Lynn's Industrial Protocols over IP

Monday, January 29, 2007

Do you need a special Industrial Ethernet hardware?

This is a bit off topic, but someone asked me about “Industrial-Grade” Ethernet designs. Well, I’m not a hardware expert but have better than average exposure to issues of grounding and surge management. Plus I’ve been involved in many detailed discussions about how things may break Ethernet devices with engineers from Rockwell and other industrial suppliers. Bottomline is standard Ethernet designs are quite robust - the main thing I see hurting careless designers is PCB via or tracks violating the 1500v isolation of the Ethernet magnetics. This means a customer actually testing high-voltage noise or surges could see arcing across the isolation barrier which causes product reboots or even failure.

The $9.95 Mindset:
Any detailed discussion about “special Ethernet for Industry” first starts with the fact that customers can buy 10/100 computer Ethernet adapters from any big-box store for $9.95. So users have this perception that the increment for Ethernet is small & cheap. While they may not expect you to sell Ethernet products for $10 more than serial products, they won’t be happy to hear that your Ethernet product is $300 more. I will tie this together below, but the bottom line is the closer you can match your Ethernet hardware to the market norm, the lower your over all costs will be.

Who Pays for Extra Software Work?
Of course, that $9.95 computer Ethernet card doesn’t include:
  1. Microsoft’s ROI on TCP/IP and network stack work
  2. the OPC server cost to add Ethernet drivers in place of serial drivers
  3. tool vendors need (ie: your need) to rewrite serial-based tools to become network-based tools.
Plus don’t forget that the customers are NOT really just asking for “Ethernet” – Customer X will want web pages for configuration, Customer Y will want SNMP for remote management, and Customer Z will want strong 128-bit AES encryption suitable for general US-Government usage. So unlike the $9.95 PC card, you must hope your customers help pay for a whole lot of added, ongoing engineer work.

What is the Market Supply Sweet-Spot?
Go online and look at the cost of hard-drives – a 300GB (300,000MB) drive is in the $75 range, while an old 20MB drive (Meg, not Gig) costs about $140. We all understand this oddity – there is high demand for 300GB drives and virtually no demand for old legacy 20MB drives needed for repair. Even trying to buy a 40GB (Gig) drive today is hard. The market has what people call a “sweet spot” – a range of product features and capacity which is the cheapest and easiest to buy. Product builders trying to use components that are better than (or even worse-than) the market sweet-spot have disproportionally higher costs than builders using components in sync with the market sweet-spot.

The same thing happens for Ethernet components – for example buying magnetics rated at 1500v isolation (normal IEEE commercial spec) is very cheap while trying to source magnetics with 2500v isolation can cost an order of magnitude more. So while your company could define a number of electrical improvements for an “Industrial Ethernet” interface, you have to weigh this against the added cost and supply headaches of buying against the grain – of ignoring the gigantic “sweet-spot” for commercial-grade Ethernet components that enable creation of that $9.95 PC Ethernet adapter.

What is Your Manufacturing Sweet-Spot?
Just as the world market has a sweet-spot, so does your own in-house production; just ask your purchasing department. Adding Ethernet is NOT just the cost of adding a few new chips - the NIC, MAC/PHY, magnetics, and RJ45 connector. You may need to upgrade your whole basic hardware design away from a simple 8-bit CPU with 64KByte of memory to a 16 or 32-bit CPU with several MByte of memory. For example, Digi’s basic Device Server platform has a 32-bit CPU, 4MB flash, and 8MB RAM. Few of our products really need this much horsepower, but putting for example 8MB of RAM into all products is cheaper given purchasing logistics and reliability of supply than buying a mix of 2, 4, and 8MB chips. In fact, today we are looking at the cost tradeoff in shifting the basic design from 8MB to 16, 32, or even 64MB. Yes, 16MB (or 64MB) will cost more than 8MB, but given some products need 16MB (or 64MB) there are both tangible and intangible benefits to moving a larger volume of products up the curve to retain supply-chain advantages. This is especially true of FLASH and RAM chips which frequently suffer feast-and-famine availability cycles.

