Here is the question every engineer eventually asks: how can you trust an Ethernet packet to stop a hydraulic press? Standard Ethernet drops packets. Switches buffer and re-order them. Cables get unplugged mid-cycle. Yet CIP Safety lets a safety PLC monitor an emergency stop button over the same network that carries your I/O and your HMI traffic — and certify the whole arrangement to SIL3, Cat 4, PL e.
The answer is not magic. It is a careful set of cross-checks that assume the network is hostile and verify every safety message independently. The standard network is the “black channel” — untrusted, unrated, possibly broken at any moment. The safety layer on top is what makes the safety claim valid.
This article walks through how CIP Safety actually works: the Black Channel principle, the safety objects that live in every device, what a SafetyOpen connection looks like compared to a standard Forward_Open, how Time Coordination Messages catch delays, and the safety identifiers (SNN, SCID, CFUNID) you will see in your engineering software. For the broader CIP context, see our CIP Protocol Complete Guide.
Table of Contents
What CIP Safety is in one paragraph
CIP Safety is an extension to the Common Industrial Protocol that adds functional safety to standard CIP networks — EtherNet/IP, DeviceNet, and Sercos III. It runs as an additional application-layer protocol on top of CIP, certified to IEC 61508 SIL3, EN ISO 13849-1 Cat 4 PL e, and IEC 62061. The safety logic lives in the end devices, not the network. Standard and safety devices share the same wire, the same switches, and the same cables. CIP Safety has been deployed since 2005 and is the most widely installed safety network protocol in North America.
The Black Channel principle
This is the core concept and the one most articles skip.
A safety network does not need a safety-rated network. It needs a safety-rated way of using a network. CIP Safety treats the underlying CIP communication path as untrusted — a black box that can corrupt messages, drop them, reorder them, repeat them, or insert new ones. The safety layer on top assumes all of that can happen, and validates every message independently.
If the standard network is working fine, the safety layer adds a tiny bit of overhead and everything runs. If the standard network fails — packet loss, switch outage, cable cut, latency spike — the safety layer detects it within a defined time and forces the device into a safe state. The standard network is not certified. Only the application-layer logic in the endpoints is certified.
This is why CIP Safety can:
- Run over standard Ethernet switches with no safety rating
- Coexist with non-safety traffic on the same wire
- Be routed across multiple CIP networks (EtherNet/IP → DeviceNet → EtherNet/IP)
- Use unmodified Ethernet cabling
The price you pay is the certified safety logic at each endpoint, which is more complex than non-safety CIP.
The seven errors any safety protocol must detect
IEC 61784-3 — the standard for safety-related communication over fieldbuses — lists seven communication error types that a safety protocol must detect and handle. CIP Safety covers all seven through specific mechanisms:
| Error type | CIP Safety mechanism |
|---|---|
| Corruption | Safety CRC plus complement data bytes |
| Unintended repetition | Monotonic time stamps and sequence numbers |
| Incorrect sequence | Time coordination and sequence verification |
| Loss | Time expectation (timeout detection) |
| Unacceptable delay | Time coordination message round-trip checking |
| Insertion | Connection ID combined with Safety Network Number (SNN) |
| Masquerade | Safety Configuration ID (SCID) and SafetyOpen authentication |
Each safety message carries enough redundant information that any one of these failures triggers detection — and detection means the safety connection drops and the device goes to its safe state.
The safety objects every CIP Safety device has
A standard CIP device has objects like Identity, TCP/IP Interface, and Connection Manager. A CIP Safety device has additional safety-specific objects:
| Class ID | Object | What it does |
|---|---|---|
| 0x39 | Safety Supervisor | Manages the device’s safety state machine (idle, configuring, running, abort). Every CIP Safety device has one. |
| 0x3A | Safety Validator | Validates incoming safety data (consumer side) and packages outgoing safety data (producer side). |
| 0x3B | Safety Discrete Input Point | One channel of safety discrete input. |
| 0x3D | Safety Discrete Input Group | A group of safety discrete inputs handled together. |
| 0x3F | Safety Discrete Output Point | One channel of safety discrete output. |
| 0x41 | Safety Discrete Output Group | A group of safety discrete outputs. |
| 0x44 | Safety Analog Input Point | One channel of safety analog input. |
| 0x4B | Safety Stop Functions | Implements certified stop functions (STO, SS1, SS2). |
| 0x4C | Safety Limit Functions | Implements certified limit functions (SLS, SLP, SDI). |
The Safety Supervisor is the boss. It controls when the device exchanges safety data, accepts new configurations, and decides when to drop into safe state. It also holds the SNN and SCID (we cover these below).
The Safety Validator is where the seven-error detection happens at runtime. There is one Validator per safety connection — and every safety connection produces or consumes data through a Validator instance.
