent mode these buffers are invisible to the user.
Speed
In normal usage the M48 uses a 19.6608MHz crystal for a basic 4800 baud transparent throughput.
However, elementary communication theory tells us that ultimate range and S/N performance
(sensitivity) relates to bandwidth, and hence data rate.
In applications where ultimate range matters more than a fast data path the M48 can be fitted with
slower clock crystals. In these cases the fundamental link data rate is reduced (and other timing
parameters scaled accordingly) but the link range will be increased.
Crystal frequency
19.6608MHz
9.8304MHz
4.9152MHz
2.4576MHz
Data rate 4800 baud
Data rate 2400 baud
Data rate 1200 baud
Data rate 600 baud
Sensitivity +0dB
Sensitivity +3dB
Sensitivity +5dB
Sensitivity +7dB
[ slower clock frequencies will also allow operation on lower power supply voltages than the basic +5v
normally specified, See Microchip PIC16F648A data sheet for more information ]
Radiometrix Ltd, M48A-434-NiM2 modem manual
page 4
RADIOMETRIX [ RADIOMETRIX LTD ]
