500 Series plug-in interface: Difference between revisions

==Signal Inputs and Spoiler Switch Hole==

The input impedance of 500-series scopes presented to the plug-in at pins 1+3 is approximately kept constant by adding shunt capacitors at the input depending on the tube type used. Early, scopes like the [[536]] and the [[555]] use a [[12BY7]] differential amplifier in the input stage, and the plug-in is loaded with the grid input capacitance with 2x 27 Ω (555) or 47 Ω (536) in series. Later, medium speed scopes like the [[545B]] and the [[549]] use dual input followers with a [[6DJ8]] tubes and 2x 47 Ω in series at the grids, so to keep the plug-in loading approximately constant a 3.9 pF capacitor is placed across pins 1 and 3 in the scope.

High-speed scopes designed for 1A-series plug-ins and up to 50 MHz bandwidth use a even lower input capacitance dual follower implemented with [[12AT7|12AT7s]] and series grid resistors of 22 Ω. For compatibility with older plugins designed for a higher capacitive load a series network of 7.5 pF and 82 Ω is shunted accross pins 1 and 3 by means of a switch. This spoiler switch is activated by all slower letter plugins. High speed plugins like the [[1A1]] have a hole in the back plate to allow for the switch plunger to pass through and access the full system bandwidth of the scope. Scopes and adapters known to use this spoiler switch are the [[544]], [[546]], [[547]], [[556]], and the [[81A]].

==Internal Trigger signal==