User's Manual

Chapter Three Operation
Man_455U Rev 1.0 Page 19
When the 455U unit detects data in the input buffer, it initiates a radio message. The radio
message will end when the number of transmitted bytes reaches the maximum message
length (configurable by the user), or if the input buffer becomes empty.
If the configured serial data rate is the same or more than the radio data rate, then data is
transmitted as soon as it enters the input buffer - data “streams” from the input buffer to the
radio port. If the serial rate is less than the radio rate, then the transmission will be delayed
for a period to allow sufficient data to build up in the input buffer to avoid the radio emptying
the input buffer before a complete serial message has been input. This delay is configurable
separately for the RS232 and RS485 serial ports.
The radio transmission will stop when the input buffer is empty or when the radio has
transmitted the maximum number of bytes (user configurable - maximum 1024 bytes). If
there is still data in the input buffer, the 455U will start another radio transmission.
If error checking is configured, then a 16 bit CRC error-check is added to the end of the
transmitted data packet. The receiving module will receive the full data packet and check the
CRC before outputting the data.
The maximum size of the data packet is configurable by the user (maximum is 1024 bytes).
If less data than the maximum size is input to the 455U, then the 455U will transmit the
actual data input. If more data is input than the maximum size, then the 455U will transmit
multiple packets until all of the data is transmitted.
Because of radio start-up delays, the effective radio data rate will be lower than the
transmitted data rate. If you are sending large blocks of data, and the serial rate is equal or
more than the radio rate, we recommend that you use CTS/RTS flow control to prevent the
input buffer from overflowing.
3.2.1 Character Type
The 455U may be configured by the user to recognize the following types of characters - 7 or
8 data bits, even or odd or no parity, 1 or 2 stop bits.
Most applications will require the character type to be the same at each 455U modem in the
system. Nevertheless, the character type may be configured to be different at different 455U
modems. Data is transmitted by radio as an eight-bit byte without stop or start bits. If the
input data is 7 data bits, then the byte transmitted by radio comprises the 7 bits plus a zero bit.
Input characters with 8 bits are transmitted as just the 8 data bits, with no parity. Because the
data is transmitted without parity, the user may configure CRC error checking to be added to
each transmitted data packet. Data is output at the destination module based on the character
type configured at that module - that is, the start/stop bits and parity is added to the radio data.
Use the AT&A command to set the RS485 port Character type. AT&B to select the RS-232
port character type.
3.2.2 Serial Data Rate
The communications baud rates supported on both the RS232 serial port and the RS485 serial
port are 600, 1200, 2400, 4800, 9600, 14400, 19200, 28800, 31250, 38400, 57600, 76800,
93750, 115200 and 187500 baud - the user selects one of these rates during the configuration of
the modem. The RS232 and RS485 ports may be configured with separate data rates. ATC
command selects RS232 baud rate. ATR selects the RS485 baud rate.