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Q36, Q37, Q38, and Q39 form an RS flip-flop. | Q36, Q37, Q38, and Q39 form an RS flip-flop. | ||
When input overload is detected, Q32 turns on, supplying current to the base of Q36. | When input overload is detected, Q32 turns on, supplying current to the base of Q36. | ||
The collector voltage of Q36 and Q37 goes to around 0 V. | The collector voltage of Q36 and Q37 goes to around 0 V. | ||
The pin 13 "MODE INPUT" pin is normally grounded (e.g., in the [[485]]. | The pin 13 "MODE INPUT" pin is normally grounded (e.g., in the [[485]]. | ||
In this state, Q38 and Q39 are cut off and their collectors are pulled up by R38. | In this state, Q38 and Q39 are cut off and their collectors are pulled up by R38. | ||
That causes a current to flow through R53, into the base of Q37, | That causes a current to flow through R53, into the base of Q37, | ||
which keeps the collectors of Q36 and Q37 close to ground potential. | which keeps the collectors of Q36 and Q37 close to ground potential. | ||
So, once an overload condition is detected, the flip flop stays in the | So, once an overload condition is detected, the flip flop stays in the | ||
left-side-active state even after the input is removed. | left-side-active state even after the input is removed. | ||
[[File:Tek m94 output logic.png|600px]] | [[File:Tek m94 output logic.png|600px]] | ||
When the excessive input signal has been removed and the operator wants to re-enable the 50 Ω input, | |||
pin 13 of the M94 is momentarily allowed to float high. | |||
In the 485, this is done by the operator pressing the impedance selector button on the front panel, S125B. | |||
Letting pin 13 float allows current to flow through R45, into the base of Q39. | |||
The resulting collector current in Q39 pulls the collector of Q38/Q39 low, | |||
which stops the current in R53, thus turning off Q37. | |||
Q36 is also off because the excessive input has been removed. | |||
So the collector of Q36/Q37 is pulled high by R36, which also turns on R38. | |||
This leaves the flip-flop in the stable right-side-active state. | |||
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