5-5

Decoder Operation

The Decoder requires a single clock with a frequency of 12

times the desired data rate applied at the DClock input. The

Manchester II coded data can be presented to the Decoder

in one of two ways. The BOI and BZI inputs will accept data

from a differential output comparator. The UDI input can only

accept noninverted Manchester II coded data (e.g. from

BOO of an Encoder through an inverter to UDI).

The Decoder is free running and continuously monitors its

data input lines for a valid sync character and two valid

Manchester data bits to start an output cycle. When a valid

sync is recognized (1), the type of sync is indicated by the

CDS output. If the sync character was a command, this out-

put will go high (2) and remain high for sixteen DSC periods

(3), otherwise it will remain low. The TD output will go high

and remain high (2) - (3) while the Decoder is transmitting

the decoded data through SDO.

The decoded data available at SDO is in a NRZ format. The

DSC is provided so that the decoded bits can be shifted into

an external register on every low-to-high transition of this

clock (2) - (3). Note that DECODER SHIFT CLOCK may

adjust its phase up until the time that TAKE DATA goes high.

After all sixteen decoded bits have been transmitted (3) the

data is checked for odd parity. A high on VW output (4) indi-

cates a successful reception of a word without any

Manchester or parity errors. At this time the Decoder is look-

ing for a new sync character to start another output

sequence. VALID WORD will go low approximately 20

DECODER SHIFT CLOCK periods after it goes high if not

reset low sooner by a valid sync and two valid Manchester

bits as shown (1).

At any time in the above sequence a high input on DR during

a low-to-high transition of DSC will abort transmission and ini-

tialize the Decoder to start looking for a new sync character.

2ND HALF

1ST HALF

0

1

2

3

4

5

6

7

8

16

17

18

19

SYNC

SYNC

15

15

14

13

12

11

10

14

13

12

11

10

21

0P

21

0P

15

14

13

12

4

3

2

1

0

VW

SDO

CDS

TD

BZI

BOI

DSC

TIMING

FROM PREVIOUS RECEPTION

UNDEFINED

2

1

3

4

HD-6408