Bill Greer, senior product specialist, Contemporary Controls
Various networking technologies specify that stations must be separated by some minimum distance. Occasionally a customer is surprised about the lack of a minimum length requirement for their Cat 5 cable that connects one Industrial Ethernet device to another.
Network cable minimum length specifications arise because of an impedance mismatch between elements in the signal path. When electrical energy is transmitted through a cable, some of it is not absorbed by the receiving station and is instead 'reflected' back to the source. This reflected energy disturbs the purity of the signal - and if the effect is of sufficient magnitude, the signal can be degraded to the extent that its information is corrupted or even useless. If source and destination transceiver impedances match the impedance of the cable itself, there is no reflected energy and signal energy is delivered completely.
Reflected signal is always an issue in a bus topology because only the end devices have transceiver impedance values that match the impedance of the cable. Mid-bus device transceivers must have higher impedance values to avoid 'loading' the cable (preventing proper signal delivery to devices further along the bus). Thus, a mid-bus device transceiver presents an impedance mismatch to the cable and inevitably creates some degree of reflected energy. To keep the reflected energy to acceptable levels, a bus protocol will specify a minimum length of cable that separates two bus devices.
Industrial Ethernet uses a star topology in which devices exist only at each end of a cable. With this topology, both transceiver impedances match the cable impedance and no appreciable signal reflection occurs. This lack of signal reflection is true whether the devices are separated by 100 m or 100 mm. Therefore, there is no need to specify a minimum cable length.
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