ZERO
FUEL FLOW
X-OVERCOUNTERAC THREEDET
PHASE
RPM
FUEL
START

TACHO PORT COUNTER
LEFT EICAS COMPUTER STOP PULSE
TACHOMETER TO RIGHT EICAS COMPUTER
 ú

 31-41-00
 ALL  ú ú 09 Page 36 ú Jan 28/02

EICAS Analog Inputs Schematic
Figure 9

A

757
MAINTENANCE MANUAL

 (a)  
Analog signals are received from engine sensors and airplane subsystems. These parameters are received as dc voltage and current inputs, ac voltage inputs, variable frequency and pulse sensing inputs, and temperature resistance bulb inputs.

 (b)  
All analog inputs are paralled to both computers except temperature resistance bulb inputs. The left system temperature parameters are received by the left computer only. The right temperatures are received by the right computer only. These parameters are then shared using the crosstalk bus.

 (c)  
In the case of engine oil temperature, there are two temperature sensing bulbs on each engine. One bulb is connected to the left computer, and the other is connected to the right computer. This enables engine oil temperature parameters to be displayed even in the event of an EICAS computer failure.

 (d)  
Analog inputs enter the computer and are all multiplexed down to a common A/D converter. Each input is read at a rate required by its particular characteristics.

 (e)  
Filtering and/or smoothing of the signals is done mostly under software control. Each type of signal is filtered differently. Considerations are made for iteration rate and frequency response requirements. They are also made for resolution requirements and noise characteristics of the input data.

 (9)  
Analog Discrete Inputs (Fig. 10 and 11)

 (a)  
Signals sent by airplane subsystems for the generation of EICAS messages are analog discrete inputs. These include all alert, status, and maintenance messages.

 (b)  
Thirty-two analog discretes are received as program pin inputs. These program pins define airplane and engine model configuration and also define all customer selected display options.

 (c)  
Airplane switch inputs are received from the air/ground and nose compression systems #1 and #2. These signals are used by the computers for display format and central warning interface inhibits. Other switch inputs are received from the Master Caution Light Reset and the IDG Temp Switch.

 (d)  
The EICAS display units send failure discretes for DU Beam Test Fail, DU anomaly, and DU Overheat to both computers (Fig. 12).

 (e)  
Each computer has spare discrete inputs that can be allocated for any category using only software modification. These spare inputs provide for growth in the system.
　
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