When fieldbus pioneers expressed concerns about single points of failure in simple two-wire buses, Relcom engineered and patented the "SpurGuard," a compact assembly that could be substituted for pluggable terminals on their wiring blocks. Any "spur" (what fieldbus calls a single-pair drop to a device) fitted with a "SpurGuard" could sustain a water-logged or otherwise short-circuited condition without cratering the whole network.
SpurGuards were expensive, so Relcom and its competitors began to design and manufacture "couplers," wiring blocks pre-fitted with short-circuit protection for each spur. Since about 2009, end users have been able to choose from a wide selection of couplers that are registered and tested by the Fieldbus Foundation. So can couplers be procured on the basis of cost only?
There are distinctions. If you're creating a wiring block or "brick," you have to make choices regarding how many spurs you can wire to each brick.
See Also: Fieldbus Couplers Protect Automatically
Pepperl+Fuchs has one of the wider selections of bricks, which can be purchased with 4, 6, 8, 10 or 12 spurs per brick. P+F and Phoenix Contact go a step further to optimize scalability in their lines of snap-together modular coupler hardware. Both P+F's RM-SP* line and Phoenix Contact's FB-ET lines allow you to build up couplers with between two and 26 spurs. A pluggable bus rail extends the bus for up to a half-dozen more modules, each extending the base unit in increments of four spurs. These solutions may provide the optimal solution for those who want to minimize junction box size.
I used to think the original Relcom "Megablock" had a comforting ruggedness due to its physical mass, but Relcom's Cyrus Kelly explains the "heft" came from "potting"—encasing the entire electronics assembly in a substance that sealed the circuitry from its environment. Cyrus's research revealed that potting was no better than the significantly more cost-effective technique of "conformal coating" —sealing the circuit with what amounts to a coating of a similar non-conductive substance, such as acrylic lacquer. In other enhancements to the second generation of bricks, Relcom engineers reduced the component count by 35%, which can be broadly tied to a proportional increase in mean time between failures (MTBF). So perhaps "heft" isn't an infallible indicator of reliability or ruggedness.
MooreHawke's couplers are extremely rugged. The case of the TG200 series is metal, and an eight- or 12-block spur block might break a toe if you dropped it on your foot. MooreHawke also has unique "auto terminator" circuitry that adds the required impedance-matching terminator at the furthest end of the trunk automatically. While other couplers may add a short-circuit load of 50 mA to 60 mA when a spur is shorted, MooreHawke couplers add only a few, once a short-circuit is detected.
See Also: Wire, Cable and Connectors Are Crucial
R. Stahl offers another line of couplers that have low-quiescent short-circuit current. This can be a factor when one is computing the maximum power budget for a segment: The 30 mA you save could facilitate adding an additional device or allow test equipment to be connected without causing communication errors. Stahl couplers also have special accommodations to limit inrush current when powering up an entire segment, and circuitry to tolerate more than one short circuit without an increase in the total current load.
Cobalt Process doesn't believe in salesmen and caters to savvy end users who know what they want and don't want to pay for marketing overhead.
Arguably, any of the certified couplers will be one of the most reliable components of one's fieldbus networks. But there are ample features and distinctions that have their appeal for individual users and cultures.