Trigger Source Switching Circuit

Trigger signal selection is accomplished by using the SOURCE switch (S440) to enable one of three triggering signal paths (internal, external, or line) to the Trigger Level Comparator circuit. With S440 set to INT, the inhibiting voltage is removed from R438, causing both U421B and diode CR440 to be biased on. The internal trigger signal is then passed from the emitter of U421B through diode CR440 to the Trigger Level Comparator and Auto Trigger circuits. The SOURCE switch prevents the line and external triggering signals from reaching the Trigger Level Comparator by reverse biasing diodes CR444 and CR448 and also by reverse biasing Q414 and CR418 through R417.

When S440 is set to LINE, U421B and CR418 are biased off through R438 and R417 respectively, while CR444 is enabled by removal of the inhibiting voltage from R446. Similarly, with S440 set to EXT, the external trigger signal is selected by biasing off CR444 and U421B through resistors R444 and R438 respectively and by enabling Q414 through the removal of the inhibiting voltage from R417.

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