SafetyOpen vs Forward_Open
In standard CIP, you open an I/O connection with the Forward_Open service. In CIP Safety, the equivalent is SafetyOpen — but it carries much more data and is validated end-to-end.
A SafetyOpen request includes:
- Connection size, RPI (Requested Packet Interval), trigger type — same as Forward_Open
- SNN (Safety Network Number) of the originator
- CFUNID (Configuration Owner Unique Network Identifier)
- SCID (Safety Configuration Identifier) of the target
- Safety connection parameters (Ping Interval, Time Coordination behavior)
- Cryptographic-style validation that the target matches the configured safety device
If any of this does not match what the device expects — wrong SNN, wrong SCID, wrong configuration — the SafetyOpen is rejected and no safety data flows. This is how CIP Safety prevents masquerade attacks. A spoofed device cannot replay a captured SafetyOpen because the SNN/SCID combination uniquely identifies the originator-target pair.
There are two flavors:
- SafetyOpen Type 1 — Configuration is delivered as part of SafetyOpen. The safety PLC sends both the connection parameters and the device configuration in one shot.
- SafetyOpen Type 2A / 2B — Configuration is loaded separately (typically by a configuration tool), then SafetyOpen establishes the connection. The SCID verifies that the configuration in the device matches what the safety PLC expects.
Type 2 is more common in larger systems because the configuration tool can validate the safety logic offline.
Safety identifiers — SNN, SCID, CFUNID
These show up in every CIP Safety engineering tool. Knowing what they are saves hours of “why does the safety connection refuse to open” troubleshooting.
| Identifier | Length | What it is | Where it comes from |
|---|---|---|---|
| SNN (Safety Network Number) | 6 bytes (10 hex digits) | Unique identifier for a safety network or subnet | Auto-generated by the safety PLC (timestamp + manual) or set by the engineer |
| SCID (Safety Configuration Identifier) | 4 bytes timestamp + 2-byte CRC | Identifies a specific safety configuration in a device | Calculated from the configuration data — changes every time the configuration changes |
| CFUNID (Configuration Owner Unique Network Identifier) | SNN + Node Address | Identifies which originator owns a safety device’s configuration | Combines the configurator’s SNN with the configurator’s node address |
In practice:
- Every safety device on a network has an SNN. Devices on the same safety subnet share an SNN; devices on different subnets do not.
- Every safety configuration has an SCID. Change one parameter, the SCID changes, and existing safety connections must be re-opened.
- Every safety device knows its CFUNID — who configured it. If a second safety PLC tries to claim it, the device rejects the claim because the CFUNID does not match.
When you commission a new safety device, you have to set the SNN. Most engineering tools auto-generate it. When you replace a failed safety device, the new device needs the SNN of the network it joins (usually entered during commissioning or read from a configuration card).
Time Coordination Messages — catching delays
Standard CIP I/O messaging just exchanges data at the RPI rate. CIP Safety adds Time Coordination Messages — a periodic round-trip exchange between consumer and producer used to detect timing problems.
Here is how it works:
- The consumer (the device receiving safety data) sends a Time Coordination Message periodically.
- The producer receives it, processes it, and sends a Time Coordination Message Response back.
- The consumer measures the round-trip time and compares it to its Time Expectation value.
- If the round-trip exceeds the expectation — or if a response is missing entirely — the safety connection fails and the consumer goes to safe state.
This catches delays that would otherwise let stale data look fresh. A safety message that takes 800 ms to arrive when the safety reaction time is 100 ms is just as dangerous as a corrupted message. Time Coordination catches it.
The Time Expectation is set at configuration time, based on the application’s safety reaction time. Setting it too tight causes nuisance trips. Setting it too loose violates the safety reaction time requirement. This is one of the harder commissioning parameters to get right.
How CIP Safety data actually flows
Let’s walk through what happens when a safety PLC reads an emergency stop button over EtherNet/IP.
- Configuration phase. The engineer configures the safety PLC, the E-stop device, and their connection in the engineering tool. The tool generates SNNs, SCIDs, RPIs, and Time Expectations. The configuration is downloaded to the PLC and to the safety device.
- SafetyOpen. The safety PLC issues a SafetyOpen to the E-stop device. The device validates SNN, SCID, and CFUNID. If everything matches, the Safety Supervisor moves to RUNNING state.
- Cyclic exchange. Every RPI (typically 5–20 ms), the E-stop device produces a safety message containing the button state plus integrity data (sequence number, time stamp, Safety CRC, complement bytes).
- Validation on consumer side. The safety PLC’s Safety Validator checks the message: CRC valid? Sequence in order? Time stamp consistent? Complement matches? If any check fails, the safety connection drops and the PLC sees the E-stop as “pressed” (fail-safe).
- Time Coordination. Periodically (typically several times per second), the consumer initiates a Time Coordination round-trip. If it times out, the connection drops.
