Encore Systems Automation Blog

Replacement of Older Simatic TI 500/505 Series Power Supplies

2011-01-19T12:00:00.000-08:00

Many 2500 Series™ users have recently expressed concerns about aging Siemens 500 and 505 series power supplies in their facilities, and have asked if CTI has recommendations regarding power supply replacement.

CTI recommends considering replacement of PLC power supplies beginning at about 10 years of operation. This might seem like an unusual recommendation given the typical operating life of modern industrial electronic equipment. However, it is founded in well-understood science and in many years of experience with our customers.

Almost all DC power supplies employ some kind of energy storage on-board to smooth out voltage ripple on the outputs while in operation. In addition, PLC power supplies typically include an extraordinary amount of energy storage to allow the system to ride through short brownouts, and to allow the system time to do a safe shutdown in the event of power loss. For both technical and cost reasons, most manufacturers use electrolytic capacitors to provide the needed energy storage. This type of capacitor also offers better electrical performance and more energy storage per unit volume.

Power supplies from both CTI and Siemens/TI employ electrolytic capacitors for energy storage.

Why should you consider replacement of power supplies which use electrolytic capacitors after 10 years? Electrolytic capacitors use an electrolyte, an ionic conducting liquid, as one of its plates, to achieve a larger capacitance per unit volume than other types. Unfortunately, in all such capacitors the electrolyte degrades over time due to evaporation of water and resulting change in conductance. When the conductance changes outside the design parameters of the circuit the operation of the product is compromised.

What things affect the life of electrolytic capacitors?
The primary factors are temperature and voltage. A power supply operated near or above its rated temperature will have a shorter life on the capacitors. A power supply exposed to frequent overvoltage transients will have a shorter life. In addition, capacitors “on the shelf” (i.e. power supply in storage) have an accelerated degradation compared to power supplies operated at normal ratings. A secondary factor is operating the capacitor with high ripple currents – from extended operation of the power supply at or beyond its rated current.

What are symptoms of failing capacitors?
Lower hold-up time which results in nuisance shutdowns because of no brownout resistance. Increased ripple in power supply output which can result in unpredictable operation of modules on the backplane.

My power supplies are operating without incident. Why should I replace them?
Two reasons;

First, the biggest problem associated with reduced energy storage is reduced holdup time. You’ll never know that the energy storage of your power supply is degraded until you have a brownout and a resulting shutdown of your PLC system. Also, if the energy storage is not in specification the power supply will not provide the correct timing of the shutdown control signal to the PLC. Result: when power is restored, the PLC will not restart automatically but instead will come up with a fatal error.

Second, over time the voltage ripple on the power supply will increase, and at some point it will cause a problem with operation of the PLC system.

For a more in-depth discussion of electrolytic capacitor reliability, see “Reliability and Length of Life” in this Wikipedia article: http://en.wikipedia.org/wiki/Electrolytic_capacitor

I hope this explanation is helpful to understand the reason for this recommendation for replacement of power supplies. Experience has shown that customers who employ this kind of replacement strategy enjoy far fewer nuisance shutdowns due to power supply problems. In most plants, the increased reliability more than offsets the cost of this replacement. If you have any questions please do not hesitate to contact us.

Simatic is a registered trademark of Siemens AG.

PLC WorkShop for Siemens 505 Outperforms the DOS Alternative.

2008-01-17T08:57:00.000-08:00

Compared to the DOS alternative, FasTrak’s PLC WorkShop™ for CTI 2500 and Siemens 505 ™ offers a superior PLC programming experience. Because it supports all Windows® operating systems—from Windows 95 through Vista—WorkShop is highly accessible software offering exceptional ease of use. WorkShop also supports networked and laser printers, a feature that is not available in DOS programming software. Speed is provided with a full 32-bit Windows implementation, and all methods of communications are supported, including serial (COM1 –COM4), USB, Ethernet, H1, FMS, Modem, AutoBaud, and NITP.

Workshop 505 utilizes NT security management (Windows Authentication) with password protection, providing complete administrator control and auditing of all user operations. WorkShop also supports the entire functionalities of the latest PowerMath CTI 2500 and 555/575 CPUs.

