5 Technical Background on RS485

5.3 Polarization

In RS485 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. When the termination resistors are of the 120Ω types, the polarization resistors shall have

about 750 to 1000Ω.

The Polarization Resistors must not be too small, because they add current to the signals, and a

transmitter has to act against that. A typical transmitter provides up to 60 mA of current in high

and low level.

5.4 2-Wire Scheme

In many conﬁgurations a very simple cabling is required. RS485 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 own or receive foreign data at a given time, so it is operating in

half-duplex mode.

Shown in ﬁgure 24 are three devices, RS485 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, so the cabling is 3-wire in reality.

Figure 24: 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 RS485 addresses are deﬁned by that protocol, as well as bus access.

June 2009 USB-COM PRO User Manual 28

1 2 ... 23 24 25 26 27 28 29 30 31

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