Build a "hot-swap" interface circuit

In applications that require wholeboard power cycling or actual board “hot-swapping,” maintaining a highimpedance interface at power-off as well as the power-up and power-down cycles becomes a critical design problem. Ideally, system designers can create a high-impedance, low-capacitance interface to minimize any possible disruptions of system data resulting from inserting or removing a board.

At the heart of the circuit shown, which meets all of these design criteria, is the Fairchild Switch FST6800. It provides 10 bits of high-speed CMOS TTL-compatible bus switching. The low on-resistance (4 Ω) of the switch allows inputs to be connected to outputs without adding propagation delay or generating additional ground-bounce noise.

The FST6800 precharges the B-Port to a selectable bias voltage (V_BIAS) to minimize live-insertion noise. The FST6800 is organized as a 10-bit flowthrough switch with an active low —OE control pin. The —OE control pin, when driven to the active low state, connects the A and B Ports together.

The bias voltage is set via a 3.6-V Zener diode stack and resistors. This circuit draws about 10 mA of current and has sufficient capacity to power several FST6800 V_BIAS networks. In cases where the backplane V_CC and board V_CC are separated and the FST6800 must be connected to the board or local V_CC supply, the diode stack on the V_BIAS pin will prevent sourcing the local V_CC supply to the backplane V_CC.

The —OE voltage is set by either a pull-up resistor or on-board control logic. A simple pass gate shifts control between the two selections. Pass-gate control can include provisions for board-maintenance functions, low V_CC, and board-level RESET.

When the —OE voltage is pulled up to a valid V_IH, the NC7SZ125 singlegate-buffer will drive an “OK to Remove” LED on the board.

In applications where the board V_CC and the backplane V_CC aren’t hardwired together, the FST6800 should be connected to the backplane V_CC. This ensures that the backplane V_CC and the board V_CC won’t be coupled together through the FST6800’s internal V_BIAS circuits.

See associated figure