The MX641 is a low-power, system-selectable Dual Subscriber Pulse Metering (SPM) Detector designed to indicate the presence, on a telephone line, of either 12kHz or 16kHz telephone call-charge frequencies. It is designed for PBX and PABX line-card and remote telephone installations. Under Processor control via a common serial interface, each channel of the MX641 will detect call-charge pulses from a telephone line and provide a digital output for recording, billing or security purposes. A common set of external components and a stable 3.579545MHz Xtal/clock input ensures that the MX641 adheres accurately to most national "Must and Must-Not" decode band-edges and threshold levels. For non-Processor systems a preset sensitivity/system input allows external channel level and system setting The digital output is pin-selectable to one of three modes: (1) Tone Follower mode: logic level for the period of a correct decode. (2) Packet mode: respond/de-respond after a cumulative period of tone or notone in a fixed (intrinsic hardwired period that is not user controlled) period. (3) High-impedance output: for device multiplexing. The MX641 requires a 5V supply and is available in the following packages: 24-pin SOIC (MX641DW) and 24-pin PDIP (MX641P)