Canon SPEEDLITE 540EZ Service Manual page 39
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Also See for SPEEDLITE 540EZ:
- Instruction book (37 pages) ,
- Manual (36 pages) ,
- Parts catalog (12 pages)
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Part 2: Technical Information
4.4 Determining Pre-flash and Aperture (A-TTL mode)
* In the ATTL mode, the {/stop computed based on the light reflected from the subject
by pre-flash (infra-red light) and the {/stop computed by the camera are compared to
determine the appropriate {/stop. There is no pre-flash during bounce flash.
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
When SW-1 on the camera is pressed, the TV signal is input to pin No.6 of IC7.
At the same time, the EFID signal indicating the camera mode is input to pin No.9
of IC7 from the camera.
* If the camera is in the ATTL mode, pre-flash described below is carried out. In
other modes, pre-flash is not carried out.
First of all, the trigger signal (High) for pre-flash is output from pin No.37 of IC7.
When this signal is input to the gate of SCR2, SCR2 turns ON, and C16 (for pre-
trigger) starts to discharge.
At this time, the voltage raised by T2 is applied to XE1, and pre-flash is carried out
by the load of C19 (for pre-flash).
The light reflected from the subject by pre-flash is input to PD2, and is relayed
from pin No.8 of IC6 to pin No.55 of IC7.
IC7 outputs distance information to the camera from pin No.7 (AVEF line) based on
this information. (At the camera, the aperture (AV) is determined based on this
information taking existing light, ISO, mode and other factors into consideration.)
The aperture (AV) determined at the camera is relayed to pin No.9 of IC7 by the
EFID line to determine the f/stop.
* Signals on the AVEF line are relayed to EOS series cameras as digital signals,
and to the T-90 as analog signals. Judgment of EOS and T-90 is carried out
according to the terminal voltage of EFID. If the voltage is 2.2V or more, the
camera is judged to be an EOS series camera.)
¢ Flow of Operation:
(1) > (2) > (3) > (4) > (5) > (6) > (7) 5 (8)
Figure 2-9 Circuit 4
2-17
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