The DMX512 has a characteristic impedance of 120Ω, but it's not impedance matched?

The DMX512 has a characteristic impedance of 120Ω, but it's not impedance matched?

*The content of this article is based on publicly available data. We welcome corrections from industry veterans if there are any errors.

We often say that

DMX512 requires a cable with a characteristic impedance of 120Ω,

and the term "impedance matching" is frequently encountered... but is this term accurate when applied to DMX512?

The DMX512 is a low-impedance output.

In fact, the RS485 chip used by the DMX512 has a low-impedance output!

(The DMX512 evolved from RS485)

For example, the Analog Devices MAX485 datasheet mentions that for Differential Driver Output (with load) used with RS485:

R = 27Ω

MIN 1.5V MAX 5V

Therefore, the output is not 120Ω, but its measurement standard corresponds to a 120Ω load.

(Figures 19, 20)

The cable requirement is also 120Ω (Figure 21)

Image courtesy of MAX1487-MAX491: Low-Power, Slew-Rate-Limited RS-485/RS-422 Transceivers Data Sheet (Rev.11). Copyright belongs to the original author.

The input internal resistance of a single receiver is >12kΩ

The most commonly used parameter for evaluating RS485 chips is the unit load (UL).

A unit load capable of supporting 32 terminals is called 1UL, and its equivalent input resistance is >12kΩ.

Internal resistance can be divided into different levels: 1UL, 14UL, and 1/8UL, to drive a corresponding number of daisy-chain lights.

The standard stipulates that the input resistance of one unit load (1 UL) must be >12kΩ.

As can be seen from the table below, the equivalent input resistance of the terminals is very high.

Below is a summary of the internal resistance of common RS-485 receivers on the market.

The DMX512 is a one-to-many control bus structure.

Next, we'll calculate the parallel resistors to see

what the resistance value will be when the load reaches a certain quantity.

We can observe three UL unit load configurations.

When the rated number of parallel connections is reached, the resistance value is 375 Ω,

rather than the wellknown 120 Ω

At this point, we would like to remind you of an essential component in the DMX network — the termination resistor!

Section 4.9 of the standard states: "The DMX512 data link shall be terminated to eliminate ringing and signal reflection, so as to prevent malfunction of welldesigned systems. To comply with this standard, all devices connected to the DMX512 data link shall operate in accordance with the specifications stipulated by the manufacturer after the data link is terminated. The terminator shall have an impedance of 120 Ω (+5%/-10%) and be placed between Data+ and Data."

Now let’s add the termination resistor and test it out! Please note the 375 Ω value mentioned above.

After reaching the nominal parallel resistance of 375Ω, adding a 120Ω terminating resistor will result in approximately 90.91Ω.

Conversely, using a 120Ω terminating resistor can create a parallel resistance of approximately 110Ω, with the following possible values: 8 (1UL), 32~64 (1/2UL), and 64~128 (1/2UL).

* The DMX specification ANSI E1.11-2008 (R2018) states:

"The terminator shall be a 120 ohm +5%/-10% impedance placed between Data+ and Data-."

The terminating resistor should be a 120 Ω +5%/-10% impedance, placed between Data+ and Data-.

                            In other words, a resistance range of 108–126 Ω is permitted.

SVP55835110 Termination Resistor 

This connector features both 3pin and 5pin terminals, which can be used according to actual requirements.

Then, what is the magnitude of the current?

According to MAX485 data, VOD2 with load is MAX 5V / MIN 1.5V.

ANSI E1.11-2008 (R2018)

states Ground Referenced Transmitter Characteristics as 6/0V

and Isolated Receiver characteristics as +12 / -7 VDC.

Therefore, substituting 12V into a 120Ω ohmmeter, we get I = 100mA.

(Note that this refers to the voltage withstand capability at the receiving end; the actual current will be lower.)

At 5V, I = 41mA.

At 1.5V, I = 12.5mA.

This is equivalent to the current seen at the cable tip.

Therefore, the requirement for cable cross-sectional area is not high.

Below, we will discuss these points in more detail.

Please continue reading if needed:

Why low-impedance transmitter and high-impedance receiver?

1/32, 1/64, and 1/128UL loads refer to the "unit load" of the RS-485 chip.

RS-485 requires a sufficient voltage difference (typically greater than 200mV) to be established at the ends of the transmission line even when multiple points are connected in parallel (daily daisy-chain). If the chip's internal resistance is high, the voltage will be pulled down when connecting a large number of lights.

Therefore, the low internal resistance output impedance at the driver end is to generate sufficient current!

A similar example is that the amplifier has very low internal resistance, which can accommodate multiple relatively high-impedance speaker units.

From the perspective of the source—multiple receivers—this is not impedance matching: At the frequency of DMX512/RS-485 (250kbps), the chip's output impedance is not designed to "match" the cable to deliver maximum power, but rather to have enough "energy" to drive the cable (actually charging and discharging) and drive all receivers.

Why are terminating resistors needed more when there are fewer light fixtures?

