Data cables should be shielded twisted 120 ohm pair rated for EIA485 uses at 250 kilo-baud. The shield is not current carrying and must be connected to power supply common only once in a system, usually at the power supply. Power cables must be sized to ensure that when voltage drops on both the supply and common lines are included the power to the module remains above seven volts at all times.
In many cases Cat5 or better Ethernet cable can be used. Unless the cable is run in metallic enclosures, shielded Cat5 is preferred.
Another VERY important parameter when sizing distribution cable is the voltage drop on the power supply return (common) wire. Any drop in the common return appears as common mode voltage on inputs to these modules. EIA485 has inherent common mode voltage of about two volts and a positive common mode limit of 12 volts. Ground return drops of greater than five volts should be avoided. Ground return drops of more than eight volts will cause unreliable operation. See the page on figuring the voltage drop.
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