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F
FU
UN
NC
CT
TIIO
ON
NA
AL
L D
DE
ES
SC
CR
RIIP
PT
TIIO
ON
N
The HT640/S encoder begins a three-word transmission cycle
when the transmission enable line (TE) is pulled high. This cycle
will repeat itself as long as the TE line is held high. Once TE falls
low, the encoder output completes its final cycle and then stops as
shown in the Encoder/Decoder Timing diagram. When a transmis-
sion enable signal is applied, the encoder scans and transmits the
status of the 18 bits of address/data serially in the order A0 to A9,
D0 to D7.
The status of each address/data pin can be individually preset to
logic high, low, or floating. The floating state data input is interpret-
ed as logic low by the decoders since the decoder output only has
two states. The address pins are usually set to transmit particular
security codes by DIP switches or PCB wiring, while the data is
selected using push buttons or electronic switches. The floating
state allows the HT640/S to be used without pull-up or pull-down
resistors on the data and address input lines.
Power On
Standby Mode
Transmission
Enabled?
Yes
No
3 Data Words
Transmitted
Transmission
Still Enabled?
3 Data Words
Transmitted
Continuously
Holtek HT640/S Data Sheet
Page 4
B
BL
LO
OC
CK
K D
DIIA
AG
GR
RA
AM
M
+33 Divider
Data Select
& Buffer
+18 Counter
& 1 of 18
Decoder
18
Transmission
Gate Circuit
OSC2
OSC1
Oscillator
DOUT
Sync.
Circuit
Trinary
Detector
TE
A0
A9
D0
D7
VDD
VSS
Pilot Period
(6 bits)
1/6 bit
Sync. Period
Address Code Period
Data Code Period
D
DA
AT
TA
A S
ST
TR
RU
UC
CT
TU
UR
RE
E
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