All small companies quickly learn – often the hard way – that during market shortages, it is the small volume purchases that get last delivery. During a chip famine low-volume purchasers will NOT be able to buy sufficient chips at any price to maintain their production. The higher your volume of a part, the lower is your price and perhaps more importantly the more reliable is your supply. So when you start to add Ethernet products and reduce sales of non-Ethernet products, you may find you need to upgrade the CPU design of some your non-Ethernet products to gain or retain reliability of parts supply.

How Robust is Commercial-Grade Ethernet?
So far I have been saying that trying to create special Ethernet hardware for industry may be costly and not very cost-effective. Worse, your average commercial-grade Ethernet is already very robust when compared to RS-232, RS-485 or USB serial. Ethernet uses a transformer-isolated signal with differential pairs, plus has nice, low-level, hardware-supported error detection. Given the high signal frequency, low signal voltage and isolation transformer, trying to add extra surge protection greatly complicates product ground design and weakens the signal, shortening the supported cable length below the 100m length customers have etched within their minds. So trying to boost your Ethernet spec for an industrial design gives questionable gain for the extra cost and lost profits. Plus your customers won’t likely perceive a market differentiation that they are willing to pay for if you say you have better isolation, etc.

A Note on Shielded Ethernet Cables:
Many industrial users start out assuming STP (Shielded Twisted Pair) is better than UTP (Unshielded Twisted Pair) for Ethernet. Oddly enough, STP has proven a bit like ABS brakes in private automobiles; despite lots of hoopla about saving lives when the US government forced ABS brakes into cars, insurance industry records continue to show it has had no measurable impact in real world road deaths. It seems while an expert driver can be helped immensely by ABS, your average idiot or careless driver still reacts to skidding situations in ways ABS brakes cannot fix.

The same appears true for STP cables and Ethernet. I have seem many discussions where industrial users tried STP cables and found the system only works reliably when they lay temporary UTP cables across the weld-shop floor! I suspect the main problem is traditional IT groups have used and measured STP success in terms of preventing Ethernet cable emissions affecting other equipment. This is not the same as using STP to prevent external interference from affecting the Ethernet signal. So ignoring the issues old truisms of a floating shield is worse than no shield and a shield grounded at both ends and creating a ground loop is worse than no shield, it appears that only experts and a very detailed system design results in STP Ethernet working better than UTP. My recommendation is to use optical fiber whenever you really worry about noise interfering with UTP Ethernet.

Vibration and RJ45:
Field tests of RJ45 connectors have shown them very bad in areas of high vibration. This is actually very easy to see for yourself - take any RJ45 connector with pins facing down and wiggle it up and down. What happens? That little finger-catch / lock acts as a pivot point and you are actually scrubbing the gold-flash contacts of the connector against the socket contacts. Metal-against-Metal; quess the result. Tests on industrial robot arms have shown even high-quality gold plated RJ45 connectors self-destruct in months or even weeks. If you expect vibration, better look for alternative connectors - such as any of the many (way too many) IP67 locking designs.

Industry and CAT 5, 5e, and 6:
Another insteresting twist to the commercial evolution of Ethernet is tests of bulk cable shows that CAT 5 is the likely the best for industrial use where the noise rejection properties of the twisted pair (differential signal) is desired. This is because - so I have been told - one of the tradeoffs IEEE allowed for CAT 5e and 6 is to allow less consistancy within the wires of a pair. After all, few Ethernet systems ever see serious external interference, so things which improved speed outweighed things which reduced noise rejection in abnormally high noise conditions. Several large automation companies tried to bring up the idea of a CAT 5i with IEEE which emphasised better noise rejection and special jacket plastic, however ... it appears it went no where. If the big computer, networking, and cable vendors don't see the value, it cannot happen through IEEE.



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