Canon A-1 Repair Manual page 48

is input into the CPU by way of the switch based on
the Gray Code and information switches.
1) Preselected value in either the shutter speed
priority (TV) or aperture priority (AV) AE mode.
2) Film Speed (SV)
3) Lens maximum aperture (AVO
+
AVC)
4) Self-timer setting
5) Information of whether the lens is stopped down
or set for manual or automatic control
The bus line system used for the A-l is based on the
time sharing system in which a great deal of infor-
mation is transmitted through a few lines and is
considered the method best suited for a small-sized
aparatus like a camera.
Taking an example of its functions, analog information
of the subject brightness metered by the A-l is
converted into B-bit (one word) digital information
and is expressed to the minimum unit of I/BBV.
Eight lines are necessary to get eight-bit signals
transmitted at the same time to the micro-computer
for computation. In other words, each bit of the
eight bits is ranked in the order of I/BBV, 1/4BV,
1 / 2BV, IBV, 2BV, 4BV, BBV and 16BV.
Taking the case of the signal "01001100" for example,
the resulting digital information of the subject bright-
ness becomes 6.25BV (1/4 + 2 + 4
=
6.25BV). This
information is input into the micro-computer to be
computed together with .other digital information like
the film speed, shutter speed or aperture and
maximum aperture of the lens used to compute the
necessary exposure information. To make it possible
to transmit the B-bit signals through a single line,
the signal of each bit must be sent with a time
interval and also must be received with the same time
interval to be able to identify the signal to be
received.
In the A-I, 240 micro-seconds is considered as one word
and divided into B time zones (bits) names TBO - TB7
(Time Bus 0-7).
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