Down at the Bit Level

Discrete manufacturing and some traditional process areas depend on vast numbers of small bit-level devices to monitor and perform critical functions. These devices do not generate scalar data, but simple on-off duties. Normally these are connected to PLCs using hard-wired I/O, but users can employ some of the same wire and labor saving methods that instrumentation engineers use, especially fieldbus-like architecture.

Bit-level sensors include a huge range of products and functions. The common element is that they have a simple binary function of on or off. They include:

• Limit switches;

• Proximity sensors;

• Photo sensors;

• Pressure or flow switches;

• Current sensors, etc.

Bit-level actuators also have on or off functions:

• Motor starters;

• Relays;

• Solenoid valves, etc.

Bit-level safety devices include:

• Light curtains;

• E-stop buttons;

• Safety gates, etc.

The traditional method of wiring bit-level devices was, and often still is, hard wiring single devices to an I/O card (or relay cabinet in electromechanical systems) each with its own cable. This works but is wire and labor intensive. Nevertheless, this method is still the most deterministic and eliminates any external sources of latency. It is also suited to small installations where the population of devices is relatively low. Various wiring companies offer specialized cabling and connectors that simplify the task, but ultimately each device has to have its own wire running back to the I/O point. Hard-wired systems are reliable, but when there is a problem there are no diagnostic capabilities to assist with troubleshooting.

There are networking architectures that use wiring topologies similar to an instrumentation fieldbus that specialize in bit-level devices and the type of data they process. These bit-level bus networks reduce wiring and add troubleshooting capabilities that hard-wired systems cannot match. If you are considering such an approach, there are two main open systems from which to choose: AS-Interface and CompoNet.

AS-Interface has proven versatility

Since its introduction in 1994, AS-Interface has established itself as the most heavily used bit-level bus and has become a virtual standard due to its ubiquity. It is known for its simplicity, adaptability and exceptionally wide range of available products. This can be attributed possibly to the large group of suppliers and users that was involved in its design.

Dr. Helge Hornis, a member of the AS-Interface board and manager of the Intelligent Systems Group at Pepperl+Fuchs, says, “There are four things that drive AS-Interface applications: it is a networking technology, it is a hardware technology, it is a safety solution, and it is an installation technology. All the elements are thought through for real-world use. A technician has everything required to make complete applications easily and economically.”

Bit-level networks can integrate a mix of sensing and actuating devices. AS-Interface can also include safety elements.

The system architecture is very simple conceptually but serves critical functions:

• A scanner interfaces with a PLC or other control device to form a segment.

• A single two-wire cable carries power and signals to device modules. The cable has a distinct shape to assist in maintaining correct polarity throughout the installation.

• The wiring topology can take effectively any shape with drops in any configuration as long as the total cable length per segment does not exceed the maximum allowed value. Large networks are constructed by combining individual segments.

• Special self-piercing connectors and terminal blocks make wiring simple and positive.

• Each device is assigned an address. This can be done using a handheld addressing unit, a PC or function keys on most scanners or gateways. The scanner addresses the devices in order. There is no other software or programming necessary.

• The normal scan interval is 5 ms. The scan time for all devices and the per-device packet update time is about 150 microseconds. (It is possible to build devices that transmit larger amounts of data by sending this information in multiple packets. While it is still easy to calculate the deterministic update time of the full network, it is not 5 ms under all circumstances. For example, assume there are 62 I/O nodes with 4 inputs and 4 outputs each. The full update time of the network for all 248 inputs is 10 ms. The 248 outputs are updated in 20 ms.)

• Individual generic devices do not connect directly to the bus cable. At each point where there is a device or cluster, a device module is installed on the cable with connections for (typically) four input and four output devices. Input devices send signals (sensors) and output devices receive signals (actuators). Under the newest specification, modules that support extended addressing are offered and have 4 in and 4 out. This is possible since even very old scanners and gateways can communicate with them; simply do not address them to a B-address and things work as before.

• There are devices with an embedded decoding chip that are designed for direct connection to the bus cable without a device module. These includes encoders, photoelectric and inductive proximity sensors, stack lights, e-stops, motor starters, pneumatic modules, valve position sensors, push button modules, etc.

• The maximum power available to each segment is 8 A.

• The maximum cable length per segment is 100 m (300 ft.) and multiple segments can be combined using repeaters.

