Elpro Technologies 905U-D User Manual download pdf (Page 53)

Chapter Six Troubleshooting

Page 53 © January 2011

&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 905U-D 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

57600 15 -90

115200 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.

6.2.2 Bit Error Rate Test (BER)

BER may be tested in one direction by typing AT&T0 (BER master) at the sending unit and

typing AT&T1 (BER slave) at the receiving unit. The sending unit will repeatedly send pseudo-

random frames, and the receiving unit will check these frames for errors. If a terminal is

connected to the sending unit, the pseudo-random data is displayed as it is transmitted. If a

terminal is connected to the receiving unit, then the test results are displayed.

An example of the receiving unit’s display is here.

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Elpro Technologies 905U-D User Manual Page 53