Xylem H-4400 - Manual (Page 4)

1-2 RS-232 Data Radio
H-4400
Master
Bubbler
Repeater
Hill
1.2 Using the H-4400 as a Repeater
The spread spectrum radios require “line of sight” between the transmitter and receiver. For
installations where this is not practical a H-4400 can be configured to operate as a repeater.
The radio in the H-4400 has an internal buffer on its RS-232 input. If the RS-232 transmit and
receive signals are connected together the radio buffers, then re-transmits (echos) the RS-232
string. Bursts of up to 64 characters can be buffered. The repeater system works as follows:
1. The master station is powered up when the data recorder asserts DTR.
2. The data recorder sends a short RS-232 message string.
3. The master radio transmits the message string.
4. The repeater receives the message string.
5. The message string is routed back to the transmitter of the repeater with the loopback
connection
6. The repeater radio re-transmits the message.
7. Both the master and slave stations receive the message.
This scheme works because data transfer is only unidirectional. The echo back to the master
station is ignored by the data recorder.
Note: Depending on conditions the radio signal from the master station may sometimes be
received directly by the remote slave station. In this case the slave will receive two copies
of the RS-232 message, one from the master, the other from the repeater. When using the
radio link for a point-of-sale display, this will not cause a problem. The display will simply
update twice with the same message. This simple radio link cannot be used with more
than one repeater, the RS-232 message would bounce endlessly back and forth between
additional repeaters.