- Application logic. The safety PLC reads the validated E-stop state and runs its certified safety logic. The output goes through the same kind of safety connection to a safety contactor or drive.
All of this happens on a standard EtherNet/IP wire alongside non-safety I/O, HMI traffic, and SCADA polls. The safety layer does not know or care what else is on the network.
CIP Safety on the wire — Wireshark
CIP Safety messages look like normal EtherNet/IP UDP packets in Wireshark, but with an additional safety payload structure that Wireshark can dissect. Useful filters:
cip # All CIP messages
cipsafety # Only CIP Safety frames
cip.service == 0x54 # SafetyOpen Type 1
cip.service == 0x55 # SafetyOpen Type 2A
cip.service == 0x56 # SafetyOpen Type 2B
cip.service == 0x5A # SafetyCloseFor the standard EtherNet/IP transport layer, see our Wireshark for EtherNet/IP guide. For the port mechanics, see CIP Protocol Ports: How EtherNet/IP Uses Ports 44818 and 2222 — CIP Safety uses the same UDP port 2222 as standard implicit messaging.
A typical CIP Safety I/O message contains:
- The standard CIP I/O header (connection ID, sequence count)
- The actual safety data
- The complement of the safety data (each bit inverted)
- A 32-bit time stamp
- A 16-bit or 32-bit Safety CRC
- Connection identifiers tied to the SNN
The complement-and-CRC combination is what gives the Hamming distance required for SIL3 certification. A single-bit corruption is detected by either the CRC or the complement mismatch. A multi-bit corruption that fools the CRC must also produce a valid complement, which is statistically negligible.
Common CIP Safety problems and what they mean
Most CIP Safety field issues are configuration mismatches, not protocol bugs.
| Symptom | Likely cause | What to check |
|---|---|---|
| SafetyOpen rejected, connection refuses to open | Wrong SNN or SCID — configuration mismatch | Re-download the safety configuration. Verify SNN matches between PLC and device. SCID changes whenever you change config. |
| Connection opens then drops randomly | RPI too aggressive for network conditions, or Time Expectation set too tight | Loosen RPI. Increase Time Expectation. Check for switch QoS issues. |
| Replacement safety device cannot be commissioned | Old SNN/CFUNID still claims the slot | Reset the new device to factory defaults. Re-enter the network’s SNN during commissioning. |
| Safety task scan time keeps growing | Too many safety connections, or RPI too fast for the safety task period | Reduce connection count or increase RPIs. The safety task period must accommodate all active connections. |
| “Configuration Owner” error in engineering tool | Two safety PLCs trying to own the same device | Only one safety PLC can own a CIP Safety device’s configuration. Reset CFUNID on the device to release it. |
| Safety device goes to safe state intermittently | Network jitter exceeding Time Expectation | Capture with Wireshark. Look at the time between safety messages. Compare to the configured Time Expectation. |
| Safety CRC errors in diagnostics | Bad cable, EMI, or marginal switch | Standard network troubleshooting. Replace cable. Check shielding. Reseat connectors. |
| Cannot route safety data across networks | Router or bridge does not support CIP Safety routing | Verify the routing device is CIP Safety capable. Not all standard CIP routers are. |
CIP Safety vs PROFIsafe vs FSoE
The three dominant industrial safety protocols are not interoperable. Knowing how they relate matters when you design a plant with mixed equipment.
| Aspect | CIP Safety | PROFIsafe | FSoE (Safety over EtherCAT) |
|---|---|---|---|
| Steward | ODVA | PI (PROFIBUS & PROFINET International) | ETG (EtherCAT Technology Group) |
| Carries on | EtherNet/IP, DeviceNet, Sercos III | PROFINET, PROFIBUS | EtherCAT |
| Cert level | SIL3 / Cat 4 / PL e | SIL3 / Cat 4 / PL e | SIL3 / Cat 4 / PL e |
| Base standard | IEC 61508 | IEC 61508 | IEC 61508 |
| Black Channel | Yes | Yes | Yes |
| Primary ecosystem | Rockwell, Sercos vendors | Siemens, Phoenix Contact | Beckhoff, Omron, B&R |
All three use the Black Channel principle. All three detect the same seven communication errors. The differences are in how they frame the safety data and how they tie into their parent ecosystems.
You cannot mix safety protocols across a network boundary. A CIP Safety device on EtherNet/IP cannot talk safety to a PROFIsafe device on PROFINET, even with a gateway. Each safety network operates as its own certified island. If you have a plant with both, the safety functions run on separate networks even when the standard data crosses between them. This is one of the practical considerations in our PROFINET vs EtherCAT comparison.
When CIP Safety is the right choice
- Rockwell-centric installations. GuardLogix, Kinetix 5700 with safety, PowerFlex 755T with safety — these are CIP Safety native.
