How the Unit Works
© Diebold Election Systems, Inc. 2002 AccuVote-OS Hardware Guide
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The boards on each side of the reader scan thirty-four columns on each ballot side. All voting
marks are aligned with these columns. The column positions are indicated by diagnostic marks
at the top of the ballots and ballot card ID marks at the bottom of ballots. Ballots are drawn
through the ballot reader assembly by means of a rubber pinch-roller drive system.
The ballot reader is connected by a twenty-conductor ribbon cable to the CPU board’s P20.
Control, status, and data signals are sent along this cable. The ballot reader is powered by a
three wire cable from the power supply. This provides a 5 volt DC logic supply and a 14 volt DC
motor supply.
2.1.1.1.
Scanning the timing marks
The columns on each ballot card are spaced ¼” on-center and the two outside columns carry only
timing marks, while the thirty-two inside columns carry all potential voting positions.
The scanning of each timing mark and voting mark position is done using visible light emitting
diodes and silicon photodiodes in a special configuration which monitors the diffuse reflectance of
the ballot surface. The red-orange emitters are Aluminum Indium Gallium Phosphide (AlInGaP)
and have a peak wavelength of 621 nanometers.
The optical elements are placed behind a quartz rod lens that concentrates the illumination and
detection along the scan direction of the ballot. The quartz rod lens also serves as a dust shield
to minimize the collection of paper dust on the optical sensors.
2.1.1.2. White
levels
Each voting channel establishes a reflectance reference level during the first ¼” of ballot travel
under each side of the sensor. This calibration is accomplished by measuring the leading edge
reflectance of the ballot. The reference level is stored for each voting channel. As the ballot is
scanned, the reflectance is measured and compared to a threshold percentage of the reference
level. Areas of the ballot with reflectance below the threshold are reported as a logic high in the
data sequence.
2.1.1.3.
Powering the infrared emitters
The visible emitter portions of the reflective sensors are powered by a multiplexed constant
current source to provide a reliable and power efficient illumination source. The voting channel
emitters are energized one at a time only when a ballot is being scanned.
2.1.1.4.
Ballot reader to CPU transmission
The data derived from the scanning of the columns of voting marks are transmitted to the CPU
board in serial fashion:
•
six ID bits for the upper reader board
•
the left hand timing bit from the upper reader board
•
thirty-two voting bits (left to right) of the upper board
•
the right-hand timing bit from the upper reader board
•
six ID bits for the lower reader board
•
the left hand timing bit from the lower reader board
•
thirty-two voting bits (left to right) of the lower board
•
the right-hand timing bit from the lower reader board
In the data sequence given above, it is important to note that the two sides of the ballot are read
in opposite directions, in the normal visual sense of reading a ballot. Even though both reader
