Siemens SINAUT ST7 - Page 84

Operating Modes and Character Recognition
© Siemens AG 2002 All Rights Reserved 6NH7811-0AA22
MD2 Modem (07/2002)
4-17
Test Operating Mode "Test f
H constant
"
Test operating mode "Test f
H constant
" can be used to perform level measurements
(see below Determination of the Receive Level). In this scenario the modem
continuously outputs the upper keying frequency of the selected baud rate to the
WAN port. The operating mode is signaled by the continuously lit "RTS" and "TxD"
LEDs.
The following illustration shows the setting on the 10-pin DIL switch of the MD2 for
the "Test f
H constant
" operating mode.
Fig. 4-12 DIL switch setting for the
"Test f
H constant
" operating mode
Determination of the Receive Level
The MD2 modem at the beginning of the dedicated line must be set to the test
operating mode "Test f
H constant
" (all other test operating modes are not suitable for a
correct level measurement). After this the level available at the other end of the
dedicated line can be checked there. If this dedicated line is a line with several
tapping points, then all of these tapping points must be included in the line (incl.
LTOPs) to achieve a correct measurement.
Note
For level measurements it is important to ensure that the line termination is set to
150 Ohm (for 9600 / 19200 bps) or 600 Ohm (for 1200 / 2400 bps) at a terminal
point or 6 kOhm at a tapping point. It is also important to carry out the
measurement without grounding since the dedicated line operates with a
symmetrical signal.
The MD2 modem can receive data up to a minimum level of –43 dB. What level is
actually available at the receiving end must be verified with an oscilloscope or level
meter. If this is not available, the level still may be determined with adequate
precision using a normal voltage measuring device. For this purpose the following
table summarizes the level values and the assigned (approximate) voltage values
(U
rms
AC voltage).
Table 4-3 Comparison of level and voltage values
Level
Voltage
value (AC)
Level
Voltage
value (AC)
Level
Voltage
value (AC)
+10 dB 2,45 V -10 dB 245 mV -25 dB 43,5 mV
0 dB 775 mV -12 dB 195 mV -30 dB 24,5 mV
-3 dB 552 mV -15 dB 137 mV -35 dB 13,7 mV
-6 dB 389 mV -18 dB 97,8 mV -40 dB 7,8 mV
-9 dB 274 mV -20 dB 77,7 mV -43 dB 5,5 mV
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