The M-8880 is a complete DTMF Transmitter/Receiver that features adjustable guard time, automatic tone burst mode, call progress mode, and a fully compatible 6500/6800 microprocessor interface. The receiver portion is based on the industry standard M-8870 DTMF Receiver, while the transmitter uses a switched-capacito r digital-to-analo g converte r for low-distortion, highly accurate DTMF signaling. Tone bursts can be transmitted with precise timing by making use of the automatic tone burst mode. To analyze call progress tones, a call progress filter can be selected by an external microprocessor.