AS-i can process analog signals (16 bit value) and encoders with 16 bit resolution in place of bit-level devices. (Bear in mind that this is not the designed purpose of the system. Users who have extensive populations of these devices should explore other networking options.) These devices must be equipped with an embedded AS-i chip and data buffer. The value is held in the device and sent back over sequential scans to be reassembled at the gateway.

Gateways are available that interface AS-i with virtually any higher level system or industrial network. This allows it to be an extension of an Ethernet protocol or instrumentation fieldbus down to bit-level devices.

AS-Interface was the first platform approved for dual-use applications, allowing operating and safety functions on the same network. This dates back to 1997, so there is a large installed base of safety networks and a wide variety of hardware available.

CompoNet for high speed

A relative newcomer to the ODVA networking group is CompoNet. ODVA supports a family of related networking protocols, including DeviceNet, ControlNet and EtherNet/IP. These applications all share CIP (common industrial protocol) network services, which are designed to connect the plant and enterprise. CompoNet is the latest offering and has been available for about one year. Omron was heavily involved in its development, but it is now an open standard within the ODVA group.

While AS-Interface strives for simplicity and ease of use, CompoNet adds both a higher level of performance and complexity. Omron aimed this at machine builders, particularly high speed electronic device assembly, although the same objectives can apply to other types of discrete manufacturing. The key elements are high-speed, high-capacity, and flexibility.

“The Japanese market has been longing for a high speed sensor and actuator network that incorporates advanced configuration and diagnostics capabilities,” says Hiroyuki Usui, chair of ODVA’s territory alliance group in Japan. “CompoNet will meet this need and is expected to gain rapid widespread adoption. As the newest member of the family of CIP Networks, CompoNet promises to become a de facto standard for sensor and actuator networks, joining DeviceNet, the number one fieldbus in the Japanese market, and EtherNet/IP, one of the fastest growing industrial Ethernet networks in Asia.”

Basic operating elements include:

• A segment can control up to 256 bit-level devices (128 input nodes and 128 output nodes) and 128 word device I/O (64 input nodes and 64 output nodes).

• I/O is typically bit level, although analog information can be sent with 16 bit value.

• Running at its highest speed, the gateway scans all devices (up to 1000 I/O points) every 1 ms. However, at this speed, cable length is restricted. For networks that are physically spread out, slower speeds are necessary.

• Flat four-conductor cable, coded to work with specialized, self-piercing connectors simplifies wiring tasks.

• Wiring topologies are flexible using flat cable.

• Device modules adjust to the gateway’s baud rate.

• Cable lengths are dependent on communication speed, ranging from 30 m to 200 m maximum using flat cable. Distances can be extended with repeaters.

• Power to devices is optional, 24 Vdc, 5 A max.

• System programming is via a computer using CompoNet software. This also supports troubleshooting.

• Devices are available from Omron, although there are few other manufacturers so far.

• CompoNet offers no safety functions at present.

“CompoNet is targeted at builders of high-speed machines, such as conveyors and electronic assembly equipment,” says Katherine Voss, executive director, ODVA. “These users need networks that deliver high speed, deterministic performance for highly distributed control of simple I/O devices combined with a network architecture that adds little ‘design and build’ overhead for device connection, the network cable or overall installation. CompoNet’s addition to the family of CIP Networks rounds out the application coverage of CIP and delivers the future-proof technology benefits of CIP for those users who seek a true bit-level network.”

Making a choice

CompoNet has some catching-up to do to achieve AS-Interface’s level of adoption. While this article is not intended to be a side-by-side comparison of the two platforms, some points do tend to stand out:

Speed: CompoNet can be faster with 1 ms scan intervals (if cabling restrictions are observed), compared to AS-Interface’s 5 ms.

Connectivity: The two are similar in regard to the number of devices that can be assigned to a given segment.

Simplicity: AS-Interface probably has a small advantage, but in many respects the two are comparable in wiring topology, use of repeaters, etc.

Safety: AS-Interface has safety capabilities approved to category 4 that are widely applied and many available products. CompoNet has no safety functions.

Integration: Both systems have wide connectivity to higher level networks, but AS-Interface has more gateways from which to choose. Users who like CIP based networks may be drawn to CompoNet, however AS-Interface has gateways for those networks as well.

Availability: AS-Interface has a larger range of products and vendors available at this time.

Bit-level applications are an excellent point to explore device level networking if you have never given it a try. The versatility these systems offer may convince you that hard-wired installations are the harder way to solve machine building challenges.

Author Information

Peter Welander is process industries editor. Reach him at PWelander@cfemedia.com .

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