TWO-WAY SERIAL COMMUNICATION OVER A SINGLE OPTICAL FIBERBy Guido Stolfi – Sao Paulo, Brazil
(Published in Electronics World Magazine - Circuit Ideas - August,2004)
is the best transmission medium when reliable communication has to be
performed in electrically noisy environments; especially when galvanic
isolation is required. However, a drawback of fiber optics is that usual
transmitters and receivers are unidirectional. Two-way communication
usually requires two fibers running in parallel, and also two pairs of
optical transmitters + receivers.
on the other hand, provides bidirectional, half-duplex optical
communication using a single fiber. It works due to a not widely known
characteristic of infrared LEDs, in that they can also work as
photodiodes, converting optical input power into electric current. Thus
the same device, optically coupled to a fiber, works as a transmitter
and a receiver.
drives infrared emitter D1 during transmission. When receiving (signal
TXD at high logic level), Q1 is cut off and D1 develops around 10-20 mV
peak over R2. U1 is a FET input op-amp, operating as a voltage
comparator (negative rail input range is required).
R4 and R5 set the slicing level. Diode D2 helps to isolate D1
from Q1’ collector capacitance.
has been tested with more than 5 meters of 1 mm plastic fiber, and was
found useful at more than 19.2 kbps. Transmit and receive levels are TTL
/ CMOS compatible.
can be obtained reducing R2; but received voltage will be lower,
requiring a high speed, lower input offset comparator to adequately
recover the data.