A refinery once swapped out PLC modules, remote I/O adapters, and communication cards chasing a random evening network failure. The actual cause was one missing 75 ohm resistor. Here is exactly why that tiny part carries so much weight.
A ControlNet termination resistor is a 75 ohm resistor installed at both physical ends of the trunk cable, and its only job is to absorb the signal before it can bounce back into the network.
ControlNet networks tie together PLCs, remote I/O racks, HMIs, drives, and process controllers that all depend on deterministic, scheduled communication to function correctly. Get the termination wrong, and none of that hardware being correctly programmed will save you.
Why Exactly 75 Ohms? It Comes Down to the Cable
ControlNet runs over RG-6 quad shield coaxial cable, and RG-6 has a characteristic impedance of 75 ohms. That single fact is the entire answer.
A termination resistor's job is to match the cable it's terminating, nothing more. Since the cable itself is 75 ohms, the resistor absorbing the signal at each end has to be 75 ohms too.
Matching condition: Z(termination) = Z(cable) = 75 Ω
The Reflection Coefficient: Why Mismatched Impedance Bounces Signal Back
Once a communication line runs fast enough, it stops behaving like a simple wire and starts behaving like a transmission line, one where a mismatch at the end genuinely reflects energy backward.
Reflection coefficient: Γ = (ZL - Z0) / (ZL + Z0)
ZL = load (termination) impedance, Z0 = cable impedance
When ZL = Z0, Γ = 0, meaning no reflection at all
Try It: Reflection Coefficient Calculator
Enter a cable impedance and a termination value to see exactly how much signal would reflect back.
Why 50 Ohm and 120 Ohm Resistors Are the Wrong Choice
Both wrong values come from the same mistake: borrowing a termination rule from a completely different cable type.
Older Ethernet coaxial systems ran on RG-58 cable at 50 ohms, so a 50 ohm terminator was correct there, just not on ControlNet's RG-6 cable. Meanwhile, DeviceNet, CAN Bus, and Profibus all run on twisted pair cable with a 120 ohm characteristic impedance, which is correct for them and wrong for ControlNet's coaxial trunk.
ControlNet vs Ethernet, CAN Bus, and DeviceNet Termination
| Network | Cable Type | Characteristic Impedance | Termination Resistor |
|---|---|---|---|
| ControlNet | RG-6 coaxial | 75 Ω | 75 Ω |
| Ethernet 10Base2 | RG-58 coaxial | 50 Ω | 50 Ω |
| CAN Bus | Twisted pair | 120 Ω | 120 Ω |
| DeviceNet | Twisted pair | 120 Ω | 120 Ω |
The pattern across every row is the same: the resistor value was never arbitrary, it always matches whatever cable that specific network happens to run on.
What Actually Happens When Termination Goes Missing
The classic explanation is an echo in a tunnel, sound travels to the end, hits a wall, and comes straight back. An unterminated ControlNet cable does the same thing electrically, and the returning signal collides with whatever is arriving next.
Flashing Red Network LEDs
Media fault and communication fault indicators are often the first visible warning sign.
Random PLC Communication Loss
Drops tend to be sporadic and genuinely difficult to reproduce on demand.
Communication Retries and Packet Corruption
Reflected energy corrupts the signal the network is simultaneously trying to receive.
Scheduled Data Transfer Failure
ControlNet's deterministic scheduling depends on clean timing that reflections directly disrupt.
These symptoms tend to worsen during motor startup, VFD switching, or heavy network traffic, which is exactly what makes a missing terminator so easy to mistake for a completely different hardware fault.
How to Test ControlNet Termination With a Multimeter
This is the fastest real diagnostic available, and it only takes a few minutes with the network powered down.
| Measured Resistance | What It Means |
|---|---|
| ~37.5 Ω | Normal, both 75 Ω terminators are present and in parallel |
| ~75 Ω | One terminator is missing |
| Infinite (open) | Both terminators are missing |
| Very low or near 0 Ω | A short circuit or an extra unintended terminator |
That 37.5 ohm figure isn't arbitrary either, it's simply two 75 ohm resistors sitting in parallel across the trunk, which is exactly what two correctly installed end terminators look like electrically.
Installing ControlNet Terminators the Right Way
Install exactly one terminator at each physical end of the trunk, never in the middle.
Use only approved ControlNet taps and proper BNC connectors.
Confirm RG-6 quad shield cable is used throughout, not RG-58 or RG-59.
Verify shielding continuity across every connector and tap.
Route cable away from power conductors, VFDs, and other noise sources.
Test resistance across the trunk before startup, expect roughly 37.5 ohms.
FAQs on ControlNet 75 Ohm Termination
What we learn today
- A ControlNet termination resistor must be 75 ohms because it has to match the 75 ohm characteristic impedance of the RG-6 coaxial cable the network runs on.
- Mismatched impedance causes real signal reflection, described by Γ = (ZL - Z0)/(ZL + Z0), and that reflection is what produces retries, corruption, and instability.
- 50 ohm and 120 ohm resistors are wrong for ControlNet specifically because they belong to different cable types, RG-58 coaxial and twisted pair, not because they are universally incorrect values.
- A correctly terminated ControlNet trunk reads roughly 37.5 ohms across it, since two 75 ohm terminators in parallel produce exactly that value.
- Many real-world "PLC hardware failures" turn out to be a single missing termination resistor, since the symptoms look nearly identical.