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-). The cable is said to be driven differentially and the receiver(s) connected to the cable read the data by observing the difference between the two signals on the two conductors, and not their absolute value.

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

A balanced signal means there is no net radiated signal along the cable length (i.e., the combination of A and B signals effectively cancel one another out). At the receiver, this offers significant immunity to external interference (any interfering signal would change both signals equally, resulting in common-mode noise that does not change the difference signal).

EIA-485 Differential Transmission

EIA-485 (previously called RS-485) is a standard that uses differential transmission to provide differential transmission using a 5V signal. The design is well suited for long-distance usage in potentially noisy environment.

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

EIA/RS-485 Transmitters and Receivers

A line driver is used as a transceiver for the bus. Typical circuits support both a sender and receiver. In DMX-512, the transmission is simplex. The sender uses this as a line transmitter and a receiver uses it as a line receiver. In Remote Device Management (RDM), both circuits are used by all devices using half-duplex.

Typical line driver for differential transmission, showing a line transmitter and line receiver.

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

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

At the receiver, the difference between the voltage on the pair of lines is used to detect the value of each received baud. 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). For EIA-485, the detection threshold is 200 millivolt (0.2V).

EIA-485 typically uses shielded cable. The bus transmitter (sender) connects the ground to this shield. This provides protection from interference, especially high frequency signals. Each receiver along the bus connects the shield from its input to its output. In this way the earthing provided at the sender is continuous along the entire length of the bus. The ground may also be used to shield the transceiver electronics - but is not used to detect the bauds that are sent.

The shield/ground wire may not be present at all in some EIA485 installations.

NOTE: The receive signal is NOT the difference between EACH line and a ground reference - as in EIA-232 or EIA-432.

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 (often known as ground loops). This design uses DC isolation, were the line levels are electrically isolated from the logic levels. This often uses a dc-dc power supply and opto-isolators.

Controller Area Network (CAN)

CAN also uses standard differential transmission, but uses three voltage levels on the lines. 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. (2020)