EPAC300-M50 SERIES USER’S MANUAL
AUGUST 2005 PAGE 40 OF 46
suppression circuitry clips power surges from inductive loads and nearby
lightning strikes, and shunts the potentially damaging voltage surge safely to
Equipment Ground.
3. Shields of communications lines, such as EIA-232, EIA-422, and EIA-485 should
be tied to equipment ground, BUT ONLY IN ONE PLACE. Usually, all of the
shields from all of the communications lines are tied together at a single point
near the EPAC M50-Series, such as the EPAC M50-Series equipment ground.
The voltage of the earth can, and regularly does jump, due to lightning, ground
connections to high-tension lines, ground connections to high horsepower
electric motor, etc. If grounded in more than one place, the shields of
communications lines can momentarily experience current surges of hundreds
of amps during a ground jump. These current surges can sometimes cause
damage, but almost always cause communications errors. Connecting the
shields to ground creates a beneficial path to drain noise from the
communications lines, but should never be used to connect two or more
Equipment Grounds.
4. "Ground" wires of communications lines, such as EIA-232, EIA-422, and EIA-
485, are really not "grounds" at all. For example, EIA calls "Ground" = Pin 7 of a
25 Pin EIA-232, such as COM1 of an IBM PC XT. EIA calls "Shield" = Pin I of the
same connector. Pin 1 is the cable shield, and should be connected to
Equipment Ground in one place. Pin 7 is the signal ground, and logic common
line of the power supply, but has nothing to do with Equipment Ground. Many of
the EIA specifications have this confusing feature. For proper installation of EIA
communications lines, use the following guidelines:
a) Always use shielded communications wire of the correct type. The Belden,
Alpha, and other wire catalogs have the EIA cables listed by the EIA
specification number, and are available with a foil shield.
b) Twist all of the shields together in one place, and connect to the Equipment
Ground at one point.
c) Treat the EIA Ground as another signal. The EIA Ground, such as EIA-232 pin
7 is used to reference the line driver at the transmitting end to the line
receiver at the receiving end. Noise on the EIA Ground can cause as many
problems as noise on a data line. Pick a cable type from the catalog that has
enough wires such that EIA Ground can be carried on a separate wire inside
the shield. Many installers use the shield as the EIA Ground, but Siemens
discourages this practice.
Figure 9::
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