SERIAL PORTS

6/22/96

Serial Port: A port on PCs used to communicate data serially -- one bit at a time. Speeds supported on unmodified serial ports range from 50bps to 115200bps, however, many CPU/Operating System combinations are unreliable above 9600 bps. "16550" type ports contain internal FIFO queues that improve but do not guarantee reliability at high speed. Serial ports are specified to the RS-232 specification which is designed to communicate over distances of at least 3-4 meters and requires the ability to connect outputs to outputs without damage to the components. The standard PC has two serial ports -- COM1 on IRQ 4 and COM2 on IRQ 3. Two additional ports are allowed by MSDOS, but no interrupts are allocated.

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.

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