Balanced Transmission

Balanced transmission uses a cable with a twisted pair of conductors. One of the pair carries the A signal (Data+) and the other the opposite signal (complement), known as the B signal (Data-).

A typical balanced transmitter drives both the A, B conductor to send each baud:

A balanced signal means there is no net radiated signal along the cable length (i.e., the A and B signals effectively cancel one another out). At the receiver this offers good immunity to external interference (any interfering signal would change both signals equally).

EIA-485 Differential Transmission

EIA-485 (which used to be called RS-485) is a standard that uses balanced transmission to provide a differential output across a 54 Ohm cable using a 5V signal. It provides reliable data transmission even under severe signal degradation across the cable and connectors. This robustness is the main reason why EIA-485 is well suited for long-distance usage in potentially noisy environment.

Transmission of a square wave signal using a balance line subject to noise.

EIA/RS-485 Transmitters and Receivers

A typical line driver acts as a transceiver for the bus. That is, it supports both a sender and receiver. In DMX-512, the sender uses the line transmitter and the receiver uses the line receiver. In Remote Device Management (RDM), both circuits are used by all devices.

Typical Line Driver for balanced communications, showing a line transmitter and line receiver.

A pair of line drivers may be connected with a length of twisted pair cable to form a bus, as shown below:

Use of a bidirectional typical driver chip, with a termination resistance, Rt. RE and DE are respectively the receive and transmit driver enable pins.

Receivers can safely detect signals with a much lower voltage, which allows the receiver to function correctly even if there are large voltage drops along the cable. At the receiver, the difference between the voltage on the pair of wires is used to detect the value of each baud. The detection threshold is 200 millivolt (0.2V). For balanced transmission the threshold is measured as the difference between the two conductors, for unbalanced transmission the threshold is relative to a common reference (ground).

NOTE: The receive signal is NOT the difference between EACH conductor and the ground wire, the ground is simply a shield from interference, it may not be present at all in some EIA485 installations.

While a simple line driver is sufficient for short-range communications, isolated transmitters and receivers are preferable for DMX-512, because they have much better protection from transient voltages and earthing problems between equipment. These use DC isolation, were the line levels are electrically isolated from the logic levels and often use a dc-dc power supply and opto-isolators.

Controller Area Network (CAN)

CAN also uses standard differential transmission, but uses three voltage levels. As in DMX, one conductor (line) carries the inverse signal to the other conductor (line), but the CAN_H signal is level-shifted so that the lowest level aligns with the highest level of the CAN_L signal. This enables arbitration, since one symbol (recessive) lets both conductors float to zero. The other symbol drives both conductors - one to the opposite polarity of the other.


See also:


Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland. (2016)