In the fields of industrial control and building automation, RS-485 communication is widely favored due to its differential transmission, long-distance capability, and excellent anti-interference performance. However, in practical engineering, the "loop impedance", which affects communication stability, is often overlooked, leading to occasional packet loss and communication interruptions of equipment. Troubleshooting such issues is both time-consuming and laborious.
This article will take a "life - close and easy - to - understand" approach to help you gain an in - depth understanding of what loop impedance is, why it is so important, and how to optimize it in design and debugging, so that RS-485 communication can be as smooth as a paved highway.
Imagine the water pipe system in your home: the water pump (driver) pushes water to the water consumption point (receiver), and then the water returns to the water pump through another pipe, forming a cycle.
Factors such as the diameter of the pipe, elbows, branches, and water pressure will all affect the smooth flow of water. The "loop impedance" in a circuit is similar: it is the comprehensive manifestation of the "resistance" exerted on the AC signal in the entire closed loop where the signal starts from the transmitting end, transmits along the differential pair, reaches the receiving end, and then returns to the transmitting end.
- Resistance (R): It is like the frictional resistance determined by the diameter of the pipe.
- Inductance (L): It is similar to the valves and elbows in the pipe, which will cause a "hysteresis" effect when the signal changes.
- Capacitance (C): It can be compared to a water tank or a water storage tank, which stores energy and releases it instantaneously, affecting fluctuations.
In the RS-485 system, the total "loop impedance" under the combined action of these three factors directly determines the quality and reliability of the signal.
RS-485 communication cables usually use 120 Ω shielded twisted pairs, just like choosing a water pipe with a constant inner diameter to ensure the minimum loss of water flow (electrical signal).
A 120 Ω resistor is connected in parallel at each end of the line to "absorb" the signal energy and avoid "echo" - just like installing a silencing valve at the end of the pipe to prevent water hammer.
When multiple devices are connected in parallel on the bus, it is equivalent to connecting multiple branches to the pipeline. The overall impedance decreases, and the signal is more likely to be "shunted", which may result in the receiving end not receiving a sufficient level.
Each connector, each TVS diode, or each protection device will add a little discontinuity, just like the joint at the pipe interface is not sealed tightly, which will cause local leakage or blockage.
Although RS-485 is differential communication, the ground wire will still form a loop, which is "uninvited" to common - mode interference. The ground potential difference between different devices is like the water level difference between different water towers in a water supply system, which will cause problems such as "backflow" or "cross - flow".
Impedance mismatch will make the signal "bounce back" like hitting a reflective wall, resulting in waveform distortion, ringing, and overshoot. In the end, the receiver cannot distinguish whether it is "1" or "0".
Unstable impedance is equivalent to increased water leakage in the pipe. When transmitting over long distances or at high speeds, the loss is more serious, and the signal may be "exhausted" before reaching the destination.
Discontinuous impedance is like a gap in the pipe, which is more likely to be "infiltrated" by external electromagnetic interference, increasing the bit error rate.
The driver will output a larger current to make up for the signal attenuation, just like a water pump running at a large flow rate for a long time will wear out faster, leading to heat generation, power consumption, and life risks.
Core principle: Maintain impedance continuity, making it as flat, constant in width, and with few branches as a paved road.
Use shielded twisted pairs with a nominal value of 120 Ω.
The shield layer should be reliably grounded: whether to ground one end or both ends should be weighed according to the actual interference environment.
The differential pair must be routed with equal length and equal spacing to avoid uneven impedance caused by one side being too long.
Differential traces on the PCB should not cross the ground plane division, and should be laid on the same layer or use a symmetrical ground plane as much as possible.
Connect a 120 Ω termination resistor in parallel at each end of the bus.
If it is necessary to suppress common - mode noise, "split termination" can be used: connect two 60 Ω resistors in series, and connect a small capacitor in parallel at the midpoint to the ground, which is equivalent to adding a "muffler" to the signal path.
Keep the receiver output at a stable known level (usually logic "1") when the bus is idle.
A pull - up resistor can be added to pull up the differential line A and a pull - down resistor to pull down the differential line B to avoid signal floating when the line is broken or no one is transmitting.
Prioritize the use of "linear topology" (straight line), and install termination resistors only at the physical ends.
Avoid star, ring, or too many long branches, just like avoiding inserting branches randomly on the main road to prevent traffic jams.
The faster (steeper) the signal edge, the more serious the reflection. For long - distance transmission, a slope - limited transceiver can be used or the baud rate can be appropriately reduced to match the "vehicle speed" with the "road conditions".
Use a differential probe to observe the voltage waveform of the A/B line, and check for ringing, overshoot, or attenuation. Compare the baud rate with the theoretical signal waveform to determine whether slope limiting or rate adjustment is needed.
Disconnect the branches section by section, observe the waveform changes, and locate the position of impedance discontinuity or common - mode problems.
Try replacing the cable, termination resistor, or adding a common - mode choke in the suspected area to see the effect of the change. Optimize the grounding layout to reduce the ground loop interference caused by multi - point grounding.
Configure TVS tubes and common - mode chokes reasonably to resist external surges without excessive signal absorption.
Ensure that the parasitic parameters (capacitance, inductance) of the protection components have a controllable impact on the total impedance.
- Only one end of the termination resistor is installed, resulting in serious reflection at the other end.
- The position of the termination resistor is incorrect, and it is not placed at the physical end.
- There are too many or too long branches, and the signal rebounds repeatedly at the branches.
- Blindly choosing non - 120 Ω cables, which have a large matching difference with the receiver.
- Ignoring the ground potential difference between devices, resulting in excessive common - mode voltage.
- Fully relying on the internal Fail - Safe of the transceiver without external bias, leading to frequent misjudgments when the line is broken.
