Physical basics

The way in which the method of electrical control panel DMX512 transmit 0 and 1, and the manner in which these signals are accepted devices, of course, it is important to understand, since 80 percent of all our problems with DMX caused by various physical distortion of the signal.

DMX512 signal is transmitted Industrial interface EIA485, better known as RS485. He is not much different from the standard interface RS232, which has a slot on your computer, but it's not quite the same thing.

Specially-equipped computers are in their architecture RS485 port for different tasks, but the formats of communication of information rather different. This can be compared with the Latin alphabet for writing Russian words - virtually all understand Russian, though a foreign alphabet, but almost all the British did not understand, although the native alphabet.

Standard RS485 uses 2 / 3 wires for transmission of high (HI) and low (LO) signal levels, which represent 1 and 0, respectively:


- positive wire (+ s)


- negative wire (-s)


- zero or ground wire (0v)

Digital unit is sent when the potential of the positive wires more potential negative - U + s> Us.

Digital zero is sent when the positive potential of the wire is less than the capacity of the negative - Us> U + s.

That is the potential difference between the two signal wires is important, not the potential difference between each signal wire and the neutral wire, which is simply a reference point. In principle, in some installations using interface EIA485 ground wire may be absent. High and low levels of capacity rather clearly describes the standard interface RS485. The potential of each of the signal wires may be in the range from +12 to -7 in the relatively neutral wire.

For example, to send one, it is necessary that the potential positive wire was +5 V and a negative -5 V. To send a zero, it is necessary that the potential positive wire was -5 V and +5 V negative

So:

What is the minimum difference potentials receiver must be taken as a change of state signal? Standard EIA485 states that it must be at least 200 mV, or, equivalently, 0,2 B. And that, again, it is important that the difference between two signal wires. Often the ground wire in the shielded wires is used as a screen, and no more. In other words, this scheme has two major advantages.

Interference (and there are usually plenty of on-site) suggests the same in both wires in one phase. This means that they increase and decrease in both simultaneously. As a result, the receiver gets the same potential difference as it was originally.

If the signal goes out of control and has options, for example, + / - 5 V, when the signal goes through a cable with a high resistance, then as a rule, both the signal wires in the cable have the same resistance, and as a result of weakening both the capacity of both wires. This means that even if the potential of each wire will fall to about zero + / - 100 mV, then the receiver will still detect the transmitted information, because the potential difference is still at the level of 200 mV. Finally, it is very bad, but not as much!