A single-chip peer-relationship optical transceiver

Abstract

communicate equally well with any other device. In addition to the user-data channels, the IC features a bidirectional channel for enabling remote loopback testing and a link-status indicator to show when two devices are properly connected by an optical fiber.A CMOS peer-relationship optical transceiver has been fabricated, permitting bi-directional digital data transmission with one optical fiber. The IC, when used with a single LED, provides two full-duplex user data channels at 19.2 kbit/s and 1200 bit/s via a time-division multiplexing of the optical signals. TRANSMIT PATHThe 19.2 kbit/s TX-Data lead is continuously sampled once every 2.5 1 / . I S. The Transmitter stores the successive samples in its 31-bit register. Since the shortest bit time of a 19.2 kbit/s signal is about 5 2 p , TX-Data is being oversampled. The oversampling is necessary to insure no more than ~L S of uncertainty in the location of any transition has been introduced by the link. Because the data is over sampled, the parallel output of the 31-bit register is continuously encoded into 9 bits. OVERVIEWThe 1C described in this paper provides two full-duplex data channels over a single optical fiber. The first channel, consisting of a transmit and receive pair, operates at any data rate up to 19.2 kbit/s over the TX-Data and RX-Data leads ( fig. 1). The other channel operates up to 1200 bit/s over the TX-Control and RX-Control leads. To accomplish the bi-directional transmission with a single fiber, the user data is continuously sampled and time-compressed into packets. The packets are then altemately transmitted to and received from the optical fiber. Since a single LED is used as both a light emitter and detector, the cost for a fullduplex optical lmk has been reduced over what has previously been possible.The IC may be broken into two sections, analog and digital. The analog section interfaces between the SV digital signals and the transmit and receive currents of the LED. The digital section handles the timing, fonnatting and multiplexing operations necessary for two devices to regularly exchange packets of data. The digital section consists of four major blocks: Protocol, Timer, Receiver, and Transmitter.