Optoelectronic differential multiplexer logic based on phototransistors/LEDs and its use in optical systems

Differential and single-ended operation are compared for optoelectronic logic based on phototransistors and LEDs. This demonstrates the advantages of operating differentially to obtain tolerant elements which do not require tuning. Fan-out levels and interconnection strategies for this logic are discussed leading to the adoption of local interconnections with a fan-out of four to support regular 2D logic arrays of 4-to-1 multiplexers. It is shown that 4-to-1 multiplexers not only minimise the number of elements required to perform a function but also minimise the number of interstage connections and the number of stages required. The flexibility and power of the logic functionality and interconnection approach are demonstrated for both combinatorial and sequential logic where the use of shared outputs leads to significant logic compaction. This considerably increases the functionality and parallelism offered by a system