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 2)  The RAM is checked by three monitors in the FCC. At power up, a write-then-read test is performed on each memory location. During flight, the RAM is in use, so two RAM locations, with complementary addresses, are checked with a write-then-read test. The in-flight test verifies the condition of the address and data lines. During normal operation, parity bits are added to words written into memory and checked when words are read from memory.
 (d) Interfacing Electronics 1) Internal I/O monitoring is performed primarily in software. If the I/O logic fails, the CAPS processor indicates a fault. The I/O processor address and data lines are checked by the CAPS processor with a write-then-read test. A failure is indicated if the CAPS processor does not receive the proper word. Wraparound monitoring is used to check, in part, the ARINC 429 I/O and analog I/O circuits. Test words are wrapped around to test the 429 UART and 429 multiplexer circuits. The MCP and MCDP outputs provide the low-speed 429 wraparounds and the FCC left cross channel output provides the high-speed 429 wraparound. The analog wraparound signals are generated and fed back into the FCC analog inputs. The analog signals wrapped around are servo position, force link position, servo drive, and commanded output. 2) The inertial reference system (IRS) input is brought in, through two separate paths. The IRS information is stored in separate memory locations, thus allowing data comparisons by the CAPS processor. If a fault exists in either path, the comparison will indicate an inequality.
 3)  The ARINC 429 outputs which are not wrapped around are checked by hardware monitors. The monitors detect failures of one side of each ARINC 429 driver output and send a fail signal to the software if a fault is detected.
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MAINTENANCE MANUAL

 4)  The A/D and D/A converter is used to check the reference voltage used in the conversion logic. The reference voltage is scaled and inputted. The digital result is compared in software, with the expected result, and a fault is indicated if the voltage is out of tolerance. The analog wraparounds cannot be used to find reference voltage faults because both the D/A and A/D converters use the reference voltage generated by the A/D converter. A separate reference is, therefore, required.
 (e)  Power Supplies
 1)  The power supply voltages are continuously monitored for overvoltage, undervoltage, and overcurrent conditions. The +5, +15, and -28 Vdc outputs are monitored by additional overvoltage monitors which latch the entire power supply in a shut-down state. The 115 Vac input power must be interrupted to reset these monitors. The voltage and frequency of the 26 Vac supply output are checked by software.
 (5)  Internal Fault Storage
 (a)  
The FCC provides permanent storage for up to five internal faults which result in an FCC invalid, and temporary storage for five words of fault data per flight leg.

 (b)  
Faults detected by FCC self-test are stored in memory for access by shop support equipment. A maximum of five faults can be stored at one time. This fault data can only be removed from memory in the maintenance shop.

 (c)  
Fault data in temporary storage is formatted for transmission over a ARINC 429 data bus to the MCDP. The FCC clears temporary data at the start of each flight leg. Data provided to the MCDP includes: 1) Fault identity and location 2) Source of fault detection 3) Fault effect on flight deck 4) Fault intermittent or constant 5) Fault occured in-flight or on-ground.

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757 FAULT ISOLATION/MAINT MANUAL
AUTOPILOT/FLIGHT DIRECTOR POWER

Autopilot/Flight Director Power - Component Index
Figure 101 (Sheet 1)

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