Amplicon USB232U, Инструкция по эксплуатации

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USB232U/485Ui
Page 13
4 USING THE SERIAL COMMUNICATIONS DEVICES
4.1 Why Isolate?
The USB485Ui provides complete electrical isolation between any signals connected to a serial
port, signals connected to the other port and the circuits of the host computer. In a compact
office or laboratory environment such isolation may not be necessary, however in a wide-field
industrial situation, serial digital communication can be disrupted by many types of
contaminating signals, most of which can be eliminated by the proper use of isolation.
Both input receivers and output drivers make use of opto-couplers to provide a signal path
between external and internal circuits without any electrical connection being made. An opto-
coupler comprises a Light Emitting Diode (LED) which is turned on by the incoming signal, and
a photo-detector that receives the light and converts it into an electrical signal. The emitter and
detector are mounted in a package that provides a light path together with high voltage
insulation between the two sides. Power to the input/output circuit is provided by an isolated
DC-DC converter.
Interfering signals can come from a variety of sources and are often greater in amplitude than
the desired signal. Such an interfering signal is 'common mode' and is effectively the potential
difference between the zero-signal reference points of the signal source and receiver. By using
the USB485Ui, this potential difference appears across the isolation barrier and not in series
with the signal as would be the case if direct connection was employed.
The two ends of a link would normally have their grounds connected together through the
signal ground line, the protective ground line and/or the cable shield. Such a connection has an
impedance such that an interfering signal caused by a difference in ground potentials is likely
to be a few volts in amplitude, maybe peaking to tens of volts. However there may be
circumstances or fault conditions in which higher common mode voltages exist, and the circuits
of the USB485Ui are designed to hold off peak common mode voltages up to 250V.
RS422 and RS485 standards provide a better level of ground noise immunity than RS232, the
maximum common mode voltages being –7.0 V to +7.0 V and –7.0 V to +12.0 V respectively
for the two standards.
However, any ground offset, noise, hum or other interference exceeding
these peak amplitudes will be detected as real signals and may corrupt the data or control
functions.
Breaking the common ground connection by the use of the USB485Ui, completely eliminates
the detection of these interfering signals as real signals with no loss or corruption of data.
In general, isolation is only required at one end of a data link.
Multi-drop or broadcast systems with many RS485 or RS422 nodes on a single circuit may
require all circuits to be isolated for error free serial data communication.
DANGER
High Voltage
WARNING
DANGER OF ELECTRIC SHOCK
When the USB485Ui isolated device is in use without the
supplied plastic case, high voltages may be exposed on the
connector and associated circuitry.