AN1857 Freescale Semiconductor / Motorola, AN1857 Datasheet - Page 13

• Manufacturer Part Number: AN1857
• Description: A 3-Phase ac Induction Motor Control System Based on the MC68HC908MR32
• Manufacturer: Freescale Semiconductor / Motorola

AN1857
MOTOROLA
is a logic 1. The logic 1 out of the SCI’s output port forces the diode in
U7 to be turned off. With the diode in U7 turned off, the transistor in U5
is also turned off. The junction of D12 and D15 are at a mark (–3 V to
–25 V). With the transistor in U5 turned off, the input is pulled to a mark
through current limiting resistor R53, satisfying the PC’s serial input in an
idle condition. When a start bit is sent from the MR32’s SCI port to the
output of the MR32’s SCI, output transitions to a logic 0. That logic 0
turns on the diode in U5, thus turning on the transistor in U7. The
conducting transistor in U5 passes the voltage output from the PC’s RTS
output, that is now at a space (+3 V to +25 V) to the PC’s receive data
(RXD) input. Capacitor C30 is a bypass capacitor used to “stiffen” the
mark signal. The output half of the circuit provides output isolation, signal
inversion, and level shifting from the MR32’s SCI output port to the PC’s
serial port. An RS-232 line driver, such as an MC1488, serves the same
purpose without the optoisolation function.
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Freescale Semiconductor, Inc.
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For More Information On This Product,
3
4
2
1
5
TXD
TXD
GND
GND
DTR
DTR
RTS
RTS
RXD
RXD
Go to: www.freescale.com
Figure 6. Optoisolated RS-232 Circuit
(+12 Vdc)
(+12 Vdc)
4.7 k
4.7 k
2.2 F
2.2 F
1 k
1 k
SFH6106
SFH6106
SFH6106
SFH6106
ISOLATION
ISOLATION
BARRIER
BARRIER
System Hardware
GND
330
330
Application Note
+5V_D
+5V_D
1 k
1 k
TxD
TxD
+5V_D
+5V_D
RxD
RxD
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