Understanding Terminator Wiring Diagrams for Bus Systems
Terminator wiring diagrams are essential when you are setting up any kind of bus network, whether that is CAN bus, RS-485, or RS-422. The core concept is simple but people mess it up constantly. A terminator is a resistor placed at each end of a bus line to absorb signal reflections. Without it, your data gets garbage. The diagram you follow dictates where those resistors go, what value they should be, and how they connect to your ground reference. The most common type you will encounter is the CAN bus terminator diagram. On a CAN network, you need a 120-ohm resistor at each physical end of the bus. That means if you have three devices on one run, the two outermost devices get the termination resistors, and the middle one does not. The wiring goes from the CAN_H pin on the first device through a twisted pair cable to the CAN_H pin on the last device, and the same for CAN_L. The 120-ohm resistor sits between CAN_H and CAN_L at both ends. That is the standard configuration, but your actual diagram may vary depending on the voltage level and the transceiver you are using. I spent about three weeks troubleshooting a CAN network in a piece of industrial equipment where everything looked correct on paper. The signals were garbage. Turns out the cable run was roughly 80 meters and someone had only terminated one end because they were following an incomplete diagram that showed a single terminator. Adding the second 120-ohm resistor at the far end brought the error rate down to nearly zero. It was frustrating but it confirmed what every decent design guide says: both ends need termination unless you have a very specific reason not to.
RS-485 Terminator Wiring
RS-485 uses a similar approach but with some differences. The bus impedance for RS-485 is typically 120 ohms as well, so you use the same resistor value. However, RS-485 is half-duplex in most implementations, which changes the dynamic a bit. The A and B lines carry the differential signal, and you place the termination resistor across A and B at each end of the cable. One thing people overlook: the termination resistor should be placed as close to the transceiver pins as possible, not just anywhere on the cable. A couple of inches of unterminated trace between the resistor and the chip can cause enough reflection at higher baud rates to create bit errors. I once had a project where a team placed the termination resistors at the junction box instead of at the actual device transceivers. At 9600 baud it worked fine, but when they bumped the baud rate to 115200 to speed up data throughput, the errors started appearing. Moving the resistors directly adjacent to the transceiver pins fixed it immediately. This is a detail you will not always see in a basic diagram, which is why having a detailed one matters.
Common Pitfalls When Building Your Circuit
The biggest mistake is assuming every device on the bus needs a terminator. It is only the two endpoints. If you put terminators on every device, you effectively place multiple 120-ohm resistors in parallel and you drop the total resistance to something like 40 ohms or less, which loads the transceivers and can cause signal degradation or even damage over time. Another issue is using the wrong resistor power rating. For typical low-speed CAN or RS-485 applications, a 1/4 watt resistor is plenty. But in high-noise environments or at longer cable lengths, the resistor can dissipate more heat than expected. I have seen 1/4 watt resistors fail after a few months in outdoor installations where temperature cycling and voltage spikes added up. There is also the matter of ground referencing. Some diagrams show a ground connection alongside the termination resistor, especially in CAN systems where the common-mode voltage matters. If your system has significant ground potential difference between the two ends, you may need additional grounding measures beyond just the termination resistor. A terminator alone will not fix ground loop issues.
Get the Full Details
Downloadable Terminator Wiring Diagram Resources
If you need a proper Terminator Wiring Diagram to reference during installation, the best sources are the manufacturer datasheets for your specific transceiver IC. NXP, TI, and Microchip all publish application notes with complete diagrams. The Bosch CAN specification document also includes detailed termination diagrams that cover most automotive and industrial use cases. You can find those freely on their respective websites. Avoid random diagram sites that do not cite their source, because the variations between different standards are easy to mix up if you are not careful. One last note on what this approach cannot do: a termination diagram will not solve problems caused by poor cable selection. Using random bell wire instead of a proper 120-ohm impedance twisted pair is going to cause issues no amount of correct termination wiring will fix. The cable matters as much as the resistors. Also, if your bus has more than two devices and long cable runs, you may need repeaters or switches, which complicates the termination scheme significantly. In those cases, a simple diagram is not enough and you should consult the transceiver manufacturer directly.