WorkShop is also backed by a broad scope of customer support options. Should a question or issue arise, you may phone us to speak directly with a knowledgeable representative; or, visit us online for additional support resources. WorkShop is further supported with comprehensive training courses led by expert instructors. Our training solutions focus on practical instruction and hands-on learning to maximize on-the-job effectiveness.

Additionally, the CTI 2500 Series PLC System has features that are exclusive to WorkShop.

These features include:

Built-in Rung Table providing direct access to network mode, which allows real-time Cross Reference and ladder network documentation.

Direct IP address configuration allowing you to set IP through serial or USB ports.

Support for new CTI 2500 specific instructions.

Support for CAMP protocol to significantly increase communication speeds.

Full use of additional PLC memory.

Contact us at Encore Systems for more information.

Selecting Wired Industrial Ethernet Devices

2008-01-13T06:28:00.000-08:00

Selecting Wired Industrial Ethernet Devices (Switches, Hubs, PLC’s and NIC’s.

Understanding Devices (Switches, Hubs, PLC’s and Computer NIC’s

Many devices support two types of negotiation. The first is called "auto-negotiation." This protocol allows two interconnected Ethernet devices (for example an industrial switch and a SCADA node) to come to some agreement over several parameters used in their communications. One parameter they negotiate is the data rate to be used. This is generally 10 Mbps, 100 Mbps or 1Gbps. They negotiate the use of half- or full-duplex. Full-duplex allows communications to exist in both directions at the same time, while half-duplex only allows communications in one direction at a time. They also negotiate the use of flow control in their communications. If flow control is utilized, then either device can request that communications be halted if the device needs time to process received frames.

Products such as the Control Technology 2500 Automation Systems PLC's support Auto-MDIX (Auto-crossover). The two devices negotiate to decide which wire pair to use for transmitting packets and which wire pair to use for receiving packets. This feature is attractive when connecting two devices without a switch or hub as this would normally require the use of a special cross-over cable. With Auto-MDIX the two devices negotiate which wire pairs to use when communicating, and this allows the use of standard (straight-through) cables when connecting two or more devices.

Fiber- Optic or Twisted-Pair (Cat5e Copper)

Most Ethernet networks are cabled utilizing twisted-pair cabling. However, there are times when communications over fiber-optic cabling is required. Fiber –optic cabling is used when signals need to travel over greater distances than the 100 m supported by twisted-pair cabling. Most fiber-optic devices can communicate up to 2 km when using a full-duplex setting. Fiber-optic cables are also used in high noise environments because their communications are unaffected by electrical or magnetic fields.

Managed vs. Unmanaged Devices

One of the key questions in choosing an Industrial Ethernet device is whether to select a managed device or an unmanaged device. A managed device is generally more expensive than an unmanaged switch, however, with this additional cost, extra features are provided. A managed device is basically a switch that supports SNMP (Simple Network Management Protocol). Most managed switches provide features and benefits beyond SNMP.

Basically, a managed device allows you to control your network. An unmanaged device will simply allow Ethernet devices to communicate on a “party line”. You connect your Ethernet devices to the unmanaged device and they usually communicate automatically. Depending on the device, there may be status LEDs to give you some feedback regarding link activity; but this is generally all you get. With a managed device you will most likely have the same status LEDs, but the managed device will let you adjust your network settings and, via a web interface or the SNMP protocol, let you monitor the network performance in a variety of ways. For example, in industrial applications that communicate in high noise environments, it is sometimes advantageous to force the data rate to 10 Mbps because noise coupled into the cables may confuse the auto-negotiation process. Most managed devices will allow you to set the data rate of each port. These harsh environments can also benefit from disabling Auto-MDIX support since this negotiation can become confused by noise.

With a managed device you can also monitor the network health. You typically can view a multitude of network parameters. Some of these parameters include the number of bytes transmitted, timeouts, communication errors, bytes received; number of frames transmitted, received; number of packet errors and port status. All of this can be viewed on a port by port basis via a web server so that you can use a standard browser to view the network status. In addition, using the iSNMP protocol available from Kepware Technologies, the status of the entire Ethernet Network can be displayed on the HMI/SCADA screens utilizing packages such as Genesis 32 from Iconics. Most managed devices also offer advanced features that enhance your real time control of the network.

