Computer Solutions
Fig 4. Thin client cuts the amount of hardware needed at the operator’s end.
have to replace that one, rather than the entire system.” Chapman responded to Merritt’s comments, saying
VXWorks is C code. “If they wrote a real time system in C, that is probably what they were using,” he said. “It is really a neat system. It creates very small programs. It even re-compiles the operating system (Unix/Linux) to optimise and minimise it for running just that program. “Try this in Windows. Go ahead and try to wipe FreeCell
off your SCADA system. You can hide it, but you cannot get rid of it. I have never had the pleasure (or pain) of using Matlab for writing real time control software. I think what it is really good for is modelling systems, and developing control algorithms from the simulated models. But once you have decided on the control method, it should be hard coded in a real-time operating system in my personal opinion.” Dick Caro, computer networking consultant and owner
of CMC Associates, argues that wired networks rarely suffer from failure of the network components, but can fail due to mechanical problems like the proverbial forklift truck that tears up the wiring. Wireless networks, particularly mesh networks such as ISA100 and Wireless HART have an in-built resilience path.
User experience
Caro says: “User experience has proven the reliability of Foundation Fieldbus networks, where most failures occur in the I/O cards, not the network. Foundation Fieldbus HSE has built-in resilience if you install parallel Ethernet signal paths, one of which can be Wi-Fi, but HSE is rarely used because most DCS suppliers do not support it. I advise my clients to be highly selective in installing resilient networks to only those situations where failure is likely.” Kohli maintains that Ethernet IP is not a good candidate
for robustness or resilience. Ethernet IP can be broken too easily, unless engineers take extreme precautions in designing and maintaining the network. They need to use industrial managed switches, and routers with special recovery protocols, not just spanning tree. “With normal switches, recovery from a ring failure can
take as much a two minutes in large networks,” assert Kohli. “Most industrial switch companies use custom algorithms that can take as less than 100ms for the same network. This is a big deal when that vat of molten steel is starting to pour. “In addition to using ring networks, the higher reliability
networks need dual paths, which in a wire system need a dual homing switch. This equipment can be just as expensive as the dedicated buses. Ethernet networks are also sensitive to loads. While you may have +250 addresses available, most will not support nearly that many, and then you need to be concerned with things like vision systems and other peripherals that may be connected requiring separate networks or load balancing. “There is a third problem that all Ethernet networks face,
like Modbus TCP/IP, ProfiNet, CCLink over Ethernet, and so on: that is that anyone, from the guy with the laptop to the corporate IT Manager, can connect to your network with the right knowledge or permission, and break it. This requires additional safety measures that the engineer may not have due to company policies. “Most systems allow things like MAC filtering or static IP
address, but often those are controlled by the IT Department. A better way is to use networks like Rockwell Automation ControlNet, which can handle the faults and still talk to the Ethernet network via CIP. I think it’s great that we can use inexpensive networks, but remember they are inexpensive for a reason, and features cost.”
Redundant fieldbus expensive
Merritt retorts that the problem with fieldbus networks is that they are so expensive and redundant fieldbus segments are even more so. He believes that is why Ethernet IP will be replacing fieldbus in the not-so-distant future, citing Rockwell Automation, Endress+Hauser and other vendors who are using Ethernet IP for process instrumentation. Merritt adds: “Sure, fieldbus cuts down on the number of
wires needed for wiring 4-20mA instrumentation, but you still have all those expensive device couplers, fieldbus I/O cards, and software from six big vendors to deal with. Then you have
www.engineerlive.com 57
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68