Interfacing is a term used in electronics when different electronic devices are connected in order to move information from one device to another. This information may take the form of instructions for the function of a device, or feedback from a device about the results of executing a function. Laboratory automation devices function to either measure and/or act to control physical process. Electronic interfacing in laboratory automation spans a very wide range of sophistication and complexity, but in general does not tend to be cutting edge, simply because the demands placed on the interface are usually moderate. The information passed through an interface may be very simple, such as a two-state signal, high or low, open or closed. Or it may be a complex, binary stream of serial or parallel data. Data rates can range from hundreds to giga-bits of information per second.
This article will present information about the various types of electronic interfaces commonly used in laboratory automation.
Electrical quantities of voltage and current are easy to measure, manipulate, and transmit over long distances, so they are widely used in analog form to represent physical variables and transmit information to/from remote locations. An analog signal refers to an electrical quantity of voltage or current, used to represent or signify some other physical quantity. The signal is continuously variable, as opposed to having a limited number of steps along its range (digital).
Voltage based signals
DC voltage can be used as an analog signal to relay information from one location to another. Circuits can be designed allowing the DC voltage level at the signal source to be an indicator of the physical state of a device, or a measurement. One common signal standard is 0-10 volts, meaning that a signal of 0 volts represents 0 % of measurement, 10 volts represents 100 % of measurement, and 5 volts represents 50 % of measurement. A more common voltage range is 1-5 volts, with 1 volt representing 0% measurement, 5 volts representing 100% of measurement and <1 volt indicating a circuit
Current based signals
It is possible through the use of electronic amplifiers to design a circuit outputting a constant amount of current rather than a constant amount of voltage. Current sources can be built as variable devices, just like voltage sources, and they can be designed to produce very precise amounts of current as an indicator of the physical state of a device or a measurement. The most common current signal standard in modern use is the 4 to 20 milliamp (4-20 mA) loop, with 4 milliamps representing 0 percent of measurement, 20 milliamps representing 100 percent, and 12 milliamps representing 50 percent. A convenient feature of the 4-20 mA standard is its ease of signal conversion to 1-5 volt indicating instruments. A 250 ohm precision resistor connected in series with the circuit will produce 1 volt of drop at 4 milliamps, 5 volts of drop at 20 milliamps. As opposed to a voltage based signals, a current "loop" circuit relies on the series circuit principle of current being equal through all components to insure no signal error due to wiring resistance.
Analog current loops are used where a device must be monitored or controlled remotely over a pair of conductors. Analog 4-20 mA current loops are commonly used for signaling in industrial process control instruments, especially when signaling must occur over long distances (1000 feet or more). The key advantages of the current loop are that the accuracy of the signal is not affected by voltage drop in the interconnecting wiring, and that the loop can supply operating power to the device. Such instruments are used to measure pressure, temperature, flow, pH or other process variables. A current loop can also be used to control a valve positioner or other output actuator. A commonly used control device called a programmable logic controller, or a PLC, is used to read a set of digital and analog inputs, apply a set of logic statements, and generate a set of analog and digital outputs.
Contact closure, TTL I/O
Already in the binary language of computers, these types of digital, or discrete, inputs and outputs (I/O) are much easier for microprocessor-based data acquisition systems to deal with than analog signals. Digital I/O is designed to deal directly with Transistor-to-Transistor Logic (TTL) level voltage changes. TTL typically sets the low voltage level between 0 and 0.8 V and the high voltage level between 2.0 and 5.0 V. Voltage levels between 0.8 and 2.0 V are not allowed. A voltage change, then, from the high range to the low range (or vice versa) represents a digital change of state from high to low, on to off, etc.
- The most common type of digital input is the contact, or switch closure. A sensor or switch of some type closes or opens a set of contacts in accordance with some process change. An applied electrical signal then determines whether the circuit is open or closed. Current flows if the circuit is closed, registering a "1" at the computer interface circuitry. Conversely, an open circuit retains a high voltage (and no current), registering a "0" at the computer interface circuitry. Laboratory automation systems often use switch closures to provide input regarding mechanical positioning of a device, and are particularly useful in error detection.
