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1
2
TXD/
Start
Bit 0
RXD
Bit
1
Start
TXD/
RXD
Bit
11.1.1.4
Mode 2
Asynchronous Mode 2 is referred to as the "address recognition mode." Mode 2 is used together
with Mode 3 for multiprocessor communications over a common serial link.
In Mode 2, the RX machine will not complete a reception unless the ninth data bit is a one. Any
character received with the ninth bit equal to zero is ignored. No flags are set, no interrupts occur
and no data is transferred to SxRBUF. In Mode 3, characters are received regardless of the state
of the ninth data bit. The following is brief example of using Modes 2 and 3. See "Master/Slave
Example" on page 11-24 for more information.
Assume one master serial port connects to multiple slave serial ports over a serial link. The slaves
are initially in Mode 2, and the master is always in Mode 3. The master communicates with one
slave at a time. The CPU overhead of the serial communications burdens only the master and the
target slave device.
1.
The master transmits the "address" of the target slave, with the ninth bit set, over the serial
link.
2.
All slaves receive the character and check whether that address is theirs.
3.
The target slave switches to Mode 3; all other slaves remain in Mode 2.
4.
The master and the target slave continue the communication with all ninth data bits equal
to zero. The other slave devices ignore the activity on the serial link.
3
4
5
Bit 1
Bit 2
Bit 3
Bit 4
Figure 11-5. Mode 3 Waveform
2
3
4
Bit 0
Bit 1
Bit 2
Bit 3
Figure 11-6. Mode 4 Waveform
SERIAL COMMUNICATIONS UNIT
6
7
8
9
Bit 5
Bit 6
Bit 7
5
6
7
8
Bit 4
Bit 5
Bit 6
10
11
Parity
Stop
or
Bit
Bit 8
A1286-0A
9
Stop
Bit
A1287-0A
11-7
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