Optical Isolation; Shield Grounding - Toptech MultiLoad II Manual

Chapter 9 – Data Communication Notes and Recommendations
To keep these problems under control, multi-dropped lines need line termination resistors installed. This can be
installed on the receiver of the line driver for the line. For RS-232 communications, a 500 to 1000 ohm resistor
is placed between the receive (RD) and signal ground (SG) wires of the line driver for the line. For RS-485
communications, a 220 to 270 ohm resistor is placed between the receive pair (RDA and RDB) wires of the line
driver for the line. Termination resistors actually weaken the driver's ability to transmit; therefore, when longer
lines are used or many devices are multi-dropped, the higher resistor values must be used. Unfortunately, the
proper value must sometimes be determined by experimentation. For RS-232, the resistor value must be large
enough to allow the transmitter to drive the voltage levels to at least +3VDC and -3VDC. For RS-485, the
resistor value must be large enough to allow the transmitter to produce a .25 VDC difference in the transmit pair,
but not large enough to disrupt the balance of the line. Too low of a resistor value will typically cause a short on
the data communications line and inhibit communications.
Please note that in RS-485 communications specifications, you will find that 100 ohm termination resistors are
recommended. This is a different type of line termination than what we are discussing here. The 100 ohm
termination is specifically for line balancing and is typically not required for baud rates less than 38,000 baud.
This is well above most PC data communications capabilities.
9.4 O I
PTICAL SOLATION
Due to the major power fluctuations that may occur in an industrial environment and the effect of lightning
storms in an area with lots of piping, we recommend that any data communications line connecting devices
powered from different AC circuits be protected with optical isolation devices.
converters that Toptech supplies provide this isolation. We also provide an RS-232 optical isolation device for
RS-232 lines. Similar devices are available from Black Box Corporation and Burr-Brown. One isolation device
must be installed on each data communications line. This is different from line drivers or short-haul modems
that require a device on each end.
Please note that any line termination used must be installed on the field side of the optical isolator. Also, optical
isolators are directional, which means that one side is for the computer (DCE) and the other side is for the field
equipment (DTE). They will not transmit data if installed backwards!
9.5 S G
HIELD ROUNDING
Due to all of the electrical noise generated by an industrial environment, all data communications cable must be
properly shielded. If not properly shielded, communications may work for a while, but you will most probably
experience intermittent communications errors and outages. Most data communications cable comes with good
shielding built in, but if not properly installed, the shielding will be ineffective. Proper installation requires that
the shield be attached to earth ground on ONE END ONLY, typically at the junction box on the computer end,
and MUST BE CONTINUOUS through all junction boxes out to the field equipment. In addition, the shield
should be taped back at each field device and should NEVER be connected to a field device. Unfortunately,
several of the field device manufacturers have terminal blocks labeled for attachment of the shield, and many of
their installation drawings indicate that the shield should be attached to the field devices.
circumstances should the shield ever be attached to a field device!
When connecting through a junction box, care must be taken that the shields be treated just like any other wire.
The shield must remain continuous across the junction box. Don't tie multiple shields together. Route each
shield across the junction box, making sure that it does not short to the box or any other point. Inside of a
junction box, wires are typically unshielded. If a junction box is installed in an electrically noisy area, near
motors, etc., the junction box must be able to provide shielding. Also, AC wiring should not be run into a
junction box that has unshielded data communications wiring.
switching off and on or that have a high current flow must be avoided. These will introduce noise into the data
communications lines inside of the junction box.
The RS-232 to RS-485
Especially, AC circuits with loads that are
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Under no