80C186EB/80C188EB Microprocessor User's Manual - CEUNES

SERIAL COMMUNICATIONS UNITTXD/RXD1 2 3 4 5 6 7 8 9 10StartBitBit 0Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7ParityorBit 811StopBitFigure 10-5. Mode 3 WaveformA1286-0A1 2 3 4 5 6 7 8 9TXD/RXDStartBitBit 0Bit 1 Bit 2 Bit 3 Bit 4 Bit 5Bit 6StopBitA1287-0AFigure 10-6. Mode 4 Waveform10.1.1.4 Mode 2Asynchronous Mode 2 is referred to as the “address recognition mode.” Mode 2 is used togetherwith 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. Anycharacter received with the ninth bit equal to zero is ignored. No flags are set, no interrupts occurand no data is transferred to SxRBUF. In Mode 3, characters are received regardless of the stateof the ninth data bit. The following is brief example of using Modes 2 and 3. See “Master/SlaveExample” on page 10-27 for more information.Assume one master serial port connects to multiple slave serial ports over a serial link. The slavesare initially in Mode 2, and the master is always in Mode 3. The master communicates with oneslave at a time. The CPU overhead of the serial communications burdens only the master and thetarget slave device.1. The master transmits the “address” of the target slave, with the ninth bit set, over the seriallink.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 equalto zero. The other slave devices ignore the activity on the serial link.10-7