Communication 3-5
becomes an important consideration. The GND wire is used both as a
power connection and the common reference for the transmission line
receivers in the modules. Voltage drops in the GND leads appear as a
common-mode voltage to the receivers. The receivers are rated for a
maximum of -7V. of common-mode voltage. For reliable operation, the
common mode voltage should be kept below -5V.
To avoid problems with voltage drops, modules may be powered locally
rather than transmitting the power from the host. Inexpensive ‘calculator’
type power supplies are useful in remote locations. When local supplies are
used, be sure to provide a ground reference with a third wire to the host or
through a good earth ground. With local supplies and an earth ground, only
two wires for the data connections are necessary.