Data Streams - Experimental Techniques - Lecture Slides, Slides of Electrical Engineering

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Data StreamsScope Use; IR remotesFlip-Flops, and other logic for lab

The H-ITT Transmitter Signal

•^ When you click the H-ITT transmitter button:– A–E: LED indicator comes on, and at same time, TWObursts of infrared light come out: LED stays on even aftertransmission stops, until button is released– * button: on

release^ of button, LED flashes and two infrared

bursts are sent– bursts last 53 milliseconds, are 9 ms apart, and have a bit-period of about 0.5 ms (about 2000 bits per second)• Let’s look at it on scope…

st^1 data packet

nd^2 data packet

Comparison of A & B first packetsDifferences are minor, showing up only near beginning & endWe will represent “high” states (light on) as 1’s, and lows (off) as 0’sNotice standard widths: choices are single- or double-width

(both for the zeros and the ones)

Decoding the A signal

Sequence starts out: 01101001001101001001001001…Notice the 01 delimiters: 01101001001101001001001001…This gives the signal its choppy appearance (never see 3 1’s or 0’s in a row)Actual data appears between delimiters: 1’s look fat, 0’s look skinny

Resulting bit-sequence for A signal (both packets) is:

button code

transmitter ID (normal and inverted)

end delimiter checksum

The Transmitter ID bytes

1 1^ 1 1

0 0^ 0 0 0

•^ Transmitter number is binary-coded in the usualsense: •^ Sum is:–^ 32768 + 16384 + 4096 + 2048 + 256 + 16 + 4 + 1 = 55573–^ this exactly the number pasted behind the battery•^ Second packet inverts all the bits to ensure data integrity

What’s with the Checksum? 1

button code

transmitter ID (normal first-packet version)

checksum

Break data into chunks of 8 bits (bytes) and add up:

Checksums provide a “sanity check” on the data integrity

Stereo Remote Control

•^ Similar to H-ITT transmitters in principle:– bursts of infrared light carrying digital information– punctuated by delimiters so no long sequences of 1’s or 0’s•^ Key differences:– signal initiated by a WAKE UP! constant-on burst– pattern that follows is repeated indefinitely until button isreleased•^

I can never get fewer than three packets…– packet is variable in length depending on buttondata packet

data packet^

data packet Docsity.com

Sample patterns for data packet

POWERVOL +VOL^ ^1234567

remote ID?

data

Magstripe Idea

•^ On magnetic stripe, N-S junctionseat their own magnetic flux lines,but N-N or S-S present externalflux lines of opposite direction–^ pattern of N-N and S-S creates +and

^ transitions– zero represented by long period– one represented by short period– zeros look fat; ones thin (signirrelevant)– two streams are produced fromthis:• a data stream• a clock– data valid when clock high

zeros ones mixed traditional

Magstripe Geometry

•^ There are up to three tracks of data– Tracks 1 and 3 typically higher-density (7-bit) alpha-numericdata– Track 2 typically lower-density (5-bit) numeric data– Track 2 used on almost every card; track 1 often, track 3seldom

track 3track 1

track 2

data direction

Mag-stripe Circuit Schematic

Unfamiliar bits

•^ You’ve seen many of the elements in the circuit bynow– AND, NOR, NOT logic•^ New pieces are down-counter, flip-flops (and latchversion thereof)•^ Also have a PIC microcontroller and an RS-232 levelconverter

74HC74: Dual D-type flop

74HC574 Octal D-flop

•^ Just 8 D-type flops with same clock, no set/reset•^ Often called a “latch” preserves data input at clock