Quatech RS-422 - Manual (Page 15)

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.
Scratchpad Registers applied X
Options Register removed 1
Interrupt Status Register removed 0
Register selected for
address 7 accesses
SPAD Jumper DLAB Bit
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.
Port 1 --- 1 if interrupt pending 0
Port 2 --- 1 if interrupt pending 1
0 (not used) 2
0 (not used) 3
0 (not used) 4
0 (not used) 5
0 (not used) 6
0 (not used) 7 (MSB)
Description Bit
Figure 9 --- Interrupt Status Register
10 DSCLP/SSCLP-200/300 User's Manual