AN1857 Freescale Semiconductor / Motorola, AN1857 Datasheet - Page 12

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

Application Note
Optoisolated
RS-232 Interface
12
In the event the control board is used in a system that does not use an
optoisolation board between the control board and the power stage,
connection to a computer terminal or PC could be dangerous. The circuit
in
interface used on the MR32 control board. This isolated terminal
interface provides a margin of safety between the motor control system
and a computer terminal or PC.
The EIA RS-232 specification states the signal levels can range from
a signal that ranges from –3 volts to –25 volts. A space is defined as a
signal that ranges from +3 volts to +25 volts. Therefore, to meet the
RS-232 specification, signals to and from a terminal must transition
through 0 volts as it changes from a mark to a space. Breaking the
isolated RS-232 circuit into input and output sections makes a simpler
explanation of the circuit.
To send data from a PC to the MR32 control board, it is necessary to
satisfy the SCI input on the MR32. In the idle condition, the SCI input
must be at a logic 1. To accomplish that, the transistor in U6 must be off.
The idle state of the transmit data line (TXD) on the PC serial port is a
mark (–3 V to –25 V). Therefore, the diode in U6 is off and the transistor
in U6 is off, yielding a logic 1 to the SCI input. When the start bit is sent
to the SCI from the PC’s serial port, the PC’s TXD transitions from a
mark to a space (+3 V to +25 V), forward biasing the diode in U6.
Forward biasing the diode in D3 turns on the transistor in U6, providing
a logic 0 to the input of the SCI. Simply stated, the input half of the circuit
provides input isolation, signal inversion, and level shifting from the PC
to the MR32’s SCI port. An RS-232 line receiver, such as an MC1489,
serves the same purpose without the optoisolation function.
To send data from the MR32 control board to a PC 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 V to
–25 V). 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 V to +25 V) 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 transmit data line (TXD) on the MR32’s SCI
3volts to 25volts. A mark is defined by the EIA RS-232 specification as
Figure 6
Freescale Semiconductor, Inc.
For More Information On This Product,
is the schematic of a half-duplex optoisolated RS-232
Go to: www.freescale.com
MOTOROLA
AN1857