Modern strategies for cost-effective manufacturing mandate that more information be collected from, and sent to, equipment throughout the plant. Often, a data acquisition and control system sends a myriad of signals from remotely located production equipment (frequently from different manufacturers) to a central control location. When these data collection points are widely dispersed throughout a plant, a dedicated wire is usually run to each point. This means miles of expensive multiconductor cabling and conduit, and substantial costs to install them.
Rather than installing many wires to collect sensor data and deliver control instructions, it may be more economical to digitize the signals in the field (on the plant floor) using a multiplexing system, Figure 1.
These wiring systems reduce the cost for sending multiple monitoring and control signals either long distances or through difficult environments. How? They replace dozens of wires with a twowire data link.
Generally used in a computer-based control system, a multiplexer is installed at, or in, the controller, where it converts process signals transmitted from various locations to a serial format that can be used by the controller.
Field multiplexers convert the process signals to a digital format before long-distance transmission to and from field locations. This allows the user to take advantage of economical and noise-immune digital transmission.
In a typical plant, analog and discrete (on/off) devices (sensors and transmitters located at motors, valves, and pumps) collect process data. The analog and discrete signals from these devices are sent to an analog-to-digital (A/D) converter, then a multiplexer chip, which converts data into a digital protocol (communication standard) such as RS- 485 or RS-232, Figure 2. This protocol is essentially a concentrated version of the dozens of analog and discrete process signals. At the opposite end of the data link, a multiplexer converts the digital data back to its original analog/discrete state, or delivers a digital format directly to a controller, like a PC or PLC. The control instructions are then sent back over the same set of wires. Hard wires don’t have to be run to each device.
Some types of multiplexers can be used with programmable logic controllers (PLCs), but this type of application is best suited for adding remote I/O to existing systems because cost and setup effort may be less than for additional PLC I/O. Also, a multiplexer with electrically isolated inputs interfacing with a PLC may be less costly than adding isolated PLC inputs. The multiplexer must be able to output a protocol the PLC can understand (such as Modbus) or convert serial data back to analog form for input to the PLC.
Inputs and outputs
To simplify system design and implementation, it is usually best to choose a field multiplexer that is compatible with an array of transmitter signals and field control devices.
A field multiplexer should be able to accommodate all common analog signals: 4-20 mA, 1-5 V, and 0-10 V. This enables it to accept inputs from any temperature, pressure, level, flow, or power transmitter. Analog signals are also useful for proportional control of valves, pumps, dampers, and louvers.
Discrete signals (contact closure, TTL) are used for tripping alarms to indicate unwanted process conditions. They provide on/off control of motors and valves.
Some multiplexers process only analog or discrete signals, whereas others can process both. When both analog and discrete devices are present in the system, it may be advantageous to use a field multiplexer that accepts both types of input.
A typical 16-channel multiplexer processes data at a scan rate of up to 0.5 sec per 16 channels. Thus, each monitoring point is scanned up to 120 times per minute. The scan rate generally depends on the number of channels per system, the baud rate, and the transmission distance.
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Certain applications may not be suitable for multiplexer use. For example, sensors that count fast-moving parts on a conveyor or check for high-voltage spikes in a motor require more frequent scans, sometimes in the millisecond range. Such applications should be evaluated to determine if it is feasible to add more multiplexers to obtain the required scanning rate. Other applications not suitable for multiplexer use include those that require logic functions or data logging.
Field multiplexers are compatible with commonly used communication protocols, such as RS-232C and RS-485.
RS-232C is the most widely used communication standard for interconnecting digital devices. It is intended for connecting a single device to a host computer. This makes it a good choice where telephone modems are used in the multiplexing system, or a multiplexer module is connected to a host computer system. Without modems, RS-232C allows transmission distances under 200 ft. This is normally not long enough to make field multiplexers a cost-effective strategy. Multiple devices can be connected to an RS-232C data link in a daisy-chain configuration. However, this is risky because one defective module will shut down the the rest (like the old Christmas tree lights). Also, RS-232C is not a very noiseimmune protocol.
