Z86129 Datasheet, PDF (32/50 Pages) Zilog, Inc. – NTSC LINE 21 DECIDER

Z86129/130/131
NTSC Line 21 Decoder
PRELIMINARY
INTERNAL REGISTERS (Continued)
XDS data and header information (including START,
CONTINUE, and END commands) are passed through the
filter for the XDS class and type specified in the XDS Filter
Register. All other Line 21 data is filtered out and will not
be output, or used to generate a data available flag (DAV)
in the Serial Status Register.
To properly read filtered XDS data from the
Z86129/130/131, the master device must first write the
XDS Filter Register (05h) with its desired XDS Class and
Type information. For example, in order to extract ONLY
the Line 21 Program Rating information, the master must
write the value 61h to the XDS Filter Register. The master
should then poll the state of the DAV bit in the SSR until
DAV = 1.
As soon as DAV=1, the master may initiate a 3-byte read
in the normal manner (XDS data bytes always arrive in
pairs, so it is safe to assume that RD2=1 when DAV=1 in
the SSB). A 3-byte read always yields two data bytes,
which in this case will be the first two bytes of the Current
Class, Program Rating Type XDS data stream
encountered on Line21 field 2. The master device must
then interpret those two bytes according to EIA-608
specifications for Current Class, Program Rating Type
data. Refer to EIA-608 for data formats.
The XDS filters on the Z86129/130/131 greatly reduce the
amount of field 2 data passed on to the master device for
further processing and interpretation. However, the master
device must still interpret the filtered data stream in
accordance with EIA-608. The filtered data stream from
the Z86129/130/131 will be in full compliance with EIA-
608. In other words, the filtered data stream will contain all
the XDS command and data packets, in standard EIA-608
format, but only for the selected XDS Class and Type(s).
Note: The Z86129/130 XDS filter for Program Rating
information behaves differently than all other Z86129/130
predefined XDS filters. This change has been made to
minimize the amount of data passed through the Program
Rating XDS filter, thereby minimizing the interpretation
and communications load on the master device. When the
XDS Filter Register is set to 61h (Class=01h (Current),
Type=05h (Program Rating) the only data from Line 21
field 2 that will pass through the filter is:
1. Program Rating Packet: [xxh,00h]. The Current Class
Program Rating data byte pair as defined in EIA-608.
The program’s rating is encoded per EIA-608 in the
byte xxh.
2. The END Packet [0Fh,CHKSUM]. A two-byte packet
that includes a CHKSUM computed per EIA-608. The
checksum calculation includes the START packet
[01h,05h] even though this value was not passed
through the filter.
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