Convention assigns ports in the order 3F8, 2F8, 3E8, 2E8. Not all BIOSes do that and any missing port will assign the higher ports to unexpected addresses thereby utterly confusing most software. Hardware suppliers advocate sharing interrupts between COM1/COM3 and COM2/COM4, but software rarely supports such sharing properly. External modems are connected to a serial port and require a 16550 port for high speed connections. Internal modems replace a serial port and require that any device with the same port/IRQ provided with the PC be disabled. Wiring of the serial port and flow control of serial data is an arcane art. PC Serial ports have either DB-9 or DB-25 pin female connectors. For each type of socket, there are two pinouts -- Data Communication Equipment (DCE) and Data Terminal Equipment(DTE). There are two types of serial cables that look just alike and are utterly incompatible. Straight through (modem) cables are wired straight through to support DCE to DTE connections. Null Modem cables have wires crossed over in the cabling to support DCE-DCE or DTE-DTE connections. Neither has any relation to modems except that modems almost always require straight through cables. The keyboard and PS/2 type mouse ports are modified serial ports.
Return To Index Copyright 1994-2008 by Donald Kenney.