Omega Engineering RS-422/485 manual Why use an ISP?, OMG-ULTRA-COMM422, Page

Technical Description

Why use an ISP?

The answer to the polling inefficiency was the Interrupt Status Port (ISP). The ISP is a read only 8-bit register that sets a corresponding bit when an interrupt is pending. Port 1 interrupt line corresponds with Bit D0 of the status port, Port 2 with D1 etc. The use of this port means that the software designer now only has to poll a single port to determine if an interrupt is pending.

The ISP is at Base+7 on each port (Example: Base = 280 Hex, Status Port = 287, 28F… etc.). The OMG-ULTRA-COMM422will allow any one of the available locations to be read to obtain the value in the status register. All four status ports on the OMG-ULTRA-COMM422are identical, so any one of the four can be read.

Example: This indicates that Channel 2 has an interrupt pending.

Bit Position:	7	6	5	4	3	2	1	0
Value Read:	0	0	0	0	0	0	1	0

OMG-ULTRA-COMM422 Page 16