Listing 2-The XR-88C681A DUART goes into multidrop 9-bit operation and interrupt generation when a character is received.

// Full DUART Register Definitions 
#define MODE1 ((volatile char *) 0x00 + DUARTBASE)	// Mode 1 Register -- Channel A 
#define MODE2 ((volatile char *) 0x00 + DUARTBASE)	// Mode 2 Register -- Channel A 
#define TXDONE ((volatile char *) 0x01+ DUARTBASE)	// Status Register -- Channel A
#define CLOCK ((volatile char *) 0x01+ DUARTBASE)	// Clock Select Register - Channel A 
#define CMD ((volatile char *) 0x02+ DUARTBASE)	// Command Register - Channel A 
#define INTMASK ((volatile char *) 0x05+ DUARTBASE)	// Interrupt Mask Register
#define TXHOLD ((volatile char *)0x03+ DUARTBASE)	// Transmit Hold Register
#define ResetToMODE1() *CMD = 0x10 | (*CMD & 0x0F)	// Macro to force access to Mode Register 1
void Initialize21KplusDUART(void) {	// 21k processor configuration 
*SYSCON21K &= BANKS8K;	// Set bank size
*WAIT21K &= BANK3MASK;	// Clear Bank 3 bits
*WAIT21K |= WAIT6BANK3;	// 6 Waits to set
// XR-88C681A Channel A Configuration
*CLOCK = 0x99;	// 4800 Baud
*CMD = 0xD5;	// TX and RX enabled
ResetToMODE1();	// Access first-mode register
*MODE1 = 0x1F;	// 9-bit char, multidrop mode 
// Interrupt asserted on RXRDY
*MODE2 = 0x07;
*INTMASK = 0x02;	// Activate interrupt	
}	

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