Vision Systems VScom 200E PCIex Manual de usuario descargar pdf (Pagina 48)

7 Proper Cabling in RS 485 and RS 422

The Polarization Resistors must not be too small, because a transmitter has to act against them.

A typical transmitter provides up to 60 mA of current in high and low level.

With the isolated models the polarization option may have additional advantage. As said above

RS 485 requires a connection of GND signals, otherwise the signals at the receiver may be out of

bounds of the Common Voltage Range. Since on SI-models the driver and polarization voltage are

isolated, the received signals are now in the required range. As a result only two wires are required

for data transmission.

Attention: this is the situation if the isolated VScom Industrial Card is the only device with

polarization active.

7.4 2-Wire Scheme

In many conﬁgurations a very simple cabling is required. RS 485 allows for so called 2-wire cabling

as shown below. Several devices are connected in parallel to the wires, which is called bus topology.

Each device can either send or receive data at a given time, so it is operating in half-duplex mode.

Shown in ﬁgure 26 are three devices, RS 485 speciﬁes up to 32. The data lines are named as Data+

and Data-, a positive diﬀerential voltage is the state for a transmitted One. The GND is also

connected between all devices as required

Figure 26: 2-Wire Cabling Scheme

The resistors P1 and P2 are for polarization, T1 and T2 are for the termination function. Polar-

ization of Data+ and Data- appears only once on this net, the termination is at the physical ends

of the cable.

All devices appear the same on the cable, they have the same function. There is no Master or

Slave deﬁned by the hardware. Such functions are implemented by way of the data transmission

protocol. Also RS 485 addresses are deﬁned by that protocol, as well as bus access.

See 7.3 for isolated VScom Industrial Card

July 2011 VScom Industrial Card User Manual 48

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Vision Systems VScom 200E PCIex Manual de usuario Pagina 48