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| What's
Needed for Industrial Applications |
| Most devices used in today's industrial environments
are designed for use with the RS-422/485 interface. This
means that
many users employ RS-422/485 serial communication boards
in their PCs, allowing the PCs to connect to an industrial
RS-422 or
RS-485 network and transmit data over distances up to 1.2
km. The RS-422/485 standards overcome the transmission distance
limitation by using a differential signal for transmitting
data and
control signals. In addition to distance and multi-drop issues
for industrial applications, other issues to pay attention
to are isolation, housing, wiring, power supply,
and over-surge protection. |
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| RS-422 Differential Transmission |
The RS-422 protocol greatly expands the
practical possibilities of the serial bus. It provides
a mechanism by which data can be transmitted over long
distances (up to 4,000 feet or 1.2 km) and at very high
speeds (up to 10 Mbps). This is accomplished by using differential
technology that splits each signal into two separate wires
in opposite voltage states, making this type of wiring
configuration well suited for noisy environments.
|
 |
| |
| EIA RS-422A |
| Full-duplex, Point to point |
| Differential |
| 1 |
| 10 |
| 4000 feet = 1.2 km |
| 10 Mbps |
|
| |
RS-485 Multidrop Network |
| RS-485 is an updated version of RS-422 that
is specifically designed to address the problem of communication
between multiple devices on a single data line. It is a balanced
transmission system with the ability to allow up to 32 devices
to communicate using the same data line. This is achieved
with tri-statable drivers that are usually controlled by
a programmable handshake line to ensure that only one device
acts as a driver at any given time. |
|
| |
| EIA RS-485 |
| Half-duplex, multi-drop |
| Differential |
| 32 |
| 32 |
| 4000 feet = 1.2 km |
| 10 Mbps |
|
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| Termination |
| Termination is used to match
the impedance of the receive and transmission nodes. When
impedance is mismatched, the
transmitted signal is not completely absorbed by the load
and a portion is reflected back into the transmission line.
If the transmission line and load impedance are equal, these
signal reflections are eliminated. The value of resistor
needed
to match the characteristic impedance is specified by the
cable manufacturer, with a commonly used value of 120 ohms. |
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