Philips Avalon CTS M2720A Service Manual page 27

ECG Frontend Hardware 2 Theory of Operation
A seven-pin 'D-type' socket carries all parameter related inputs and outputs. An external mode resistor,
connected to one of the pins, detects which ECG mode to set when an adapter cable is plugged in. ECG
adapter cables, used with patient modules, are also used for the fetal scalp or adult electrodes.
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