155-0076-00: Difference between revisions
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So, in a 485, while the input impedance selector button is pressed, the | So, in a 485, while the input impedance selector button is pressed, the | ||
input is disconnected from the 50 Ω attenuator. | input is disconnected from the 50 Ω attenuator. | ||
There are two cases where the input relay should be de-energized, i.e., | |||
where the input should be connected to the 1 MΩ attenuator instead of the 50 Ω attenuator: | |||
# The operator has selected 50 Ω input impedance, but input overload has been detected. | |||
# The operator has selected 1 MΩ input impedance. | |||
The first case is described above. | |||
It corresponds to the flip-flop being in the stable left-side-active state. | |||
In the second case, where the operator has selected 1 MΩ input impedance, | |||
pin 13 is allowed to float high. | |||
This results in Q44 turning on and shunting current away from Q42, | |||
thus preventing the relay from being energized, leaving the input connected to | |||
the 1 MΩ attenuator. | |||
Note that if pin 13 is allowed to float high, | |||
the state of the flip-flop is of no consequence as far as the MODE RELAY output is concerned. | |||
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