The TAD Integrated Crosspoint Array-Application and Performance in Wescom's 501 EPABX

TAD is a mnemonic for threshold-activated diode or telephone access device. It was designed at Wescom for its new 501 telephone electronic private automatic branch exchange (EPABX). A single TAD is a composite p-n-p-n diode. 45 TAD's with associated exclusive path control have been integrated into a 5 × 9 monolithic crosspoint array and packaged in a 16-pin DIP. The 501 matrix is a 150 × 135 rectangular matrix comprising nine 7 × 11 in printed circuit boards with 50 DIP's per board. The significance of the TAD as a replacement for previously used crosspoint switches occurs due to its cost savings and size reduction. For a small EPABX, crosspoint minimization techniques using blocking and/or a multiple stage array are not needed, since a singlestage rectangular array is very practicable. Furthermore, the system architecture is simplified, which results in a further size and price savings for the basic system. The individual TAD is a two-port device which has common marking and transmission terminals. Insertion loss of a fired TAD is 0.1-0.2 dB when terminated in 600 Ω impedances. Voice frequency crosstalk isolation between adjacent paths exceeds 100 dB. Marking of the anode occurs by turning on a current source whose voltage changes from a quiescent 0 V to + 13 V dc while marking of the cathode occurs by activating a current sink whose terminating voltage linearly ramps from +15 V dc to +4 V dc. In a multiplestage application, definite available paths are selectable with a simple system control.