The gigabit interface converter (GBIC) is a compact transceiver for high-speed data communications. The receiver portion of an optical GBIC provides a loss-of-signal (LOS) indication. The MAX3264, MAX3265, MAX3268 and MAX3269 are limiting amplifiers with LOS that are well suited for use in GBIC modules. This design note describes a method that ensures reliable LOS operation under all operating conditions.
Figure 1 shows a host board and a GBIC module, with connections for TX_DATA, TX_FAULT, TX_DISABLE, RX_DATA, and RX_LOS. Figure 2 shows more detail regarding the receiver. If the optical signal is removed from the receiver, LOS is asserted high and the SERDES enters a loop-back mode. In some systems, the LOS indication is used to operate a squelch function, which prevents unwanted oscillations from occurring on the host board. To ensure error-free system operation, the LOS indication should be accurate under all operating conditions, including the unlikely situation where all GBIC pins are connected except VCC.
Figure 3 shows the circuit formed by a host board, and GBIC containing a MAX3265 limiting amplifier when VCC is not connected. Although VCC_RX is not connected to the host board, the gain amplifiers inside the IC turn on at approximately one diode voltage (0.8V). Current will flow from VCC_HOST, through the pull-up resistor, the ESD diode, and the amplifier, to ground. The LOS_RX output will be biased at approximately 1.2V, an undefined TTL state. Because the amplifiers are not operational, the output data contains errors, and LOS should be asserted high (>2.4V). In this example, the LOS output is not accurate.
Figure 4 shows a circuit that corrects this problem. The external NPN transistor inserted between the /LOS\ pin and the host pull-up provides a high-impedance RX_LOS output when VCC_RX is not connected.