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This Technical Note describes some nonintuitive ramifications of working with a
battery-powered computer.
[Oct 01 1992]
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It Feels Like I'm Floating
You may have problems connecting RS-232 from PowerBook to PowerBook or from
PowerBook to other battery-powered device if your connection is not properly
grounded. This will result in unacceptable noise in your signal or a complete
failure in the two devices to interpret the transmitted voltage levels
correctly. Serial communications using RS-232 voltage levels depend on
reference to ground. Because the PowerBooks are freestanding battery-powered
devices, they are properly grounded with respect to RS-232 only when the cable
converting their RS-422 to RS-232 also connects to a proper signal ground and
shield ground. (RS-422 uses balanced-pair signals and does not have this
problem.) This is a common cause of your program getting stuck in the vSyncWait
routine as the serial driver is waiting for character transmission to complete
while the SCC waits for an appropriate DTR or CTS that can't be seen due to a
lack of proper ground reference.
To remedy this problem, you will need to make sure the cable you are using
connects the signal and shield grounds to both devices and that at some
point these grounds are connected to an actual grounding source (grounding
strap, ground plug, or similar "metal pipe into the planet"). Plugging in the
wall power adapter will have no effect on this problem--it provides no
grounding to the PowerBook.
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AT or Not AT?
PowerBook modem has been designed with a minor deviation from the Hayes modem
command set standard. The Hayes command set deviation is documented on page 88
of the FaxModem user's guide. Many programs begin their modem initialization by
issuing a modem break command. For these programs, you will need to change the
"+++" string in your communication programs to "+++AT<Return>" (6
characters). Additional information about the entire supported AT command set
is now available in the PowerBook Modem Guide on the Developer CD.
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You Light Up My Life (The Charging Process)
The Power Adapter port on the PowerBook has been engineered with a mechanical
switch that detects the presence of the adapter. Software detects the insertion
of the plug into the PowerBook Power Adapter port, and, during the attempt to
charge the battery, the lightning bolt icon that appears in the Battery desk
accessory toggles on to indicate that the charging apparatus is attempting to
bulk charge the battery. In the status byte returned by BatteryStatus, bit 0
indicates the presence of the physical plug in the power connector. Bit 1
indicates if the charging circuit is set to bulk rate or trickle.
There is no way to determine if the attached charging apparatus is providing
current, as the circuit can read only the voltage level of the entire system.
Additionally, it might be possible to write a utility that, over time (seconds)
and while also watching the power state of the various subsystems, could
determine if the unit was gaining a charge. Although a difficult programming
problem, this might make a great third-party product.
There are two phases to recharging the battery. The first is bulk recharge and
the second is trickle charge. The bulk recharge will bring the battery to an 80
percent charge. It takes 2 hours to get to this 80 percent level when the unit
is not being used. The full current level available from the power adapter is
used in this phase of the recharge cycle. When the 80 percent level has been
reached, the computer will start a trickle recharge. This is a reduced current
that allows the battery to reach 100 percent charge without the danger of
overcharging.
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When I Snap My Fingers, You Will Forget...
When the PowerBook enters sleep mode, all subsystems except the memory and the
Power Manager are turned off. This means the SCC, being disconnected from
power, will no longer be able to drive the DTR lines state despite the setting
of csCode 16 to the serial driver to hold DTR.
In addition, when the power is dropped to the internal modem its current
settings will be lost also. To work around this if you expect the modem to
remember settings across sleeps, you should install a Sleep Queue task to read
out the settings just before sleep and then restore settings on system
wake-up.
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Snap, Crackle, Pop
To further power conservation and enhance battery life, the system software
detects whether or not the sound chip is being used. If it hasn't been used for
a while, power is disconnected from the sound circuitry in order to conserve
power. During this process, a click may be heard from the speaker as the
speaker coil deenergizes. You can defeat this sound by plugging a minijack into
the Sound Output port, thus disconnecting the internal speaker. The electrical
discharge and subsequent sound are unfortunately the other side of an
engineering trade-off made to help maximize battery life.
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NiCad Myths and Nightmares
There really is a "charge memory" problem with Nickel-Cadmium batteries!
The trouble is that to get that effect you need to get your battery into a
geosynchronous orbit, in microgravity, and subject it to extremes of heat and
cold while charging for precise times over a period of months. Also, the result
of a true "charge memory" effect is a reduction in voltage by a few tenths of a
volt, not a change in the amp-hour duration of battery charge. In satellites
where this is a concern, a reconditioning cycle is run every year or so! This
is not a reasonable concern even if your work routine is the same day to day.
The problem is that, for fear of causing a memory problem in NiCad batteries,
many people attempt to fully discharge then fully recharge their batteries.
This tends to accelerate electrolyte loss and to create the possibility of
reverse polarizing individual internal cells--resulting in the reduced battery
capacity that these same people claim as evidence of the "memory" effect. You
should therefore never attempt to run your battery down for the purpose
of "reconditioning" it.
Another possible problem is charging or discharging the battery too fast. In
the first case you can generate excessive heat and cause the loss of
electrolyte and in the second (by shorting the terminals) you can cause the
loss of electrolyte and generate excessive heat. The charger circuit in the
PowerBooks as well as the stand-alone battery chargers from Apple are designed
to bulk charge the battery as fast as possible without overheating, then to
trickle charge up to their full charge.
In general use, you should plug into your wall adapter whenever it is
available, whether using the lead-acid battery in the PowerBook 100 or the
NiCads in the 140 and 170, and not worry about possible memory effects. If you
are looking for a third-party battery charger, stay away from bulk chargers and
those that "provide" discharge/recharge cycles.
Despite what the manual says, you should never intentionally discharge your
battery other than in the course of normal use.
References
Inside Macintosh, Volume VI, Power Manager
Macintosh PowerBook Family Developer Notes
"Getting the Most Out of Nickel-Cadmium Batteries" by Ken Stuart, QST
Magazine, February 1992
PowerBook Modem Guide available on the Developer CD
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