Advanced Device Features

Features such as Quality of Service (QoS), Trunking, Virtual Local Area Network (VLAN), port mirroring, fault relay, IGMP snooping, redundancy and SNMP are normally only found on managed devices.

Quality of Service

Quality of Service is the ability of the device to apply a higher priority to certain ports or packets. A switch can use the port on which the packet arrived to determine the priority (port QoS) or it can use a tag within the packet to determine its priority (IEEE 802.1p and 802.1Q). These features are useful in establishing determinism on Real Time Ethernet Remote I/O Networks.

Trunking

Trunking is two or more ports grouped together and acting as one logical path. This can be used to increase the bandwidth between two switches. Also, in some cases, this dual path can provide some redundancy. For example, if two 100 Mbps switches are interconnected by two cables, then the bandwidth between these two switches can be 200 Mbps.

VLAN

VLANs allow a switch to logically group devices and to isolate traffic between these groups even if the devices all share the same physical switch. For example, if the switch was being used for both SCADA communications and Multicast Peer to Peer communications, two VLANs could be created to isolate the SCADA communications from the multicast communications. Some switches also allow devices to be located on multiple VLANs. This is sometimes called overlapping VLANs. This can be used if one device, a SCADA system for example, needs to communicate to both the MES System and the PLC OPC Server , this device would then exist in both the office VLAN and the factory VLAN. This would isolate traffic between the remaining MES Systems and multicast devices, but allow the SCADA system to communicate on both networks.

Port Mirroring

Traffic isolation — one of the advantages of using a switch instead of a hub — also becomes a disadvantage when trying to debug a communications problem. A switch will only send frames to those devices in the conversation. This helps lessen network congestion. However, if you want to view all frames transmitted on the network, this feature becomes an issue. Many managed switches offer a feature called port mirroring. Port mirroring allows one port of the switch to monitor the traffic sent/received by one or more ports of the switch. With this feature a PC running a protocol analyzer program can capture traffic from one or many ports after port monitoring has been enabled. Protocol analyzers are popular problem-solving tools for Ethernet networks.

Fault Relay

Many switches offer a fault relay to monitor link status of specific ports or the power status of the switch. These dry contacts can then be connected to an input to the PLC and control system. This is useful if you want to alert the control system or SCADA/HMI package when communication to one or more Ethernet devices has failed.

IGMP Snooping

In Ethernet networks there are three types of frames — broadcast frames which are delivered to all devices in the network, directed frames which are sent to one specific device, and multicast frames which are sent to a group of one or more devices. Some Ethernet protocols utilize multicast frames to send data to multiple devices at the same time, such as peer to peer messaging on a PLC network. These protocols generally create a large amount of multicast traffic. Switches with IGMP (Internet Group Multicast Protocol) snooping can automatically send the multicast frames only to devices that have requested these frames. This keeps the multicast frames from flooding devices (broadcast storm) that have not requested these frames. Some devices may be unable to perform their normal activities when they receive a large broadcast storm of unwanted multicast traffic. This multicast filtering can be important in large EtherNet/IP networks.

Redundancy

Redundancy is a popular feature in managed Industrial Ethernet devices. Basically, they provide the ability to interconnect these devices in a manner such that if one interconnecting cable were to fail, another cable or set of cables would take over. The time in which this recovery takes place is called the recovery time. There are two popular IEEE redundancy standards, Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol (RSTP). STP (IEEE 802.1D) is the older, slower-to-recover protocol. RSTP (IEEE 802.1w) is the newer, faster-to-recover version of STP. STP generally can recover from a fault within 30–60 seconds. RSTP, generally, can recover in 1–2 seconds.

Because of these long recovery periods, many industrial switch vendors have created their own proprietary ring redundancy protocols. These can usually recover in less than 300 milliseconds. In these networks the switches are connected in a ring. These protocols generally select one switch-to-switch link in the network to be disabled. This is the backup link. When another switch-to-switch link fails, the backup link is enabled, thus repairing the network.

