Quatech RS-232 - Manual (Page 11)

4.2 Enabling the Special Registers
The ESC(LP)-100 contains two unique registers, an Interrupt Status
Register and an Options Register. 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 3.
Options Register 1
Interrupt Status Register 0
Register selected for
address 7 accesses
DLAB Bit
Figure 3 --- 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
ESC(LP)-100, as shown in Figure 4. 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
Port 3 --- 1 if interrupt pending 2
Port 4 --- 1 if interrupt pending 3
Port 5 --- 1 if interrupt pending 4
Port 6 --- 1 if interrupt pending 5
Port 7 --- 1 if interrupt pending 6
Port 8 --- 1 if interrupt pending 7 (MSB)
Description Bit
Figure 4 --- Interrupt Status Register
ESC(LP)-100 User's Manual 5