Agilent Technologies 5517B Operation And Service Manual page 87

8-97. The dividers, consisting of A3U4B and A3U6, convert the 100 Hz oscillator signal into the desired periods. The
first stage of the divider is U6, a dual 4-bit counter, which divides the 100 Hz by 256. The last output of A3U6 at Pin
8 has a period of 2.56 seconds (nominal). The output of A3U4B at Pin 13 has a period of 5.12 seconds while the
output at Pin 9 provides a period of 25.6 seconds. These two outputs are ANDed by A3U2C to produce a signal called
DISABLE which is high 2.56 seconds out of every 25.6 seconds. DISABLE is used by the subtracting sample and
hold circuit and the power amplifier
8-98. State Machine (E)
8-99. The state machine, consisting of A3U3, U4A, and U8A, determines the four states of controller operation.
The warmup mode is divided into two states, preheat and heater qualified. The optical mode is divided into
locking and locked states (see
locked is described in the following paragraphs.
8-100. Upon power-up, the power-on reset circuit places the state machine in the preheat state and the power
amplifier turns the heater full on.
8-101. A3U8A samples the HTR OK signal on the falling edge of DISABLE to allow sufficient time for the heater
coil to cool to the rod temperature. See
temperature rises to the proper value, the HTR OK signal from A3U15 causes the output of A3U8A (HTRQ) to
go high. The state machine enters the heater qualified state and the rear panel READY (and A3DS1) LED
blinks approximately twice per second. The rate of blinking is determined by the output of A3U6B which is
Figure 8-18. State
Diagram
Figure 8-3,
Flow Diagram for State Machine). The sequence from preheat to
Figure 8-5,
Machine Timing
Warmup Mode Timing Diagram. When the heater