SNMP

SNMP is the Single Network Management Protocol that allows network management and SCADA applications to communicate with a device in a standardized way and request status information and set configurations. Most managed devices support this protocol and support similar data, so one application can communicate with multiple devices in a similar fashion. This allows one SCADA application to monitor multiple devices at the same time, thus providing a global view of the network status. Network management applications provide many features such as the ability to map the network, the ability to receive error frames from the devices, etc. There are also applications available from Kepware Technologies which convert SNMP data into OPC data for use in SCADA/HMI systems.

Protocol Required Features

In some cases industrial equipment vendors or industrial protocols require features only available on managed devices. For example, several EtherNet/IP I/O vendors recommend the use of devices that support IGMP snooping. Also, your hardware may suggest that your switch supports QoS or VLAN. Consult us for any suggested or required device features.

Harsh Environments

This is where Industrial Ethernet devices really differentiate themselves from commercial devices. Industrial Ethernet devices are designed for harsh environments that are not favorable to commercial devices. These environments include temperature extremes, high vibration and severe electrical noise. As commercial environments are generally room temperature, most commercial devices are designed for a very small temperature range. Also, some commercial devices utilize fans to help in cooling. This could be a problem in many industrial environments due to dust and dirt that could accumulate in the fans, not to mention the low MTBF of most rotating parts.

Environmental concerns can also be as simple as mounting and power supply issues. Most commercial devices are designed for 19-inch rack mounting or table top mounting, but this usually is not acceptable for many industrial environments. Normally, control panels utilize DIN-rail mounting or and require 24 VDC Power.

Future-Proofing

How do you future-proof a system? Let's say, for example, a small SCADA system was put together using an unmanaged switch. Initially, this may be an acceptable situation. However, as more Ethernet devices are added, the system becomes more complex and the need for a managed switch may become clearer. Having a managed switch in place initially may help future-proof your system so as more devices are added, you will retain the ability to fully control and monitor your network. Also, as you add Ethernet devices you may find that QoS can be useful to help prioritize the packets due to the increased traffic load of your network. You may also want to utilize VLANs to help isolate devices due to the increased number of SCADA nodes on your network.

In the future, Ethernet protocols may require QoS standards such as 802.1p/802.1Q to help achieve higher priority for packets traveling on the network. VLANs can be used to isolate devices which may be sensitive to the higher levels of traffic as the network grows in size. Having a device with these features now helps to future-proof your system.
If you are considering moving to EtherNet/IP networks in the future, it might make sense to use a switch that supports IGMP snooping. It is possible that the initial system is small and isolated from other networks and does not require the use of IGMP snooping, however, later it may become desirable to interconnect this system to the SCADA network. At this point, without IGMP snooping, you may have a large amount of multicast traffic being sent to the SCADA network. A device with IGMP snooping could help to block this traffic. Also, a device with VLAN support could be utilized to block this traffic from entering the SCADA network.

Determinism Issues

Ethernet devices can support a deterministic system. The question you need to ask is what type of response and jitter is acceptable in your system. If you send a packet from one device to another, what is the maximum response time and what is the maximum acceptable jitter? Most Ethernet devices utilize store-and-forward when processing a packet. This means that the entire packet is received and then re-transmitted. If you cascade several devices, the delay in storing-and-forwarding these packets increases for every switch. Also, each device has a small amount of internal latency which adds to this propagation delay.

If your system is communicating at 100 Mbps, then the time for the store-and-forward of each frame is generally on the order of 10 microseconds for small packets and 130 microseconds for maximum-sized packets. Also, the jitter is fairly small, approximately 1 microsecond per switch. This delay is generally very small compared to the latency of most Industrial TCP/IP devices. And protocols such as EtherNet/IP are adding IEEE 1588 (Precision Clock Synchronization Protocol for Networked Measurement and Control Systems) to help with synchronizing Ethernet connected devices.

What should I choose?

As you can see, there are many factors to consider when purchasing an Industrial Ethernet device. We have discussed a number of features found on many Industrial Ethernet devices. Please contact us at http://www.encore-sys.com to determine the features supported on various Industrial Ethernet devices.