PC Hardware in a Nutshell, 3rd Edition

Multiport Serial Adapters

Each standard serial port occupies one IRQ, which limits how many standard serial ports can be installed. Most computers need one or two serial ports at most, so this is seldom a problem. But there are applications—such as running a terminal server under Linux—for which it is desirable to have eight, 16, or more serial ports available. That’s clearly impossible if each serial port requires one IRQ. The solution is a special board called a multiport serial adapter.

These adapters are available in ISA and PCI form, and are installed in the PC just like any other expansion card. They provide from four to 32 serial ports using only one IRQ. The serial port connectors reside in a separate external box that connects to the multiport serial adapter via multiple 8P8C (“RJ-45”) connectors or a single proprietary octopus cable with many conductors.

Such adapters are available in two forms: “dumb” multiport serial adapters are little more than a collection of UARTs and connectors, use 8-bit transfers, generate an interrupt for each byte transferred, and depend on the host PC for processing power. “Smart” multiport serial adapters include a microprocessor (typically an 80286 or 386) that offloads serial interrupt processing tasks from the host PC. Smart adapters usually buffer 1 KB of data or more between transfers, greatly reducing the interrupt burden on the host system. Nowadays, most multiport serial adapters are of the smart variety.

Dumb terminals have become rare ...