Westermo RM-455U-D - Manual (Page 13)

455U-D Radio Modem User Manual
Page 14 © May 2010
Messages sent with encryption have an additional 16 bytes of data added to the start of the
radio message (the initialization vector).
1.5 Serial and Radio Data
The 455U-D module provides a full-duplex RS232 serial port and half-duplex RS485 serial
port. The radio communications is half-duplex. Many applications use full duplex RS232
communications but do not require full duplex - the protocol used operates at half-duplex and
will operate with the 455U-D without problems. If an application really requires full duplex
communications, then the 455U-D should not be used.
Data input at the serial port is placed into the input buffer. This buffer will store up to
8Kbytes of data, and CTS/RTS control can be configured on the RS232 port to prevent
overflow.
When the 455U-D 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. Alternatively a Transmitter Hold up time can
be configured (S-Register S27) to keep the transmitter keyed up between characters.
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 1020 bytes). If
there is still data in the input buffer, the 455U-D will start another radio transmission.
An error-check can be added to each radio message - this is a user-configurable selection. 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 1020 bytes). If
less data than the maximum size is input to the 455U-D, then the 455U-D will transmit the
actual data input. If more data is input than the maximum size, then the 455U-D 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.
1.5.1 PLC-Mode
The 455U-D will operate most efficiently when the serial data rate is higher or the same as the
radio data rate. If the serial data rate is less than the radio rate, there is a risk that the radio