User's Manual

CDH Communications
4
Operating States
Scanning
When the radio is neither transmitting nor receiving, it is continually scanning for
valid transmissions. From the Scanning state the radio can either transmit, if data is
presented on the serial port; or receive, if data is presented over the air. The radio will
return to scanning when data is no longer present either on the air or on the serial port, or
if a receive error is detected.
Transmitting
When data is presented to the serial port, the radio must first initialize the RF link;
it will then send the incoming data over the air. Initialization of the link takes
approximately 50 milliseconds (ms), this is the latency between the transmitter receiving
data through the serial port, and the receiver transmitting data out the serial port. During
this time, all receivers within range will sync up with the transmitter. While the RF
channel is being initialized, the radio can buffer up to 48 characters before it must stop
receiving characters. The radio will assert the CTS control line when approximately 32
characters have been received to notify the sending device to stop sending data. This
assertion point was chosen to accommodate PC’s with FIFO serial ports that can buffer
up to 16 characters and sends these characters even after CTS assertion.
After initialization, the radio will start to send any buffered data. Data is sent in
packets up to 32 bytes long. As soon as the radio starts sending a packet the CTS line is
de-asserted and more data can be sent to the radio. At this point as long as data is
continuously sent to the radio there should be very little if any assertion of CTS and data
can be continuously sent to the radio.
Data is sent in what could be considered raw mode. Meaning no CRC or check
sum is appended to the data packet. Also no attempt is made to verify whether the packet
was received (no acknowledgement). Checks that are made on a packet to verify it is a
good include: 1) a 16-bit network identifier that is used so that multiple networks or
radios can operate in the same vicinity, 2) a byte count of the packet length, 3) extensive
checking of bits to make sure they are valid.
Due to this raw mode it is suggested that the data sender use a check sum or CRC
and acknowledgement from the receiving radio to verify data transmission.
Receiving
When a scanning radio detects a valid transmission, the radio is locked in receive
mode until no more data is detected or a receive error occurs. This means that this radio
will not be able to transmit any incoming data until it returns to scanning mode. It is
important to note that all radios within range of a transmitter and with the same network
address will receive the transmission and send it out their serial ports. Any networking or
addressing will be handled outside of the radio, this is called a Broadcast, or Transparent
protocol.
After the radio is in receive mode it will receive a packet and transmit it out the
serial port. A packet will be rejected if: 1) the network identifier is incorrect, 2) the byte
count is incorrect or 3) there is a bit error in the data packet. A packet that is rejected is
discarded and is not sent to the serial port.
If there are gaps in receiving data the receiver automatically returns to scanning
mode. If the receiver has buffered data coming in the serial port this data will be sent
after the radio returns to scanning mode.