WMO-800S Radio Modem Module
Page 8 June 2010
The operation of the WMO-800S radio modem is relatively simple. As data is received at the
serial port, the data is transmitted on the radio channel. Up to 530 bytes of data can be
transmitted in one transmission. The radio transmission commences when the first data byte
is received, and ends when there are no more data bytes in the input buffer, or when the
number of bytes transmitted equals the maximum message length (user configurable - default
530 bytes). If more than 530 bytes is input, the WMO-800S unit will transmit the first 530
bytes; then the next 530 bytes, and so on until all of the data has been transmitted.
Because the radio data rate could be less than the input serial data rate, an input memory
buffer of 2Kbytes is provided. The RS232 connection provides CTS control to prevent the
buffer overflowing. There are no data flow control signals for RS485.
A radio channel cannot provide as secure a data channel as a wired connection. The WMO-
800S uses a radio band with a low level of natural or industrial noise, however there is a
chance of interference from other users of the unlicensed radio channel. We recommend that
the flow of data over the radio channel is controlled by using error detection and
“handshaking” - that is, returning an acknowledgment transmission if a data packet is
received on the radio channel without error. This function can be performed by either the
host devices or the WMO-800S modules. The modules may be configured by the user to
operate in one of two modes. In transparent mode, it is assumed that the host devices control
the flow of data. In controlled mode, the WMO-800S units control the flow of data.
1.2 Transparent Mode
The default configuration of the WMO-
800S modem is transparent mode - the
modules are set in this mode at the factory.
In transparent mode, the WMO-800S
provides no control of the data
transmissions (no error correction). Input
data is simply transmitted by radio and
every other WMO-800S unit in that system
which receives the transmission will output
the data. This mode relies on the host
devices to perform the “handshaking”
function, and re-transmitting serial data if the data is corrupted (no “handshake”). It also
relies on the host devices to include any addressing necessary in the data. In this mode,
modules are not configured with a unit address. Data is “broadcast” - every other WMO-
800S in the system will receive the data and output the data to their individual host devices.
The user may configure the WMO-800S modems to add error checking to each data packet
transmitted - if error checking is configured, data will not be output if it is received without a
correct error-check. This feature provides additional protection against corruption of the data
during the radio transmission. If error-checking is not configured, then the data received by
radio will be output without checking for errors.
HOST
800S
DATA
HOST
800S
DATA
HOST
800S
DATA
