User's Manual

16 TransNET OEM Integration Guide 05-3946A01, Rev. C
Antenna Aiming
For optimal performance, directional antennas must be accurately aimed in
the direction of desired transmission. The easiest way to do this is to point the
antenna in the approximate direction, then use the Remote radio’s
RSSI
command (Received Signal Strength Indicator) to further refine the heading
for maximum received signal strength.
In an MAS system, RSSI readings are only meaningful when initiated from a
Remote station. This is because the Master station typically receives signals
from several Remote sites, and the RSSI would be continually changing as the
Master receives from each Remote in turn. Adjust the antenna for the highest
(most positive) value to ensure the greatest communication reliability.
Antenna SWR Check
It is necessary to briefly key the transmitter for this check by placing the radio
in the
SETUP mode (Page 52) and using the KEY command. (To unkey the
radio, enter
DKEY; to disable the SETUP mode and return the radio to normal
operation, enter
Q or QUIT.)
The SWR of the antenna system should be checked before the radio is put into
regular service. For accurate readings, a wattmeter suited for 1000 MHz is
required. One unit meeting this criteria is the Bird Model 43 directional watt-
meter with a 5J element installed.
The reflected power should be less than 10% of the forward power
(2:1 SWR). Higher readings usually indicate problems with the antenna,
feedline or coaxial connectors.
Data Buffer Setting—MODBUS™ Protocol
The default setting for the data buffer is
OFF. This allows the radio to operate
with the lowest possible latency and improves channel efficiency.
MODBUS™ protocol and its derivatives are the only protocols that should
require the buffer to be turned on. See “BUFF [ON, OFF]” on Page 42 for
details.
NOTE: The BUFF ON setting may introduce high latency. For time-critical MODBUS
TM
ap-
plications, buffering can also be achieved by setting the RXD delay value. This lowers
the latency, but may not be as robust as BUFF ON. The desired RXD value can be ap-
proximated by the following:
RXD value = (9600/BAUD value) * HOPTIME value * REPEAT value * SAF multi-
plier. (The SAF multiplier is 1 for SAF OFF and 2 for SAF ON.)
As an example, with 9600bps, HOPTIME 7, REPEAT 3, SAF ON, the RXD delay
should typically be set to 42. ([9600/9600] * 7 * 3 * 2 = 42)
Hoptime Setting
The default hop-time setting is
7 (7 ms). An alternate setting of 28 millisec-
onds may be used to increase throughput, but at the cost of increased latency.
More information on the
HOPTIME command can be found on Page 45.