This is easier to understand from a signal integrity perspective!

When a signal travels through a 120Ω cable, encountering an "open circuit" (infinite impedance) at the end, a significant amount of the signal will be reflected back.

With many lights: The input capacitance and impedance of each light absorb and reduce some signal energy, resulting in relatively weaker reflections.

With few lights: Almost no energy is lost, leading to extremely strong reflections. These strong reflected waves can interfere with the original signal stream, causing malfunctions in the lights.

An easier way to understand this is:

The terminating resistor acts like a "green channel" for the signal voltage compared to other loads.

The driver has a path with the lowest resistance to this side.

However, other loads have already received the signal (voltage).

The digital 1 signal (from the charged cable) is released quickly through a low-impedance path, achieving an effect similar to eliminating reflected waves.

Therefore, the DMX512 is essentially:

"Low-impedance drive + overall impedance matching of the transmission line termination"

120Ω cable + 120Ω termination resistor is to suppress transmission line reflections.

DMX cables have relatively lenient requirements for characteristic impedance.

Compared to high-speed digital signals (such as USB 3.0 or 10G networks),

the DMX512 is relatively less demanding, but it still has certain requirements for the cable!

Here's why:

1. When the DMX512 exceeds a certain distance, the cable's distributed capacitance increases sharply.

This causes slow I/O charging and discharging, resulting in digital signal jitter.

Therefore, carefully selecting a low-capacitance cable is crucial!

2. Because the characteristic impedance (110~120Ω) is formed by both "distributed inductance" and "distributed capacitance,"

the distributed inductance of twisted-pair cables does not change much, while the key control parameter for characteristic impedance is the capacitance value.

Therefore, while requiring a specific capacitance value, the characteristic impedance must also be very close to the required value!

If you use cables with high capacitance, discontinuities in characteristic impedance will appear on every segment of the cable, causing signal reflections along the cable.

In practice, the selection of DMX cables must consider economies of scale.

Therefore, 110Ω characteristic impedance twisted-pair cable is also a very good choice.

Currently, the physical layer used in twisted-pair adapters/cables is 110Ω characteristic impedance network cable.

This is also permitted by standards for fixed installations.

The physical layer used in twisted-pair adapters/cables is a network cable with a characteristic impedance of 110Ω.

Compared with low-impedance transmission and high-impedance reception of audio signals

Audio (low impedance transmit, high impedance receive): The goal is precise voltage, and cable reflection is not considered (because the audio wavelength is too long, the cable length is negligible relative to the wavelength).

Number of terminals: Audio is usually point-to-point, but one-to-many is also supported; however, the number of terminals that can be connected in parallel is limited, not as large as DMX! But low-impedance transmission and high-impedance reception can indeed cope with one-to-many usage scenarios!

The DMX512 needs to consider the 250Kbps transmission line effect.

From the comparison table, we can see that the output impedance of the DMX driver is approximately 1/6 of that of audio.

The ultra-low impedance is designed to increase the number and distance of lights that can be driven.

Although it uses low-impedance transmission and high-impedance reception, because it involves digital high-frequency transmission,

the 250 kbit wavelength is short, so the transmission line effect will affect the transmission quality after hundreds of meters.

Therefore, impedance matching must be addressed by setting the characteristic impedance and using terminating resistors,

to solve the reflection problem.

Therefore, the DMX512 transmission control network should be understood as:

「Low-impedance driver + transmission line and termination that maintain characteristic impedance」

A 120Ω cable characteristic impedance + a 120Ω terminating resistor + other high-impedance receivers

are used to create:

a 120Ω cable characteristic impedance + an overall load of approximately 120Ω

effectively suppressing transmission line reflections.

Overall, it's a 27Ω:120Ω source-to-receiver

 "signal-flowable structure"

with a 1:4 resistance ratio, rather than a P2P point-to-point 1:1 power impedance matching.

in conclusion:

The DMX512 network is not suitable to be called "impedance matching" because:

1. It is not a P2P transmission structure (impedance matching is typically used for point-to-point transmission).

2. The receiver's network can change and be added or removed (there is an upper limit, depending on the UL internal resistance).

3. However, the overall receiver resistance can be adjusted to be close to 120Ω through the terminating resistor.

4. A drive output resistance far below 1/4 of 120Ω is a reasonable drive structure.

5. Using 100~120Ω low-capacitance twisted-pair cable, common in modern telecommunications standards, is technically feasible and can be mass-produced at economies of scale.

The DMX512 is not a typical「P2P point-to-point precision impedance matching system」,

Instead, it's a multi-point differential signal bus with a total rated impedance based on "transmission line diameter + termination resistor matching". Other receivers utilize higher resistance to "share" the signal voltage, thereby acquiring the DMX signal for operation. The termination resistor, with its resistance matching the characteristic impedance of the transmission line, eliminates signal reflection!

DMX512

It is a compromise of low-impedance transmission and high-impedance reception.

The transmission path maintains the characteristic impedance.

Industrial control bus anti-reflection design.