Quatech SSCLP-300 - Manual (Page 17)

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
DSCLP/SSCLP-200/300 User's Manual 17