(3) General rules for the use of the CFDS
 (a) comment messages When in the SYSTEM REPORT/TEST function, after each action on a line key or function key, the CFDIU displays the message WAIT FOR SYSTEM RESPONSE on the MCDU scratchpad line (last line), this informs the operator that his request has been taken into account. As a general rule, if the CFDIU has not detected the response from the system interrogated within 3 seconds, it replaces this message by the NO RESPONSE PRESS RETURN message. In both cases, the CFDIU adds these comments on the scratchpad line without deleting the page displayed on the MCDU.
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 (b)
 CFDIU time out In the SYSTEM REPORT/TEST function, the CFDIU time out is 15 minutes when there is no action from the operator on the MCDU.

 (c)
 RETURN rules The return in the menu hierarchy is performed, in all cases, using the 6L RETURN line key. Each sub-system connected to the CFDIU and to the MCDUs in Menu Mode manages the return in the hierarchy by means of the RETURN control. The CFDIU must also assign the 6L line key to the RETURN control each time it is necessary. The transition from the main menu of a sub-system to the last CFDIU menu is ensured by the CFDIU which displays its last menu if:

 -
the sub-system displays its main menu

 -
the 6L line key is pressed.
 When the CFDIU displays its menu, it sends a LOG OFF to the
 sub-system which returns to its Normal Mode.
 The transition is transparent for the operator.

R **ON A/C 001-049, 051-099, 101-105, 151-154, 201-207,
 F. CFDS Backup Modes
 (1) First back-up mode The CFDIU consists of two distinct channels: normal channel and back-up channel. In normal operation, the normal channel of the CFDIU ensures all maintenance functions and the back-up channel is not energized. In the event of a CFDIU normal channel fault:
 -
If this fault is serious (power supply board or micro): it is automatically and immediately switched to the CFDIU back-up channel. In addition, the back-up channel is power supplied on the ground only (ground/flight discrete). When the CFDS item is selected in the MCDU MENU, the CFDS BACKUP MODE page is directly displayed on the MCDU. In flight: No functions are possible (not supplied). On the ground: The only function possible is the SYSTEM REPORT/TEST function, for the aircraft main systems only. (Ref. Fig. 055)

 -
If the fault in the CFDIU normal channel is not serious (interface board), automatic switching is not performed, so as not to lose the other CFDS functions for the systems not affected by the fault. However, the CFDS adds a BACKUP MODE item in its menu, on the ground only, enabling manual switching to the back-up channel on the MCDU.

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 CFDIU - BACKUP MODE Through MCDU - On Ground
 Figure 055

R 1EFF : 001-049, 051-099, 101-105, 151-154, 1 31-32-00Page B19 1201-207, 1May 01/05 1 1 1CES 1 This switching gives the operator access to the systems affected by the fault in the normal channel. When the switching has been performed by the operator, the CFDIU is placed in the previous configuration (automatic switching).

 NOTE : Only the MCDU1 or 3 is connected to the CFDIU back-up channel
____
 and can alone be used in BACKUP MODE.

 After switching to the back-up channel (automatic or manual), return to the normal channel is not possible using the MCDU. The return can only be performed after a CFDIU power cutoff longer than 1 second. A CFDIU fault is indicated by a class 2 warning to the ECAM.
 (2) Second CFDS back-up mode: the test plugs In the event of total CFDIU failure (normal and back-up channels), it is still possible to read the maintenance information sent by certain systems by means of two test plugs located in the avionics compartment, on either side of rack 80VU.
 (a) Characteristics of the test plugs
 -Round plugs
 -Connected to a limited number of systems among the main aircraft systems (located essentially in racks 80VU and 90VU)
 -46 buses are connected (certain systems have two buses connected)
 -The side 1 and 3 systems are connected to plug 1 and side 2 systems are connected to plug 2, which ensures system segregation
 -Directly shunted on the system output buses, without passing through the CFDIU
 -28VDC power supply: Plug 1 on sub-busbar 103PP (C/B 9TW) and
 plug 2 on sub-busbar 204PP (C/B 10TW).
 (Ref. Fig. 056)

 (b) Operational use of the test plugs
 -On the ground only.
 -
The buses are read by means of an ARINC 429 bus reader.

 -
The only maintenance data that can be read on these buses are the data permanently transmitted by the systems, i.e. the data transmitted in Normal Mode, i.e. internal and external faults that occurred during the last flight.
　
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