- A simple digital output provides a means of turning something on or off. Applications range from driving a relay to turning on an indicator lamp to transmitting data to another
Digital I/O cards
- Data I/O cards that plug into the chassis, or the USB, RS-232 or GPIB port of a desktop PC or a PC-compatible industrial computer have made measurement and control extremely economical for the typical lab automation user. Many types of analog and digital signals can be input or output directly by
Serial data is any data that is sent one bit at a time using a single electrical signal. In a serial interface the serial port takes 8, 16, 32 or 64 parallel bits from the computer bus and converts it as an 8, 16, 32 or 64 bit serial stream. In contrast, parallel data is sent 8, 16, 32, or 64 bits at a time using a signal line for each bit. In theory a serial link needs only two wires, a signal line and a ground, to move the serial signal from one location to another. But in practice this doesn't really work for very long, as some bits get lost in the signal and alter the ending result. If one bit is missing at the receiving end, all succeeding bits are shifted resulting in incorrect data when converted back to a parallel signal. Two serial transmission methods are used that correct serial bit errors. With synchronous communication, the sending and receiving ends of the communication are synchronized using a clock that precisely times the period separating each bit. By checking the clock the receiving end can determine if a bit is missing or if an extra bit (usually electrically induced) has been introduced in the stream. The alternative method (used in PCs) is to add markers within the bit stream to help track each data bit. By introducing a start bit which indicates the start of a short
Serial communication is used for all long-haul communication and most computer networks, where the cost of cable and synchronization difficulties make parallel communication
|Serial Interface Quick Facts|
|Number of Device Connections||Maximum Distance|
|RS232||20 Kbps||1||50 feet @ 20Kbps, 3000 feet @ 2.4Kbps|
|RS485||10 Mbps||32||4000 feet|
|CAN||1 Mbps||30||328 feet @ 12 Mbps, 3330' @ 1 Mbps|
|USB 2.0||480 Mbps||127||10 feet (up to 60' with hubs)|
|IEEE1394c||3200 Mbps||63||15 feet (250' with repeaters)|
|Ethernet||10 Gbps||255||328 feet (longer with hubs, switches)|
The 2008 ALA Survey of Technology Providers asked a dozen prominent laboratory automation technology providers about the electronic interfaces used in their products. The results indicate that RS-232 still is the most widely used electronic interface, with Ethernet a close second. Projections for 5 years out show Ethernet becoming the dominating interface technology. It should be noted, however, that the demise of RS-232 has long been predicted, but has yet to happen.
|2008 ALA Survey of Technology Providers|
|Interface||% of devices using interface in 2008||% of devices using interface in 2013 (predicted)|
|RS-232 / RS-485||45%||31%|
|Bluetooth / Wireless||8%||21%|
The Electronics Industries Association (EIA) recommended standard RS-232-C is a standard originally devised for serial binary data signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment) in 1969. It has since become commonly used in computer serial ports. Since 1969, manufacturers adopted simplified versions of this interface for applications that were impossible to envision in the 1960s. The current
RS232 is still widely used in laboratory automation, as well as other industries. For laboratory automation systems, the interface provides sufficient speed for most interface needs and is simple and low cost. One drawback is the ability to connect only one device per RS232 port. Relatively inexpensive port expander cards or external expansion boxes are readily available to overcome this limitation. Another limitation is that most current default PC configurations do not include an RS-232 interface port. Adapters are available to convert newer serial ports (e.g. USB) to RS-232 outputs. A major drawback is and always will be the lack of true standardization.
In telecommunications, EIA-485 (formerly RS-485 or RS485) is an electrical specification of a two-wire, half-duplex, multipoint serial communications channel. The architectural difference between
EIA-485 is used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices, such as surveillance cameras or card readers. It has minimal presence in laboratory automation.
Universal Serial Bus (USB)
The Universal Serial Bus (USB) is a serial bus standard to interface devices to a host computer. USB was designed to replace many legacy serial and parallel interfaces and allow many peripherals to be connected using a single standardized interface socket. The design of USB is standardized by the USB Implementers Forum (USB-IF), an industry standards body formed in 1995 to support and accelerate market and consumer adoption of USB-compliant peripherals. The USB 1.0 specification model was introduced in November 1995. The current (as of 2008) USB 2.0 specification, with a design data rate of 480 megabits per second, was released in April 2000 and was standardized by the USB-IF at the end of 2001. A draft specification for USB 3.0 (data rate of 4.8 Gbit/s (600 MB/s)) was released by Intel and its partners in August 2008. According to Intel, bus speeds will be 10 times faster than USB 2.0 due to the inclusion of a fibre-optic link that works with traditional copper connectors. Products using the 3.0 specification are likely to arrive in 2009 or 2010.
