No-fuss data communications have contributed much to the world of data acquisition. From telephone modems to wireless networks, a broad variety of specialized data communication devices are available to suit every communication need.
Using telephone wires, twisted pairs, or fiber optic cable, modems transmit and receive serial, digital data. You may be familiar with the current 56 kbps (actually, it's 57,600 baud) controversy on the Internet, with two competing, incompatible suppliers claiming that they can achieve such data rates over commercial phone lines. Such a modem has been available for years in the industrial world, but it requires special isolated circuitry. Achieving higher data rates for serial data often requires fiber optics. Examples of modems available for data acquisition include:
Limited distance modem:
Low-cost (USD$86) general purpose modems can be used to transmit non-critical data up to 12 miles. As distance increases, transmission speed decreases. At 12 miles, speed is 1.2 kbps. At one mile, speed is 19.2 kbps, and at one half mile, 57.6 kbps. It requires four wires (two twisted pairs) for full duplex operation. To achieve the highest speed of 57.6 kbps, special high-speed, optically coupled circuits are required.
Fully isolated modem:
When data is important, it must be protected against noise, common mode voltage problems, and electrical transients. Optical isolation and dc-to-dc transformer coupling create an isolation barrier of 1000 V rms (root mean squared). An isolated modem has the same transmission speed as a limited distance modem.
Signal-powered fiber optic modem:
In particularly nasty environments with much electrical noise, a fiber optic modem can be used. Signals on fiber optic cable are immune to EMI and RFI. Simple, low cost (USD$127) fiber optic transmitter/receivers can be plugged into the RS-232 ports at both ends of the link. The transmitters take their power from the RS-232 port, and transmit data at 19.2 kbps for up to 2.2 miles.
High-speed fiber optic modem:
This device transmits RS-232, RS-422, or RS-485 data at speeds up to 5 Mbps 1.2 miles over a fiber optic link. As above, the transmitter/ receiver plugs into the data port at both ends. For more distance, a repeater can be installed.
Before the advent of reliable radio data transmission for reaching remote locations, communications required hard wiring. Wires had to be strung on utility poles or underground. This is neither cheap nor convenient. Today, transmitting data by radio-frequency (RF) links is becoming widely acceptable. RF links are available in several varieties:
Requiring line-of-sight between antennas, microwave systems can transmit data up to 32 miles. Such a system consists of an operator base station that can communicate with one or more remote terminal units (RTUs). Each RTU has a microwave two-way radio and an antenna. Data acquisition equipment at the remote site connects to the RTU via RS-232 or RS-485 ports. Typical cost of an RTU or base station is USD$3,000 to USD$5,000.
These narrow-band units operate in the 900-MHz band, and work at distances to 1 mile. With antennas or repeaters, distance can reach 90 miles at speeds to 115 kbps with a response time of 5-15 milliseconds between units.
When interference is present in the 900 MHz band from telephones or other 900-MHz devices, spread spectrum radios "frequency hop" from 902 to 928 MHz to get away from the offending interference. At distances under one mile, line of sight is not always necessary for communications. Spread-spectrum radios can achieve full duplex uncompressed data rates of 115.2 kbps.
If data acquisition needs cannot be met by 900-MHz devices, devices that operate on licensed frequencies can connect up to 10,000 remote units. The limited availability of frequencies keeps these systems out of metro areas. Utilities in outlying areas are big users of licensed systems.
Trunked or cellular:
For hilly, mountainous, or city settings where line-of-sight systems are impractical, trunked radio is a solution. The radio signal is transmitted through repeaters by the same service providers used by taxicabs. Cellular telephones also can be used to transmit data in such situations.
As noted above, any data acquisition system might mix and match components and transmit data over multiple interfaces. Protocol converters help a great deal in the interfacing of various hardware. Some of the wide variety of converters available include:
IEEE-488 to RS-232/422:
Provides a transparent interface between two dissimilar devices. For example, it allows an IEEE 488 instrument to communicate with an RS-232 printer or plotter. Conversely, a PC with an RS-232 port could communicate with an IEEE-488 device. Such converters are available with one port for device-to-device communications, or with multiple ports.
IEEE-488 to Mac:
Connects to SCSI port on Macintosh computers, allowing the computer to control up to 14 IEEE-488 devices.
RS-232 to RS-422:
Converts an RS-232 signal to RS-422, with complete signal isolation. It allows an RS-232 device to get onto an RS-422 multi-drop system. RS-422 supports up to 32 devices.
RS-232 to RS-485:
Similar to above, this converter lets an RS-232 device get onto an RS-485 multi-drop system. RS-485 supports up to 64 devices.
An auto-dialer monitors conditions at a remote site and, if an alarm condition develops, alerts people or other equipment. It is a small, self-contained data acquisition system that monitors one or more key digital or analog sensors. In almost all cases, it has automatic dial-out capability that will continue to call a series of phone numbers until someone answers. It then delivers a preprogrammed voice alarm message.
At the low end, systems with voice-only outputs start at about USD$395. One such a system monitors one temperature and three contact closure inputs. When an alarm condition develops, it dials multiple phone numbers and delivers a prerecorded voice message. It continues to call until someone properly acknowledges the message.
As features are added, costs and capabilities increase:
Additional analog and contact closure inputs allow more systems to be monitored.
Digital speech recording allows multiple messages to be recorded and assigned to various alarms.
An RS-232/485 interface allows the system to call up a computer or controller to deliver an alarm message and transmit data.
A built-in data-logger function stores data about the event for later analysis.
Local digital and analog outputs allow alarm and control functions to be programmed.