083730300A DCN8101 Theory of Operation 163
6.4.5.6 EXTERNAL DIGITAL I/O
This External Digital I/O performs two functions: status outputs and control inputs.
STATUS OUTPUTS
Logic-Level voltages are output through an optically isolated 8-pin connector located on
the rear panel of the analyzer. These outputs convey good/bad and on/off information
about certain analyzer conditions. They can be used to interface with certain types of
programmable devices (See Section 2.3.1.3).
CONTROL INPUTS
By applying +5VDC power supplied from an external source such as a PLC or Data
logger (See Section 2.3.1.5), Zero and Span calibrations can be initiated by contact
closures on the rear panel.
POWER UP CIRCUIT
This circuit monitors the +5V power supply during start-up and sets the analog outputs,
external digital I/O ports, and I
2
C circuitry to specific values until the CPU boots and the
instrument software can establish control.
6.4.6 I
2
C DATA BUS
I
2
C is a two-wire, clocked, bi-directional, digital serial I/O bus that is used widely in
commercial and consumer electronic systems. A transceiver on the motherboard converts
data and control signals from the PC-104 bus to I
2
C. The data is then fed to the relay
board and valve driver board circuitry.
6.4.7 POWER SUPPLY/ CIRCUIT BREAKER
The analyzer operates on 100 VAC, 115 VAC or 230 VAC power at either 50Hz or
60Hz. Individual units are set up at the factory to accept any combination of these five
attributes. As illustrated in Figure 6-14, power enters the analyzer through a standard
IEC 320 power receptacle located on the rear panel of the instrument. From there it is
routed through the ON/OFF Switch located in the lower right corner of the Front Panel.
A 6.75 Amp circuit breaker is built into the ON/OFF Switch.
AC power is distributed directly to the sample gas pump. The bench and GFC Wheel
heaters as well as the GFC Wheel receive AC power via the relay board.
AC Line power is converted stepped down and converted to DC power by two DC power
supplies. One supplies +12 VDC, for valves and the IR source, while a second supply
provides +5 VDC and ±15 VDC for logic and analog circuitry. All DC voltages are
distributed via the relay board.
General Safety Hazard
Should the AC power circuit breaker trip, investigate and correct the
condition causing this situation before turning the analyzer back on.
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