- Plants already using EtherNet/IP for standard I/O. Adding CIP Safety to an existing EtherNet/IP network requires no new wiring or switches. The safety devices just attach.
- Machine builders selling into North American markets. CIP Safety dominates US automotive, packaging, and material handling. Buyers expect it.
- Sercos III motion systems. ODVA and Sercos partnered specifically so CIP Safety is the certified safety layer on Sercos.
- Distributed safety across multiple cells. CIP Safety’s ability to route across CIP networks (with appropriate routers) makes plant-wide safety architectures possible.
It is less compelling on Siemens-heavy plants (PROFIsafe is native there) or pure EtherCAT machine designs (FSoE is native there). Forcing CIP Safety into those ecosystems means giving up the native engineering integration.
Certification and standards context
CIP Safety as a protocol is certified — but a CIP Safety system in your plant is not automatically certified. The certification chain has three parts:
- The protocol itself is certified to IEC 61508 SIL3 by TÜV Rheinland. ODVA holds the protocol certification.
- Each safety device carries its own SIL3 / Cat 4 / PL e certificate from its manufacturer.
- Your application — how you connect, configure, and program the safety functions — is your responsibility to validate against the applicable application standards (typically EN ISO 13849-1, IEC 62061, or sector-specific standards like EN ISO 10218 for robotics).
The protocol gives you the communication foundation. The device certification gives you the hardware foundation. The application validation is on you. Skipping the application validation does not get you a SIL3 system just because the wire is SIL3-rated. This is the most common misunderstanding among engineers new to functional safety.
For the broader cybersecurity context (which is separate from but related to functional safety), see our IEC 62443 foundational requirements article.
Frequently asked questions
What is CIP Safety?
CIP Safety is a functional safety extension to the Common Industrial Protocol. It adds certified safety communication to standard CIP networks (EtherNet/IP, DeviceNet, Sercos III), supporting SIL3, Cat 4, and PL e safety functions. Safety and standard devices share the same wire, but the safety layer runs end-to-end validation on every message.
What safety integrity level does CIP Safety support?
CIP Safety supports up to SIL3 under IEC 61508, equivalent to Cat 4 / PL e under EN ISO 13849-1. It also supports lower levels (SIL2 / Cat 3 / PL d) for applications where the higher rating is not needed.
What is the Black Channel principle?
The Black Channel principle treats the underlying network as untrusted. The safety protocol assumes the network can corrupt, lose, repeat, or insert messages, and adds enough redundancy and validation at the endpoint to detect all of these. The network itself does not need to be safety-rated — only the safety logic in the end devices is certified.
What is the SNN (Safety Network Number)?
The SNN is a 6-byte (10 hex digit) identifier that uniquely identifies a safety network or subnet. Every safety device knows its SNN, and every safety connection validates that the SNN matches what the safety PLC expects. The SNN is usually auto-generated from a timestamp during commissioning.
What is the difference between Forward_Open and SafetyOpen?
Forward_Open establishes a standard CIP I/O connection. SafetyOpen establishes a safety CIP connection. SafetyOpen carries additional parameters (SNN, SCID, CFUNID, Time Expectation) that are validated end-to-end. A standard CIP device does not understand SafetyOpen, and a safety connection cannot be established with just a Forward_Open.
Does CIP Safety need special cables or switches?
No. CIP Safety runs over standard Ethernet infrastructure — the same cables, switches, and connectors as non-safety EtherNet/IP. The safety layer handles error detection at the application level, so the network itself does not need safety certification.
Can CIP Safety and CIP Security coexist?
Yes. CIP Safety covers functional safety (preventing harm from equipment malfunction). CIP Security covers cybersecurity (preventing harm from network attacks). They are independent layers and can run together on the same device.
Why does my CIP Safety connection drop intermittently?
The most common causes are: Time Expectation set too tight for network jitter, RPI faster than the safety task period, marginal cable or EMI causing Safety CRC errors, or a switch buffering traffic in a way that exceeds the round-trip time check. Use Wireshark to measure actual safety message timing and compare to the configured Time Expectation.
Are CIP Safety, PROFIsafe, and FSoE interoperable?
No. All three are independent safety protocols. Each is tied to its parent network family — CIP Safety on EtherNet/IP and DeviceNet, PROFIsafe on PROFINET and PROFIBUS, FSoE on EtherCAT. A gateway can pass standard data between networks, but safety functions must stay within one safety network.
Do I need a safety PLC to use CIP Safety?
Yes. CIP Safety requires a certified safety controller (also called a safety PLC) as the originator of safety connections. Standard ControlLogix PLCs cannot host CIP Safety connections — you need GuardLogix or equivalent. The safety devices themselves (E-stop modules, light curtains, safety I/O blocks) are also certified products, not standard ones.