Relay Control; Power-On Sequence; Figure 4-2 Id Data Timing; Figure - Keithley 7174A Instruction Manual

CARDSEL
CLRADDR
NEXTADDR
CLK
IDDATA
Note: ID data sequence occurs on power-up only.
CLRADDR pulse occurs only once.

Figure 4-2

ID data timing

4.3.3 Relay control

The relays are controlled by serial data transmitted via the
RELAY DATA line. A total of 16 bytes for each card are
shifted in serial fashion into latches located in the 16 relay
drivers, (U101 through U113). The serial data is fed in
through the DATA lines under control of the CLK signal. As
data overflows one register, it is fed out the Q's line of that
register to the next IC down the chain.
Once all 16 bytes have been shifted into each card in the
mainframe, 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). Logic convention is such
that the corresponding relay driver output must be low to
energize the associated relay, while the output is high when
the relay is de-energized. For example, if the Q1 output of
U113 is low, relay K197 will be energized.
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D7 D6 D5 D4 D3 D2

4.3.4 Power-on sequence

A power-on safeguard circuit, made up of U409 and associ-
ated components, ensures that relays do not randomly ener-
gize upon power-up. The two ANG gates, U409, make up an
R-S flip-flop. Initially, the Q output of the flip-flop (pin 3 of
U409) is set high upon power up. Since the OEN terminals
of the relay drivers (U101 through U113) are held high, their
outputs are disabled, and all relays remain de-energized
regardless of the relay data information preset at that time.
The first STROBE pulse that comes along (in order to load
relay data) clears the R-S flip-flop, setting the OEN lines of
the relay drivers low to enable their outputs. This action
allows the relays to be controlled by the transmitted relay
data information.
A hold-off period of approximately 470msec is included in
the safeguard circuit to guard against premature enabling of
the relays. The time constant of the hold-off period is deter-
mined by the relative values of R419 and C419.
Service Information
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D1 D0
4-3