Despite becoming the interface of choice for consumer PC peripherals, USB is not highly popular in laboratory automation devices. It is most commonly found in simple devices, such as balances, whose primary market is stand-alone bench use, rather than integration into automation systems. The cable length limits without using hubs may be a contributing factor.
The IEEE 1394 interface is a serial bus interface standard for high-speed communications and isochronous real-time data transfer, frequently used in a personal computer, digital audio and digital video. The 1394 digital link standard was conceived in 1986 by technologists at Apple Computer, who chose the trademark 'FireWire', in reference to its speeds of operation. The first specification for this link was completed in 1987. It was adopted in 1995 as the IEEE 1394 standard. The interface is also known by the brand names of i.LINK (Sony) and Lynx (Texas Instruments). FireWire can connect up to 63 peripherals in a tree topology and allows peer-to-peer device communication — such as communication between a scanner and a printer — to take place without using system memory or the CPU. The newer, current IEEE 1394b (Fire standard was introduced commercially by Apple in 2003. It allows a transfer rate of 786.432 Mbit/s full-duplex via 9 pin connector, and is backwards compatible to the 6 pin configuration and slower data rates of the original IEEE 1394a standard. The full IEEE 1394b specification supports data rates up to 3200 Mbit/s over beta-mode or optical connections up to 100 metres in length. In December 2007, the 1394 Trade Association announced that products will be available before the end of 2008 using the S1600 and S3200 modes that, for the most part, had already been defined in 1394b. The 1.6 Gbit/s and 3.2 Gbit/s devices will use the same 9-pin connectors as the existing FireWire 800 and will be fully compatible with existing S400 and S800 devices. It will compete with the forthcoming USB 3.0.. IEEE 1394b is used in military aircraft and automobiles, where weight savings are important, as well as with computer vision and digital video cameras.
The most common usage of IEEE-1394 interfacing in the laboratory is with high data density imaging devices.
Controller Area Network (CAN)
The controller-area network (CAN or CAN-bus) is a computer network protocol and bus standard designed to allow microcontrollers and devices to communicate with each other without a host computer. It was designed specifically for automotive applications but is now also used in other embedded control applications . Development of the CAN-bus started originally in 1983 at Robert Bosch GmbH. The protocol was officially released in 1986 at the SAE (Society of Automotive Engineers) congress in Detroit. The first CAN controller chips, produced by Intel and Philips, came on the market in 1987. Bosch published the CAN 2.0 specification in 1991. CAN is a broadcast, differential serial bus standard for connecting electronic control units (ECUs). Each node is able to send and receive messages, but not simultaneously. The devices that are connected by a CAN network are typically sensors, actuators and control devices. A CAN message never reaches these devices directly, but instead a host-processor and a CAN Controller is needed between these devices and the bus.
CAN technology is used in automobiles, industrial control applications, and in some medical and laboratory instrumentation (Agilent, Beckman-Coulter). It is more often used as embedded control approachnot vs. a user-accessible general purpose interface methodology, such as RS-232 or USB.
A parallel interface is a link between two devices in which all the information transferred between them is transmitted simultaneously over separate conductors. Also known as parallel port. Before the development of high-speed serial technologies, the choice of parallel links over serial links was driven by these factors:
- Speed: Superficially, the speed of a parallel data link is equal to the number of bits sent at one time times the bit rate of each individual path; doubling the number of bits sent at once doubles the data rate. In practice, skew reduces the speed of every link to the slowest of all of the links.
- Cable length: Crosstalk creates interference between the parallel lines, and the effect worsens with the length of the communication link. This places an upper limit on the length of a parallel data connection that is usually shorter than a serial connection.
- Complexity: Parallel data links are easily implemented in hardware, making them a logical choice. Creating a parallel port in a computer system is relatively simple, requiring only a latch to copy data onto a data bus. In contrast, most serial communication must first be converted back into parallel form by a Universal asynchronous receiver transmitter before they may be directly connected to a data bus.
The decreasing cost of integrated circuits, combined with greater consumer demand for speed and cable length, has led to parallel communication links becoming deprecated in favor of serial links; for example, IEEE 1284 printer ports vs. USB, Advanced Technology Attachment (ATA) vs. Serial ATA, SCSI vs. FireWire (IEEE-1394).
