Vision Systems VScom 200E PCIex Manual de usuario descargar pdf (Pagina 47)

7 Proper Cabling in RS 485 and RS 422

7.1 Transmission Technique

RS 422 and RS 485 use the same balanced transmission method. Signals are not transmitted as

voltage on a single wire, as RS 232 does. Instead two wires are used; when one carries high voltage,

the other one carries low voltage. The signal is deﬁned by the diﬀerence in voltage between those

two wires. This hardens the transmission against noise. Usually twisted pair cables are used, which

further reduces the sensitivity for noise.

Typical voltages are +4V as high, about +0.5V as low. These voltages are deﬁned against the

GND signal of the transmitter. The minimum diﬀerential voltage is required as ±200mV by the

speciﬁcations of RS 422 and RS 485. The receiver detects the polarity of the diﬀerential voltage,

and thus gets a Zero (negative) or a One (positive).

To do this detection the voltages on the receivers side have to be inside the bounds of the common

voltage range deﬁned as -7V through +12V, as measured against the local GND of the receiver.

To make sure the signals meet the common voltage range, the GND of sender and receiver must

be connected somehow. To insure the signals are in the valid voltage range and the diﬀerential

voltage can be correctly sensed by the receiver the GND lines of the transmitter and receiver

must be connected. This is often forgotten. In these cases the GND connection can be indirectly

established over protective ground or over "parasite ground loops", which usually yield a bad GND

connection. For these situation the correct data receiving is not reliable or not possible.

7.2 Termination

When transmitted signals arrive at the end of a cable, they get reﬂected. They travel on the cable

some more times, which is called ringing. This can cause false reading of transmitted data. When

the reﬂections travel on the cable several times, they are damped and do no longer cause errors.

This happens earlier if the cables are short.

For long cables Termination Resistors are required. These increase the damping of reﬂections. The

value of the resistor must match the impedance of the cable, typically 120Ω. The VScom Industrial

Card provides an option to activate such resistors by jumper or DIP switch, depending on the

model.

As a rule of thumb

, when the cables are longer than

1000000

Bitrate

(one million divided by the bit rate)

in meters, you should consider Termination Resistors.

7.3 Polarization

In RS 485 the sender must activate the transmitter before sending data, and deactivate it when all

data is sent. At times when no devices send data all transmitters are inactive. As the result the

data lines are ﬂoating, and the diﬀerential voltage is undeﬁned. It may happen the next data is

not correctly recognized, because the change from undeﬁned to data signals is not detected.

To avoid such problems the data lines should be polarized by resistors. These insure the diﬀerential

voltage to be above +200mV. Typically the positive line is pulled to +5V, while the negative line

is pulled to GND. When Termination Resistors are applied, the voltage on these must be +200mV

or above. The VScom Industrial Card provides an option to activate such resistors. They are

calculated for termination resistors of 120Ω.

Assuming group speed of 100.000 km/s, 10 travels to damp out, and 10% of bit time

July 2011 VScom Industrial Card User Manual 47

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