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PDA 05 5/30/03 11:35 AM Page 57
Chapter 5 / The Electronics
This SC pin has been driven LOW in the PDA Robot circuit. However,
if you decide to modify the circuit, I recommend putting a switch on the
board or tying this line to a pin on the microcontroller. This would
allow you to set SC high or low physically through the switch or pro-
grammatically through the microcontroller. This would enable you to
hold the PDA at a much further distance from the craft when testing and
calibrating the system. You could also use the PDA as a remote control.

The guide pins on the side-view and top-view packages are internally
connected to ground, but should not be connected to the system
ground, to avoid ground loops. They should be used for mechanical
purposes only and should be left floating.
PDA Robot does not ground the guide pins. They are used only to help
secure the unit to the PCB.

Shutdown. The internal switch for the IRED in Telefunken SIR trans-
ceivers is designed to be operated like an open collector driver. Thus,
the VCC2 source can be an unregulated power supply, while only a
well-regulated power source with a supply current of 1.3 mA con-
nected to VCC1/SD is needed to provide power to the remainder of the
transceiver circuitry in receive mode. In transmit mode, this current is
slightly higher (approximately 4 mA average at 3 V supply current),
and the voltage is not required to be kept as stable as in receive mode.
A voltage drop of VCC1 is acceptable down to about 2.0 V when
buffering the voltage directly from the pin VCC1 to GND; see Figure
5.20a. This configuration minimizes the influence of high-current
surges from the IRED on the internal analog control circuitry of the
transceiver and the application circuit. Also, board space and cost sav-
ings can be achieved by eliminating the additional linear regulator
normally needed for the IRED’s high current requirements.
The transceiver can be very efficiently shut down by keeping the IRED
connected to the power supply VCC2, but switching off VCC1/SD. The
power source to VCC1/SD can be provided directly from a microcon-
troller. In shutdown, current loss is realized only as leakage current
through the current-limiting resistor to the IRED (typically 5 nA). The
settling time after switching VCC1/SD on again is approximately 50
µs. Telefunken’s TOIM3232 interface circuit is designed for this shut-
down feature. The VCC_SD, S0, or S1 outputs on the TOIM3232 can
be used to power the transceiver with the necessary supply current. If

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