Starting with version P5.5.0, external hardware reset, clock and door
open/close sensing has been returned to the the code design to provide
precise control of the chip functions.  REI have long contended that the
move to the 4-wire design was, while certainly satisfactory for most
users, not representative of the best configuration.  The bottom line is
that a 4-wire installation is easier (and by virtue of being fewer
connections takes less time) to install than a 10-wire, 7-wire, 6-wire or
even 5-wire chip.  The 4-wire implementation is simply popular because it
is the quickest implementation.  The fewer connections, the less risk of
damaging the machine.

  Over the past two years, a lot of hype and ad-copy has been spooled out
across the internet about how "advanced" 4-wire chips, using the
now-widespread PIC12C508, were so much better than the "old, obsolete"
18-pin PIC16C54 design.  The truth is, however, a bit different.  While it
is true that the internal oscillator capability of the 12C508 removed the
requirement for the external clock wire, the external clock itself is a
far more accurate signal.

  Even with the vernier-calibration technique (used in REI anti-piracy
chips but omitted from the AP code kit for legal reasons) the internal
clock was less accurate than the stable, crystal-controlled external one.
(This external clock is used on-board for a small microcontroller that
acts as a peripheral to the main CPU).  Thus, the external clock signal
wire is returned to the PIC chip setup. The 5-wire chip has this external
clock used, incidentally.

  The next issue is one of microcontroller intelligence.  A normal 4-wire
chip starts up and transmits it's data for as long as the power to the
unit is on.  While this has not been a problem to the machine, it is not
the preferred way of adding a circuit to an existing one.  A new circuit
should be as unobtrusive to the original as possible.  To make a
better-behaved chip, the hardware reset line has been enabled once again.
This way, when the machine is reset, the chip is also.  This allows a
controlled restart of the chip program at the same time the main system
restarts. To further improve the behavior of the chip, the door
open/closed signal has also been re-enabled.  Responding to the door is
crucial if the chip is to provide its data in a timely
fashion--particularly if the system is not reset or power-cycled.
 

  With these signals returned to the chip, a controlled "sleep" mode of
the chip is now possible.  Not that is wasn't possible before; it is just
that a 4/5-wire chip, once put to sleep, it is not a simple matter to
start it up again when needed.  Usually, it can only be awakened by either
power-cycling the machine or sampling the gate/data inputs for specific
information.  The problem with the latter is that the specific information
is unknown--save in the case of an anti-piracy chip, which does examine
its inputs for data signals.  A special type of 4-wire chip *could* be
made, but not until further research is done.  For the P5.5.0 series, the
chip simply "shuts down" after a timed interval.  At this point the chip
effectively no longer exists and nothing the main system does can
determine if one is there. The chip "wakes up" and starts transmitting
again on a power, reset or door cycle.

  And now the reasons for going to the extra signals is known:
The chip becomes a better device for its application.

    --The Old Crow, January, 1999