Quatech RS-422 user manual Enabling the Special Registers, Interrupt Status Register

4.2 Enabling the Special Registers

The DSCLP-200/300 contains two unique registers, an Interrupt Status Register and an Options Register. These registers are enabled when the SPAD jumper (J13) is removed (factory default). They replace the UART Scratchpad Register on accesses to register address 7.

The Interrupt Status Register and Options Register are accessed through the scratchpad location of any UART. The DLAB bit of the UART (Line Control Register, bit 7) is used to select between the two registers. The most recent write of a DLAB bit in any UART selects between the two registers as shown in Figure 8.

DLAB Bit	SPAD Jumper	Register selected for
		address 7 accesses
0	removed	Interrupt Status Register
1	removed	Options Register
X	applied	Scratchpad Registers

Figure 8 --- DLAB bit selects between special registers

4.3 Interrupt Status Register

The read-only Interrupt Status Register can be used to quickly identify which serial ports require servicing after an interrupt. Reading the Interrupt Status Register will return the interrupt status of the entire DSCLP-200/300, as shown in Figure 9. The individual bits are cleared as the interrupting ports are serviced. The interrupt service routine should ensure that the interrupt status register reads zero before exiting.

Bit	Description
7 (MSB)	0 (not used)
6	0 (not used)
5	0 (not used)
4	0 (not used)
3	0 (not used)
2	0 (not used)
1	Port 2 --- 1 if interrupt pending
0	Port 1 --- 1 if interrupt pending

Figure 9 --- Interrupt Status Register

10 DSCLP/SSCLP-200/300 User's Manual