HP 5501B, Operating And Service Manual, Page: 93 / 120

HP5501B
Service
8-33
Figure 8-19. State Machine Timing Diagram
8-99. State Machine
Ⓕ
8-100. The state machine, consisting of U3, U4B, 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 Figure 8-3, Flow Diagram for State Machine).
The sequence from preheat to locked is described in the following paragraphs.
8-101. 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.
8-102. U8A 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 Figure 8-5, Warmup Mode Timing Diagram. When
the heater temperature rises to the proper value, the HTR OK signal from U15 causes the output
of U8A, HTRQ, to go high. The state machine enters the heater qualified state and the READY
LED blinks approximately twice per second. The rate of blinking is determined by the output of
U6B which is gated through U2A and U19C to the READY LED. Blinking continues until the state
machine enters the locked state.
8-103. When HTRQ goes high; U4B changes states from reset to count. Unless reset by HTR OK
dropping low, U4B counts for 100 seconds and then clocks U3A. This sets OPTICAL high and
locks HTRQ on. The 100 second period is sufficient to stabilize the heater temperature.