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Motorola USB08 Reference Manual: User Rs232 Port

Universal serial bus evaluation board using the mc68hc908jb8.
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2.4.4 User RS232 Port

USB08 Evaluation Board
The monitor mode interface uses only one sending/receiving channel of
the RS232 transceiver IC2. The remaining channel is used for an
additional user RS232 port.
In contrast to the RS232 channel for the monitor mode interface, the user
RS232 port incorporates separate sending and receiving lines. The
PTA7 pin of the MCU is used for receiving and the PTC0 pin is used for
The MC68HC908JB8 does not have a serial communications interface
(SCI) hardware module for asynchronous serial communication.
Therefore, the necessary timing has to be generated by software.
If PTA7 and/or PTC0 are to be used, the diode D5 serves for the
adjustment of the logic levels between 5 V and 3 V. Otherwise, the
RS232 transceiver can be uncoupled from the MCU by removing the
jumpers JP1-G and JP1-H.
X3 is the user RS232 port plug connector. If this interface is attached to
a PC, a line connection similar to the monitor mode interface is
necessary. In this case, the bridges BR1 and BR2 on the downside of
the printed circuit board (PCB) (see
positions 1 and 2. For this configuration, the PC works as a host and the
USB08 board represents the device side.
The reverse case happens, if a serial liquid crystal display (LCD) is to be
operated at the user RS232 port. In this configuration, the USB08 board
is the host and the LCD module represents the device side. The
necessary RxD/TxD crossing is done by configuration of the bridges
BR1 and BR2 in positions 2 and 3. At the same time, the serial LCD can
be supplied with operating voltage by closing the bridge BR3.
This specification deviates from standard RS232 mapping.
Serial alphanumeric LCDs are offered by several vendors. In the test
configuration, the LCDs used are from the Canadian manufacturer
Matrix Orbital (
Hardware Description
Hardware Description
2-3) have to be connected in
Designer Reference Manual
Circuit Description


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