MagTek TRIPLE TRACK Delta ASIC - Manual (Page 14)

Triple Track ASIC
10
SHIFT-OUT PROTOCOL– CARDS ALLOWING 13 OR MORE CONSECUTIVE ZERO-BITS IN
THE DATA FIELD
Most card reader applications need only be concerned with ISO format cards. The ZEROES FLAG
protocol is designed to facilitate dealing with these cards easily and with a quick re-arm to read. Some
non-ISO cards are not compatible with this protocol, and must be handled as outlined below.
The secondary indication for end-of-card is triggered by a card swipe timeout (1.35 s to 3.1 s from wake-
up), failure to meet an amplitude threshold for each of the active tracks, or a full data-buffer for all of the
active tracks. This secondary indication of end-of-card is the AUXILIARY FLAG, a high to low
transition on the DATA line usually occurring after the ZEROES FLAG. The first STROBE issued in
the typical “handshake” described earlier in the PROTOCOL actually clears the ZEROES FLAG,
allowing the DATA line to return high so that the AUXILIARY FLAG event may be seen. This flag is
useful for custom data formats that allow thirteen consecutive zeroes as a valid sequence in the actual data
section of the card or between multiple data sections on the same card. The second STROBE issued in
the typical “handshake” described in the PROTOCOL actually stops the data collection process if still in
progress. For non-ISO applications that use the AUXILIARY FLAG, this STOP-STROBE should not be
issued until after the AUXILIARY FLAG has occurred, or the user’s controller has some other indication
that the card swipe is complete, such as a mechanical switch or interrupter in the card swipe path. Using
this sequence or “handshake”, the ASIC and controller can collect all the data available from a card,
independent of how many consecutive zero-bits are used in the data format.
It is always necessary to issue two high-to-low STROBE transitions as the “handshake” before the 1
st
bit
of the “preamble” can be extracted. For example, in the case where a ZEROES FLAG is never issued, the
first and only high-to-low transition of DATA represents the AUXILIARY FLAG. The first STROBE in
the “handshake” then no longer serves to clear the ZEROES FLAG, but must be issued anyway. There
will be no acknowledgement of it on DATA. Likewise, the second STROBE in the “handshake” no
longer serves to stop data collection, but must be issued anyway. There will be no acknowledgement of it
on DATA.
If the AUXILIARY FLAG occurs prior to the falling edge of the first STROBE, regardless of whether
the ZEROES FLAG has occurred, then DATA will not respond to the first STROBE. DATA will simply
stay low in this case. In other words, if there is no response to an attempt to clear the ZEROES FLAG,
then it must be assumed that the AUXILIARY FLAG has occurred.
Why use the ZEROES FLAG at all? Why not wait for the AUXILIARY FLAG and thus accommodate
all manner of data formats? First, the ASIC’s data recovery algorithm is designed to recover very low
level signals resulting from poor media and/or poor encoding and/or poor playback conditions. With high
ambient noise and at least one track of 75 bpi data, it is possible that the ASIC will not find the end of the
card by examining signal level alone, as ambient noise can exceed the signal level at which the ASIC is
capable of decoding. The consequence is that some random bits may be generated at the tail end of the
buffer for 75 bpi tracks. These random bits are 100% separable from ISO data, but the firmware required
is more complex than needed for the case where the buffer is truncated after the ZEROES FLAG
indication. Note that to ensure a “clean” buffer, it is necessary to issue the STOP-STROBE before the
physical end of the card passes the magnetic head gap. The STOP-STROBE has no effect until the
ZEROES FLAG has been issued. Note also that cards with exclusively 210 bpi data density will always