Why use an ISP?

Technical Description

polling’. This method required the interrupt service routine to ‘poll’ or interrogate each UART as to its interrupt pending status. This method of polling was sufficient for use with slower speed communications, but as modems increased their throughput abilities this method of servicing shared IRQs became inefficient.

The answer to the polling inefficiency is 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-COMM+232.PCIwill allow any one of the available locations to be read to obtain the value in the status register. Both status ports on the OMG-COMM+232.PCIare identical, so any one 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

Connector Pin Assignments

RS-232

	Name	Pin #	Mode
TD	Transmit Data	3	Output
RTS	Request To Send	7	Output
DTR	Data Term Ready	4	Output
GND	Ground	5
RD	Receive Data	2	Input
DCD	Data Carrier Detect	1	Input
DSR	Data Set Ready	6	Input
CTS	Clear To Send	8	Input
RI	Ring Indicator	9	Input

Note: These assignments meet EIA/TIA/ANSI-574 DTE for DB-9 type connectors.

Omega Engineering COMM+232.PCI

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Why use an ISP?

Connector Pin Assignments

RS-232

Name

Pin #

Mode

Omega Engineering COMM+232.PCI