INTRODUCTION
YTD423 is a high-performance communication LSI for the ISDN BRI user-network interface function (digital four-wire time-division full-duplex operation), supporting D-channel layer 1, layer 2 and HDLC controller for Bchannels, all in one 100-pin SQFP chip. YTD423 supports layer 1 (physical layer) control function conforming to ITU-T Recommendation I.430 and fully supports layer 2 (LAP-D protocol) function conforming to ITU-T Recommendations Q.920 and Q.921. ETSI (European Telecommunications Standards Institute) and several North American standard operating modes are also supported. In addition, YTD423 includes layer 3 processor interface function and 2-channel HDLC controller for B-channels, which operate in DMA transfer mode or I/O transfer mode. This gives a great advantage to mounting and functional designing of both active" (with CPU on board) terminal equipment and passive" (no CPU on board) PC cards. In order to support the U interface, YTD423 has a TTL interface (no built-in analog driver/receiver) suitable for connecting to an NT1 chip or a DSU module. S/T reference point can also be supported by connecting it to YTD421 (analog driver/receiver LSI).

APPLICATIONs
• Terminal Adapter (TA)
• Router
• ISDN PC Card
• PBX
• ISDN Telephone