155-0076-00
The Tektronix 155-0076-00 (M94) is a Tek-made input protection and probe logic integrated circuit in a 16-pin minipac package, designed by John Addis.
Probe Readout Ring Decoder
The M94 has a circuit for decoding the probe attenuation factor based on the probe readout pin. The probe readout pin contacts the readout ring on the probe input of the scope. The readout ring is pulled up to 5 volts by a 5 kΩ resistor outside of the M94. Inside the probe, there is a resistor between the readout pin and the ground of the BNC connector. This scheme allows the scope to distinguish 1x, 10x, and 100x probes, and adjust on-screen readout scale accordingly, so that the on-screen scale is referenced to the voltage at the probe tip. Letting the ring float represents a 1x probe. 11 kΩ to ground represents a 10x probe. 6.2 kΩ to ground represents a 100k probe.
When the ring is left floating, it is pulled up to 5 volts. In this case, the voltages are as shown in the schematic. Q62, Q56, and Q51 are on, and Q63, Q57, and Q52 are off. Current is allowed to flow out of the base of Q68, and consequently the darlington pair of Q69 and Q70 is turned on, sinking current from the X1 pin of the M94.
Used in
Datasheet
Links
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M94 as U80 in 485
