Hill-Rom Advanta P1600, Service Manual

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Theory of Operation
Chapter 3: Theory of Operation
Page 3 - 32 Advanta™ Bed Service Manual (man168ra)
The low-to-high transition of the inverted pulse at U4 pin 12 causes the start bit
to be clocked into the A/D as the first bit of the conversion instruction. The
remainder of the conversion instruction is sent by clocking in three more bits,
101, where 101 is the opcode for converting U4 CH 1 with respect to CH 0.
The next eight clocks at P131 pin 14 clock the eight bit A/D byte out of U4 pin
10 on the U4 pin 12 falling edge. The most significant bit (MSB) is clocked out
first, the least significant bit (LSB) last. If the voltage at CH 0 is greater than
Vcc/2, a second conversion is required. After clocking in the start bit as
described above, three more bits, 111, are clocked in, where 111 is the opcode
for converting U4 CH 0 with respect to CH 1. The next eight clocks at P131
pin 14 clock out the eight bit A/D byte as described above. C5 provides
decoupling.
Solenoid Drive
Connector P131, pins 7 - 12, routes the 24V, 50/60 Hz solenoid drive voltage
directly to the solenoid valves through P132 pins 13, 11, 8, 6, 4, and 2,
respectively. P132 pins 1, 3, 5, 7, 10, and 12 route the solenoid drive return to
P131 pins 21 - 24 through zero ohm resistor R99.
Grounds
Analog ground (Vss) and digital ground (GND) are tied together through zero
ohm resistor R29 to create a star ground configuration. This helps isolate noisy
digital return currents from the analog returns.
Capacitor C22 provides a path to chassis ground for chassis-referenced
transients on the circuit grounds.
Power Supply
Voltage regulator U8 maintains Vcc at 5 + 0.25 volts. C6 and C7 maintain
regulation stability. D2 provides a bleed off path for C6 during loss of power at
P131 pin 1 (VBB2SW). P131 pin 13 (CONN_CHK) returns Vcc to the control
processor board as an indication that the electrical connections between the
two circuits are made.
Resistors R14 and R16 voltage divides the unregulated supply VBB2SW by
0.263 to enable sampling by the A/D. Resistor R66 protects the A/D by current
limiting any transients on VBB2SW.