Chapter Six
Troubleshooting
Man_805U Rev 1.8
Page
49
&T2
Transmit & Receive BER Test. Used with a second module configured as a
transparent repeater.
&T3
RSSI Measurement. Monitors the received signal strength, and displays in dBm.
Most radio tests are carried out using the AT&T2 test as this is the easiest to accomplish.
AT&T3 - Received Signal Strength Display
This option provides for testing the radio path between two 805U units. Although a pair of
units may communicate successfully, radio communication may be affected by a range of
influences, including atmospheric conditions, changing landscape, degradation of antennas or
co-axial cable, low battery voltage etc. Fade margin is an indication of how far a radio path
can deteriorate before reliable communication becomes unreliable.
When using this feature, the current value of the received signal strength is displayed in dBm
(decibels referenced to 1 mW). This value is updated every half second. To check the radio
path between two units, force the remote unit to generate data and read the signal level from
the local terminal. The remote unit can be forced to transmit by selecting AT&T0 or by
selecting 0011 on the four DIL switches and resetting the power to the module.
Also measure the background noise by stopping transmission from the remote module and
reading the value from the terminal. The readout will jump around as the radio hops between
channels - determine the average value. For reliable operation (that is, a bit error rate of
more than 1 in 300) the transmitted signal should be better than the following margins:
Radio Baud Rate
Margin above Noise
Minimum Value (dBm)
19200
10
-95
38400
15
-90
76800
20
-85
For example, for transmissions at 19200 baud, the radio signal should be at least 10dB better
than the background noise level, or better than –95dBm if the noise level is less than –
105dBm. If the background noise is –100dBm, then you need –90dBm of signal for reliable
operation. If the background noise is –120dBm, then you need –95dBm of signal for reliable
operation.
When using directional antennas (YAGI antennas) this feature may be used to align the
antenna in the correct direction by selecting the peak signal when moving the antenna. Setup
the remote unit to transmit, and observe the signal indication while adjusting the orientation
of the antenna. A peak in signal level indicates optimum orientation of the antenna.