An early parallel interface was developed by Dr. An Wang, Robert Howard and Prentice Robinson at Wang Laboratories. The now-familiar connector was selected because Wang had a surplus stock of 20,000 Amphenol 36-pin micro ribbon connectors that were originally used for one of their early calculators. The Centronics Model 101 printer was introduced in 1970 and included this parallel interface for printers. The Centronics parallel interface quickly became a de facto industry standard.The IEEE 1284 standard superseded the Centronics interface in 1994. The IEEE 1284 standard allows for faster throughput and bidirectional data flow with a theoretical maximum throughput of 4 megabits per second, with actual throughput around 2 megabits, depending on hardware. The parallel interface remains highly popular in the printer industry, with displacement by USB only in consumer models.
IEEE-1284 interfaces have been used in the laboratory automation environment primarily for to interface imaging or printing devices (scanners, bar code printers). In many cases, the USB interface has now been added to such devices or has replaced the parallel interface.
The IEEE-488 is a short-range, digital communications bus specification. The IEEE-488 bus was developed to connect and control programmable instruments, and to provide a standard interface for communication between instruments from different sources, and is still widely used for test and measurement equipment today. Hewlett-Packard originally developed the interfacing technique, and called it HP-IB. The interface quickly gained popularity in the computer industry. Because the interface was so versatile, the IEEE committee renamed it GPIB (General Purpose interface Bus). In 1975 the bus was standardized by the Institute of Electrical and Electronics Engineers as the IEEE Standard Digital Interface for Programmable Instrumentation, IEEE-488-1975 (now 488.1). IEEE-488.1 formalized the mechanical, electrical, and basic protocol parameters of GPIB, but said nothing about the format of commands or data. .
The IEEE-488 connector has 24 pins. The bus employs 16 signal lines — eight bi-directional used for data transfer, three for handshake, and five for bus management — plus eight ground return lines. The maximum data rate is about 8 Mbyte/s in the latest versions.
The IEE-488 interface is still a standard feature of laboratory instruments built by Agilent (formerly Hewlett-Packard). Few, if any, other laboratory equipment providers have adopted the interface.
Small Computer System Interface , or SCSI (skuh-zee), is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it can connect a wide range of other devices, including scanners and CD drives. The SCSI standard defines command sets for specific peripheral device types; the presence of "unknown" as one of these types means that in theory it can be used as an interface to almost any device, but the standard is highly pragmatic and addressed toward commercial requirements. SCSI is based on an older, proprietary bus interface called Shugart Associates System Interface (SASI). SASI was originally developed in 1981 by Shugart Associates in conjunction with NCR Corporation. In 1986, the American National Standards Institute (ANSI) ratified SCSI (pronounced "scuzzy"), a modified version of SASI.
SCSI has several benefits. It supports speeds up to 320 megabytes per second (MBps). It's been around for more than 20 years and it's been thoroughly tested, so it has a reputation for being reliable. Like Serial ATA and FireWire, it lets you put multiple items on one bus. SCSI also works with most computer systems. However, SCSI also has some potential problems. It has limited system BIOS support, and it has to be configured for each computer. There's also no common SCSI software interface. Finally, all the different SCSI types have different speeds, bus widths and connectors.
SCSI is often used to control a redundant array of independent discs (RAID). Other technologies, like serial-ATA (SATA), have largely replaced it in new systems, but SCSI is still in use. Newer SATA drives tend to be faster and cheaper than SCSI drives.
Cables and connectors
A SCSI controller coordinates between all of the other devices on the SCSI bus and the computer. Also called a host adapter, the controller can be a card that you plug into an available slot or it can be built into the motherboard. The SCSI BIOS is also on the controller. This is a small ROM or Flash memory chip that contains the software needed to access and control the devices on the bus. Each SCSI device must have a unique identifier (ID) in order for it to work properly. For example, if the bus can support sixteen devices, their IDs, specified through a hardware or software setting, range from zero to 15. The SCSI controller itself must use one of the IDs, typically the highest one, leaving room for 15 other devices on the bus.
Internal devices connect to a SCSI controller with a ribbon cable. External SCSI devices attach to the controller in a daisy chain using a thick, round cable. (Serial Attached SCSI devices use SATA cables.) In a daisy chain, each device connects to the next one in line. For this reason, external SCSI devices typically have two SCSI connectors -- one to connect to the previous device in the chain, and the other to connect to the next device. The cable itself typically consists of three layers:
- Inner layer: The most protected layer, this contains the actual data being sent.
- Media layer: Contains the wires that send control commands to the device.
- Outer layer: Includes wires that carry parity information, which ensures that the data is correct.
