Relay Control; Switching Circuits; Power-On Safeguard - Keithley 2000-Scan Instruction Manual

4-12 Service Information

Relay control

Card relays are controlled by serial data transmitted via the relay DATA line. A total of three
bytes are shifted in serial fashion into latches located in the card relay driver ICs. The serial data
is clocked in by the CLOCK line. As data overflows one register, it is fed out the Q'S line of the
register down the chain.
Once all three bytes have shifted into the card, the STROBE line is set high to latch the relay
information into the Q outputs of the relay drivers, and the appropriate relays are energized
(assuming the driver outputs are enabled, as discussed below). Note that a relay driver output
goes low to energize the corresponding relay.

Switching circuits

Relays K101 through K106 and K108 to K111 perform input switching, while K107
configures the card for 2-pole or 4-pole operation.

Power-on safeguard

A power-on safeguard circuit, made up of U104 and associated components, ensures that
relays do not change state on power-up and power-down. This circuit disables all relay actuation
during power-up and power-down periods by holding the OE (output enable lines) high during
these periods.