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Technical Description

Sealevel Systems ULTRA COMM+422 Page 16

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 ULTRA COMM+422 will allow any one of the available

locations to be read to obtain the value in the status register. All four status ports

on the ULTRA COMM+422 are 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 2 ... 14 15 16 17 18 19 20 21 22 23 24 ... 28 29

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