Technical information

What To Do First Serial Solutions
AST Four Port Card.___________________
The AST Cluster card pioneered a method where a shared
interrupt register holds a status bit for each four serial ports. A
global status bit is set if any of the four ports have an interrupt
pending. This card become popular in the early days of the PC
but its inherent non expandability to more than four ports has
since caused it to fall into great decline.
The presence of multiple cards using the AST type sharing
mechanism, as used on the Flynix and other cards, is passed to
Serial Solutions using the /F parameter on the NEWCOM.SYS
line in the CONFIG.SYS file.
eg /F 5,3,4,5,6
is used to signify that the AST/ Flynix card is using interrupt
line 5, and is sharing it amongst ports COM3, COM4, COM5
and COM6.
Lynx And Quad Cards._____________________
Our Lynx and Quad port cards use the most flexible
approach, based on the original Digiboard mechanism. The
interrupts from the serial ports goes into shared interrupt
circuitry. This presents a single interrupt to the PC, the sharing
circuitry also prioritises and encodes the interrupts. There is a
shared interrupt service register, the SISR, when read it returns a
number between 0 and 255 decimal. If the number read is 255=
0ff hex then no port has an interrupt pending. If the number read
is 0 then serial port 1 on the card has generated an interrupt. If
the number read is 7 then serial port 8 on the card has generated
an interrupt. The interrupts are latched and prioritised so
preventing one port locking out the others. A linking mechanism
allows multiple four and eight port cards in the one PC to be
linked together. Thus giving a theoretical maximum of 255 serial
ports in one PC that use only one interrupt line between them.
In practice 24 or so ports, using buffered FIFO chips, in a fast
PC can provide acceptable performance.
Our Quad and Lynx cards are unique in providing
interrupts 2-7, 10, 11, 12, 14 and 15 both for individual ports
and also for the shared interrupt.
Chapter 1 Page 9