Different SCSI variations use different connectors, which are often incompatible with one another. These connectors usually use 50, 68 or 80 pins. SAS uses smaller, SATA-compatible connectors. If the SCSI bus were left open, electrical signals sent down the bus could reflect back and interfere with communication between devices and the SCSI controller. The solution is to terminate the bus, closing each end with a resistor circuit. If the bus supports both internal and external devices, then the last device on each series must be terminated.
Types of SCSI termination can be grouped into two main categories: passive and active. Passive termination is typically used for SCSI systems that run at the standard clock speed and have a distance of less than 3 feet (1 m) from the devices to the controller. Active termination is used for Fast SCSI systems or systems with devices that are more than 3 feet (1 m) from the SCSI controller.
The SCSI interface has primarily been used for the interface of high data rate devices, such as disk drives, both internal and external to the computer. In laboratory equipment, it is most often used as an interface for digital imaging devices.
Main Article on Network interfaces
A network is a collection of computers or devices connected to each other with the ability to exchange data and or share resources. A network interface is the point of interconnection between a computer and a private or public network. Networks operate using serial communication. Examples of different networks are:
- Local area network (LAN), which is usually a small network constrained to a small geographic area. Computers and devices are linked via Ethernet Cable, can be joined either directly or via a network router that allows multiple connections.
- Wide area network (WAN) that is usually a larger network that covers a large geographic area. The largest and best example of a WAN is the Internet, which is a network comprised of many smaller networks. The Internet is considered the largest network in the world.. The PSTN (Public Switched Telephone Network) also is an extremely large network that is converging to use Internet technologies, although not necessarily through the public Internet.
- Wireless LANs and WANs (WLAN & WWAN) are the wireless equivalent of the LAN and WAN.
- An intranet is a private network within an organization that uses the same communications protocols as the Internet. When part of an intranet is made accessible to suppliers, customers or others outside the organization, that part becomes an extranet.
- An internet (spelled with a lower case i) is a network that is composed of a number of smaller computer networks. The Internet (spelled with an upper case I) is the world-wide network of interconnected internets that operates using a standardized set of communications protocols called TCP/IP (transmission control protocol/Internet protocol), or the Internet protocol suite. This ultimate internet is vastly larger than any other internet and connects thousands of networks and hundreds of millions of computers throughout the world.
All networks are interconnected to allow communication with a variety of different kinds of media, including twisted-pair copper wire cable, coaxial cable, optical fiber, and various wireless technologies. The devices can be separated by a few meters (e.g. via Bluetooth) or nearly unlimited distances (e.g. via the interconnections of the Internet).
Ethernet is a physical and data link layer technology for local area networks (LANs). Ethernet is standardized as IEEE 802.3. The combination of the twisted pair versions of Ethernet for connecting end systems to the network, along with the fiber optic versions for site backbones, is the most widespread wired LAN technology. When first widely deployed in the 1980s, Ethernet supported a maximum theoretical
Ethernet uses 8 position modular connectors (8P8C), often called RJ45, based on the telecommunications standard TIA/EIA-568-B, which defines a hierarchical cable system architecture The cables usually used are four-pair or above twisted pair cable. Each of the three standards support both full-duplex and half-duplex communication. According to the standards, they all operate over distances of 'up to 100 meters'.
Ethernet interfaces are becoming more common in laboratory automation and instrumentation, although still not approaching the level of use of RS-232. Most laboratory instruments do not need the speed offered by Ethernet, but the ease of use and standardization of the modular connectors make the interface appealing. The cost of including an Ethernet
- ↑ All About Circuits Voltage Signal Systems
- ↑ All About Circuits Current Signal Systems
- ↑ Datel Application Note A 4-20mA Current Loop Primer
- ↑ Electronics Industries Association, "EIA Standard RS-232-C Interface Between Data Terminal Equipment and Data Communication Equipment Employing Serial Data Interchange", August 1969, reprinted in Telebyte Technology Data Communication Library, Greenlawn NY, 1985, no ISBN
- ↑ USB 2.0 Specification USB.org
- ↑ A first look at USB 3.0 Gizmodo
- ↑ CAN 2.0 Specification BOSCH
- ↑ Agilent 1200 Series Control Module Agilent
- ↑ Biomek FX product brochure Beckman Coulter
- ↑ How SCSI Works How Stuff Works
- ↑ Computer network definition linux.org
- ↑ The Internet - Changing the way we communicate National Science Foundation
- ↑ Connect your titrator to the ethernet Mettler Toledo
- ↑ dotLINK: Network capabilities for GC and HPLC PerkinElmer
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