RS-485 is a logical choice for most field multiplexing applications. This “party line” of communication protocols can accommodate dozens of process signals. It provides excellent noise-immunity and reliable long-distance transmission (10,000 ft or more). It also allows transmitting signals at fast baud rates: 19,200 or greater. When devices are connected in a multidrop fashion and one unit on the data link goes out of service, it does not upset communication of other modules on the link.
Two-wire twisted pair. This data link option is ideal for most field multiplexing applications. It is relatively inexpensive and easy to install. And, it is accepted in plants worldwide.
In one example, a large electric utility company equips cooling towers with sensors to monitor water temperature and level, plus fans and pumps to control these parameters.
Over time, all but one of the 128 pairs of shielded wires used to send the monitoring and control signals between the towers and the control room (about 3,000 ft) had deteriorated. The company also experienced signal inaccuracies because of the high electrical noise caused by numerous motors, transformers, and walkie-talkies used throughout the plant.
Faced with replacing the 128 wire pairs — a task that would cost thousands of dollars — the utility instead decided to use field multiplexers, Figure 3. They installed multiplexer modules in field cabinets at the foot of each cooling tower, and in a cabinet in the control room. The surviving wire pair was used as the multiplexer’s bidirectional RS-485 communication link.
The multiplexers collect 4-20 mA signals from cooling-tank level and temperature transmitters. The data is digitized and sent over the data link to control room multiplexers, which convert the data back to 4-20 mA signals, thence to analog panel meters. Control signals for regulating the fans and pumps are sent over the same data link, where the field multiplexers convert the digitized data to its original discrete (on/off) form.
Fiber optics. Sometimes, signals must be transmitted through hazardous areas such as those with explosive gases. Even a digital communication link may not be safe here. Fiber optics is an alternative. However, the cost of installing fiber-optic cable can be prohibitive when there are many points, each requiring a dedicated cable. Here is another place where a field multiplexer can help.
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Like two-wire links, the same principle is used to concentrate multiple process signals onto one link. In this case, the digital signals go through another step — conversion to light pulses — before transmission. Once converted to light, the data can be transmitted safely, via fiber optic cable, over long distances, and through an explosive environment. At the other end of the link, a matching multiplexer regenerates the original signals. Expensive, special equipment to handle the fiber optic signals is not required.
Modems and radio transceivers. In some cases, it is not cost-effective or even possible to install wires. Some field multiplexing systems can be used with modems and radio transceivers to solve unusual communication problems over long (unlimited) distances. Sending data over water between oil rigs is an example.
Though systems typically contain multiplexer modules at both ends (field and control center), field multiplexer modules can also be integrated with a personal computer (PC) and used as distributed computer I/O stations. Instead of module- to-module communication, the field multiplexer sends data directly to the PC. Once the data arrives, a variety of data acquisition/control software packages are available to perform everything from small project to full Supervisory Control and Data Acquisition (SCADA) strategies.
For example, a factory needed a lowcost way to monitor temperature and operating efficiency of 48 motors located throughout their plant. Because the operation was relatively small, a Distributed Control System (DCS) would be too costly. They wanted an affordable system that is controlled by a PC without sacrificing the high performance needed for their process.
They selected a system using six, 16- channel field multiplexing modules, 48 three-wire, 100-ohm platinum RTD transmitters, and 48 ac power transducers.
The field multiplexing modules collect motor temperature and power usage data from the RTDs and power transducers. Then, the modules convert the signals to an RS-485 protocol. The data is transmitted on a two-wire data link across the plant at 19,200 baud to a control room PC, which is equipped with an RS-485 input card. Once the data is delivered, the PC evaluates it using a small project software package.
Lori Risse is the senior applications engineer at Moore Industries-International Inc., Sepulveda, Calif.