GARTHWILSON wrote:
The external diodes would have to turn on at a lower voltage than the internal ones in order to keep the internal ones from conducting. If the internal ones start turning on at around 0.15V, I don't know how to get anything lower than that externally.
Supposedly the Schottky arrays, e.g., the 74S1053, turn on at lower voltages than the internal ESD protection diodes. The 74S1053 and its cousins were intended to suppress ringing on longish bus lines, rather than deal with ESD. Nevertheless, the 74S1053 has a pretty high forward current rating per diode and turn-on of approximately 0.4 volts. I do have to question the notion that the internal ESD diodes in a chip turn on at a lower voltage, since there are hard limits to the Schottky technology.
It should be noted that some ringing on the buses isn't necessarily an evil condition. Unless ringing is sufficient to cause circuit malfunction there's no reason to use a diode array to terminate a bus, other than perhaps on an expansion bus into which cards would be plugged.