by Bryson Barney
As our world becomes ever more dependent on flawless digital communication, engineers need to find new ways of charting calm seas when sending data. In wired networks, many clever and unique approaches have been developed to increase the robustness of data transmission from one point to another. Errors in data are measured as a bit error rate (BER). A BER measurement is simply the number of bits that are received in error over the total number of bits sent.
Figure 1: Bit Error Rate Calculation Explained
A Bit Error Rate (BER) of 1e10 means that one bit out of every 10 billion transmitted is received incorrectly. But what causes these errors?
Bit errors originate in the channel—the wiring that connects one transceiver to another. You can imagine the channel as an ocean and the data as a small boat crossing it as shown in Figure 2. Sometimes the waters are calm, but other times they turn stormy, depending on the environment.
Figure 2: Illustration of Data Transmission On a Cable with Received Errors Due to External Interference
The following are examples of what factors can cause a stormy channel.
- Electromagnetic Coupling: Cables running near motors, inverters, or other strong electromagnetic sources can pick up unwanted noise.
- Long Cables and Attenuation: Extended cable lengths act like filters, weakening high frequency components of the signal.
- Reflections: Impedance mismatches in the data path behave like mirrors, bouncing signals back down the line and interfering with subsequent bits.
- Cable Jitter: Unequal charging and discharging of capacitance between data wires can distort timing, making it harder for the receiver to latch onto the correct edge.
These conditions create a “stormy sea” for the data boat, increasing the likelihood of bit errors.
In this blog, we’ll explore techniques to calm the waters—from shielding and proper termination to encoding methods—so that the little boat of data can reach its harbor safely and reliably.
Electromagnetic Noise Coupling
Electrical noise can be coupled into a wiring system through several different mechanisms. Noise can be radiated from a source and picked up by the cable, which acts as an unintentional antenna. Noise can be capacitively coupled as well.
Counter: The easiest way to avoid coupling is to add a ground shield. This is relatively expensive, but effective. Using a tightly twisted pair of wires for the cable is effective at avoiding noise by coupling the same noise on both wires. Differential signaling uses common-mode rejection to combat coupled noise.
Long Cables or Poor-Quality Cables
The conductors in a cable have capacitance between them, and resistance across the length of the cable. The resistance and capacitance form a basic low-pass filter. The longer the cable, the more distributed capacitance. This means that the longer the cable, the more the data signals will be attenuated. The higher the signal frequency content, the higher the attenuation over longer distances.
Counter: Lowering the frequency of the data will help with attenuation. Improving the quality of the cable helps to lower resistance. Ultimately the loss of the cable will affect the distance and speed of communication.
Transmission Line Reflections
Changes in impedance throughout the network cause data edge reflections. These reflections cause constructive and deconstructive interference with other bits on the line.
Counter: Equalize the line with termination resistors. At the reflection points (any locations where a signal encounters an abrupt change or discontinuity in impedance), place a resistor between the lines that are equal to the characteristic impedance of the cable. This consumes the reflection instead of allowing it to bounce back down the line. Point-to-point connections are easier to terminate this way. For multidrop networks, termination is limited to only two of the nodes. Terminating the two furthest away nodes is the most effective. Adding more termination resistors on other nodes loads the network too much. The transceivers have limited drive strength.
Cable Jitter
Cables have distributed capacitance between the lines. Every time an edge is sent down the line, it either charges or discharges the distributed capacitance. If the data was a perfect square wave, then the charging and discharging would happen equally, but most data is not ‘0101010101….’, which means that the time it takes for an edge to be recognized and latched on the receiver side can vary. This jitter can hurt the performance of the data transmission, especially for applications that require very accurate timing, like encoders or time of flight modules.
Counter: To eliminate cable jitter, the data must be encoded to maintain balanced charging and discharging. It will not be perfect, but it can be dramatically improved by techniques like Manchester encoding.
Figure 3: Illustration of Data Transmission On a Cable Unaffected by Interference
Key Takeaway
Reliable communication depends on taming these “stormy seas”. By applying shielding, better cabling, proper termination, and smart encoding, engineers can ensure that the “little boat” of data reaches its destination safely and accurately. As you set sail may you have “calm seas, and data that always finds its harbor”. To learn more about RS-485 technology, check out ADI’s Interface Portfolio.
See the blogs in the TranscendingConventionalFieldBus series.