Introduction

Serial communication involves transmitting digital data one bit at a time. Starting with the binary telegraph code in the 19th Century, to the development of RS-232 (EIA-232) for communicating between terminals and mainframe computers in the 1960's, serial communication has been the driving force behind many advances in communication technology over the past 40 years.

Although many peripheral devices now connect to PCs via a USB port, most PCs still come with one or two male DB9 ports and one female DB25 port that support RS-232. Additional serial ports can be added when needed by installing various kinds of multiport serial boards. Serial boards that support RS-232, RS-422, 2-wire RS-485, and 4-wire RS-485 are available from Moxa.

Multiport serial boards are installed in the slots inside a PC. Slots are available for ISA, PCI, PCI-X, and PC/104 busses, with each board providing one or more serial ports. Today, multiport serial boards are commonly used for controlling external devices for different applications, including industrial automation, building automation, CNC machines, etc. Serial boards can also connect to computer servers for the purpose of monitoring and controlling the server from a remote location. The boards were once used mainly to connect many terminals and/or modems to serial ports.

Multiport boards use different types of external connectors, including DB9, DB25, and RJ45, to connect serial devices to a computer. Each physical device is connected to its own serial port, and since the space on the external part of the board is limited, there is often not enough room to fit all of the serial port connectors on the board. To solve this problem, the connectors are often on the ends of "octopus" coables that connect to the board. Another option is to use an external box (which could be rack mountable) that is connected by a cable to the multiport serial board.

Multiport Serial Boards Should Provide the Following Benefits:

Flexibility—Most multiport serial board vendors allow users to begin with a small-range system with 8 or fewer ports, and then expand up to 128 ports, depending on business growth. In addition, users have different options for control boards (PCI or ISA), connection modules (rackmount or desktop), and COM port interface (RS-232, RS-422, or RS-485). Any combination of these options can be used in one system.

Speed—Many board vendors develop their own proprietary chip that is integrated with a dedicated processor, enabling the board to transmit and receive data at up to 921 Kbps full duplex across all ports simultaneously. This is two to four times faster than the speed available using standard UART technology.

Reliability—Along with providing a fast throughput speed and reduced load on the host system CPU, the multiport serial board vendor should also produce boards that are reliable. By designing boards with a long MTBF (Mean Time Before Failure), users can be assured that the multiport serial board will provide many years of uninterrupted service.

Efficiency—Many multiport serial boards use 128-byte FIFOs, which is larger than the standard 16550 UART. Experience has shown that 128 bytes is enough to keep data from getting backed up at the serial controller level, while still conserving CPU resources.