Serial Communication Standards

In a “serial” port, data is transmitted digitally over a single data line. Bits of information are transmitted one at a time. This contrasts with parallel data interfaces, where digital information is transferred from one to eight bytes at a time across 8, 16, 32, or 64 data lines. Many digital I/O boards can be used as parallel interfaces.

Synchronous vs. Asynchronous

Two basic techniques are used in serial communications to reduce or eliminate errors. Asynchronous systems use start & stop bits to define the beginning and end of each byte (or each word of 5 to 8 bits), and many asynchronous systems add a parity bit to further improve accuracy. Synchronous serial communication is rarely used in PCs and PC-based systems, but is used by some programmable logic controllers. Here, the sending & receiving devices are synchronized with a clock that times each bit in the message. If either end of the communication loses its clock signal, communication ends. As long as the clock signal continues, the receiving end can detect missing or extraneous bits.

RS-232 Standards

The RS-232 standard was developed for applications connecting one piece of Data Terminal Equipment (the PC) and one piece of Data Communications Equipment (a data transmitter). The standard calls for single-ended data and control signals over a maximum cable length of 50 feet at a maximum rate of 20kbps (CyberResearch has interfaces that support 920kbps rates).

RS-422 Standards

RS-232 systems are limited in distance & speed primarily because RS-232 ports use single-ended voltage lines for data & control signals. RS-422 overcomes this problem by using differential voltage pairs for data transmission. Whereas the single-ended data lines used by RS-232 suffer voltage drops and pick up noise when extended much over 50 feet, the differential between the RS-422 voltage pairs remains constant over distances up to 4,000 ft. The twisted pairs used in RS-422 are also relatively immune to crosstalk, supporting speeds to 100kbps over long distances. In addition, the RS-422 standard permits full-duplex operation, so RS-422 systems can serve one PC and up to 10 receivers. EIA-530/RS-530 is a defined standard for RS-422 that specifies the pinout on a DB-25 connector & specific signal timing.

RS-232/422/485

The DB-9 male serial connector shown above is commonly used by RS-232/422/485 devices.

 RS-485 for Multiple Serial Devices

RS-485 is a version of RS-422 that has been optimized to allow up to 32 serial drivers and 32 serial receivers on one multidrop line. RS-485 typically runs in half-duplex mode, using the same pair of wires for transmitting and receiving data. Any serial device used on an RS-485 network needs to have some intelligence to know when it is being addressed. RS-485 can be an economical way to set up a “peer-to-peer” network. However, it can be slow if many devices are “talkers,” because only one device at a time can send data. Supports networks 4000 ft. long at up to 100kbps.

Understanding Data and Control Lines

RS-232, 422, & 485 serial ports have two data lines. These are referred to as TD (Transmit Data) & RD (Receive Data). RS-422 devices running in “full-duplex” mode will have two pairs of data lines: TD+, TD–, RD+, and RD–. In this mode they can send and receive data at the same time. In “half-duplex” mode, transmit and receive data are shared on a single pair of lines: TD+/RD+ and TD–/RD–. A device configured for half-duplex operation cannot transmit & receive at the same time. RS-485 usually runs in half-duplex mode. RTS (Request-to-Send) and CTS (Clear-to-Send) are signals used within the PC to enable or disable the TD lines.

USB

There are two types of USB ports: the port found on the PC chassis or hub (above left),
and ports found on USB-based peripheral (above right).

USB Simplifies Data Collection

The Universal Serial Bus (USB) solves a number of data-collection problems inherent in older systems:

• Speeds of 480, 12, or 1.5Mbps.
• Hot-swap devices (swap with power on).
• Devices powered from the USB port.
• Up to 127 devices on one port.

There are two USB standards currently in common use. USB 1.1 offers both a high-speed mode (data transfers at up to 12Mbps) and a low-speed mode (1.5Mbps). Newer high-speed devices support USB 2.0, with data rates up to 480Mbps. USB 2.0 is fully backwards-compatible, supporting both USB 1.1 and USB 2.0 devices.

Low-power devices can draw +5V from the USB line, instead of external power.

USB divides attachable devices into two major classes: Functions are devices such as peripherals that add functionality to the host. Hubs can provide additional USB sub-ports. A “compound device” combines both functions in one physical device.

Hubs are wiring concentrators that act as USB “splitters.” Most hubs convert an upstream attachment point into multiple downstream ports (usually 4~7), to share one PC port with multiple attached devices. A device-sharing hub allows multiple PCs to share one USB peripheral. Many hubs have an external power supply to provide power for attached devices, so as not to overload the +5V line on your PC’s USB port.

USB 1.1 supports cables up to 16 feet long (9 ft. max. for low-speed devices) — shorter cables are better! We recommend 6-ft. (or shorter) cables for USB 2.0 devices.

Detailed technical specifications and frequently asked questions on the USB standard are available at www.usb.org.

FireWire (IEEE-1394)

Common FireWire ports: the standard 6-pin port (left),and the compact 4-pin connector (right).

FireWire© and IEEE-1394

“FireWire” is Apple Computer, Inc.’s trademarked name for the high-speed serial bus technology that they pioneered, now codified as IEEE Standard 1394-1995. Sony Corp. uses “DigitalLink®,” though “FireWire” is more commonly used.

FireWire is currently a popular bus for hard disks, CD-RW drives, & other high data rate consumer products. FireWire offers the kind of high-speed high-accuracy data flow needed by scientists and engineers:

• Speeds to 400Mbps
• Hot-swap devices (swap with power on)
• Cable lengths up to 13 feet
• Up to 63 devices on one port
• Supports peer-to-peer connection

FireWire offers a versatile link among up to 63 devices, without the need for intermediary hardware such as hubs. Devices on the bus can communicate not only to the host PC, but also to each other.

Furthermore, devices can communicate at any mix of 400Mbps, 200Mbps, and 100Mbps. A future version of the standard will support speeds up to 1.2Gbps.

During communication, the sending device first negotiates with other devices on the line for bandwidth (no single device is allowed more than 65% of total bandwidth, and the total of all devices on the bus may not exceed 85% of total bandwidth). The sender then transmits packets of data at uniform time intervals to fill up its bandwidth allotment.

This process does not require time for data acknowledgements, nor for competing requests from other devices for bus time. Thus the need for buffering is reduced, resulting in efficient data transmission.

The standard IEEE-1394 cable carries up to 1.5A of DC power for devices on the bus. The bus reconfigures itself whenever a device is attached or detached.