The ACS401 is a complete optical-modem controller/ driver/receiver IC, supporting various user programmable, fullduplex, synchronous data rates to 128 kbps over a single fiber. Communicating modems automatically maintain synchronisation with each other such that the receive phase of one modem is lined up with the transmit phase of the other, compensating for the propagation delay presented by the link. Link lengths from zero to maximum distance, up to 95 km, are catered for automatically.