MC3PHAC Monolithic Intelligent Motor Controller, Rev. 2

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Freescale Semiconductor

Optoisolated RS232 Interface Application Example

shifting from the PC to the MC3PHAC’s serial port. An RS-232 line receiver, such as an MC1489, serves

the same purpose without the optoisolation function.

To send data from the MC3PHAC to the PC’s serial port input, it is necessary to satisfy the PC’s receive

data (RxD) input requirements. In an idle condition, the RxD input to the PC must be at Mark

(–3 to –25 volts). The data terminal ready output (DTR) on the PC outputs a Mark when the port is

initialized. The request to send (RTS) output is set to a Space (+3 to +25 volts) when the PC’s serial port

is initialized. Because the interface is half-duplex, the PC’s TxD output is also at a Mark, as it is idle. The

idle state of the MC3PHAC’s serial port output is a logic 1. The logic 1 out of the MC3PHAC’s serial port

output port forces the diode in U2 to be turned off. With the diode in U2 turned off, the transistor in U2 is

also turned off. The junction of D2 and D3 are at a Mark (–3 to –25 volts). With the transistor in U2 turned

off, the input is pulled to a Mark through current limiting resistor R3, satisfying the PC’s serial input in an

idle condition. When a start bit is sent from the MC3PHAC’s serial port, it transitions to a logic 0. That logic

0 turns on the diode in U2, thus turning on the transistor in U2. The conducting transistor in U2 passes

the voltage output from the PC’s RTS output, that is now at a Space (+3 to +25 volts), to the PC’s receive

data (RxD) input. Capacitor C1 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 MC3PHAC’s serial output

port to the PC’s serial port. An RS-232 line driver, such as a MC1488, serves the same purpose without

the optoisolation function.