Canon SPEEDLITE 540EZ Service Manual page 30

Part 2: Technical Information
1.8 Data communications
Data communications between the camera and flash unit are performed through
four contacts on the flash foot: (2) EFIDO, (3) CCC, (4) AVEF and (5) STSP. Of
these, (2), (4) and (5) are used for simultaneous 8-bit digital communications, and
(3) is used to communicate both analog and digital signals. Though the actual data
transferred between the camera and flash unit is not provided in this technical
information for secrecy reasons, one new communications function has been
added so that a camera equipped with 5-point AF can inform the 540EZ whether
or not AF auxiliary light operation corresponding to the five focusing points is
necessary. Through use of this newly added signal, the 540EZ knows to
automatically emit the AF auxiliary light when used with the EOS-1N, but prohibit
emission of the AF auxiliary light when used with the EOS 5.
Also, since the EOS-1N allows the user to select the aperture value input pitch
and provides a built-in flash exposure compensation function, the status of these
functions is communicated to the 540EZ. As a result, the 540EZ can
automatically correct the flash coupling distance range display according to
changes in the aperture input pitch and/or flash exposure compensation setting.
(2) EFIDO (1) X direct-coupled contact
(5) STSP
(6) GND (3) CCC (TV)
Figure 2-5 Flash Contacts
1.9 Power source system
1) Built-in power source
The standard built-in power source is four AA-size alkaline-manganese LR6
batteries. However, for the first time in a Canon flash unit, AA-size lithium FR6
batteries can also be used. Though the power must be turned off before replacing
batteries, the flash unit holds the contents of memory (function settings, etc.) for
90 seconds after the power is turned off. It is therefore recommended to exchange
the batteries within this 90 second time period. If more than 90 seconds elapse
after turning the power off, the flash returns to its initial default state (A-TTL,
flash compensation: 0, auto zoom: 35mm, manual output level: 1/1, multi-flash
output level: 1/128, number of firings: - - [no limit], firing frequency: 1 Hz, first-
curtain sync).
Also, even when using an external power source, the internal power source is
required for the flash control circuitry.