IEEE 1588 PTP for Machine Vision: Profiles, Clocks and Accuracy

IEEE 1588 is the standard that defines the Precision Time Protocol (PTP), a way to synchronise the clocks of devices on a network to within a microsecond, and often to tens of nanoseconds, by exchanging timestamped messages and measuring the delay between them. In a machine vision system that shared clock is what lets several cameras, lights and sensors act at the same instant and lets every image carry a timestamp you can trust.

Machine vision systems often need precise synchronisation of cameras and other sensors to capture and process visual data accurately. This is where IEEE 1588 comes in. It is supported on many of our edge AI computers and network cards, and can be enabled on our rugged Ethernet switches and managed industrial switches, so the whole path from camera to computer shares one time reference.

This guide explains what IEEE 1588 is and how its versions differ, how the message exchange works with the offset and delay formulas, the clock types and profiles, and the difference between hardware and software timestamping. It then applies all of that to machine vision with a worked timing example, compares PTP with NTP and IEEE 802.1AS, and finishes with an implementation checklist and PTP-capable products from our range.

IEEE 1588 is the IEEE standard for a precision clock synchronisation protocol for networked measurement and control systems. It describes how clocks on a network elect the best source of time, exchange timestamped messages and correct themselves for both their offset from that source and the delay of the network between them. Our FAQ on how Precision Time Protocol synchronises clocks gives the short version; this article focuses on what that timing does for cameras and sensors and how to put it into an industrial system.

PTP works over standard Ethernet, either directly at Layer 2 or over UDP/IP, and needs no separate timing cable. That is why it has replaced dedicated trigger and timing wiring in many measurement, automation, power and broadcast systems.

IEEE 1588 versions

When a datasheet says IEEE 1588 v2 it means the 2008 protocol, and most current devices interoperate on it. The international edition is published as IEC 61588.

PTP works in two stages. First, clocks on the network choose a time source using the Best Master Clock Algorithm (BMCA), comparing the quality advertised in Announce messages: priority, clock class, accuracy and stability. The winner becomes the grandmaster. Then every other clock measures its offset from the clock above it with a four-timestamp exchange.

With the four timestamps the receiver can solve for both unknowns, assuming the path is the same length in each direction:

The receiver then steers its clock to remove the offset, and repeats the exchange many times a second. Any asymmetry between the two directions, such as a switch queue that delays traffic one way more than the other, appears directly as error, which is why the network hardware matters as much as the end devices. A peer-to-peer variant uses Pdelay_Req and Pdelay_Resp messages to measure each link separately rather than the whole path.

Recent amendments to the standard add timeTransmitter and timeReceiver as alternative terms for the master and slave roles; you will see both on datasheets.

A PTP network is a tree of clocks. The role a device plays decides how well it preserves time as it passes through.

A grandmaster usually takes its time from GNSS; our FAQ on GNSS vs GPS explains the satellite systems behind it. Without boundary or transparent clocks, each switch adds a variable queuing delay that the end devices cannot see, and accuracy falls from nanoseconds to microseconds or worse under load. A managed switch with PTP support is therefore part of the timing system, not just the data path.

IEEE 1588 leaves many choices open: transport, message rates, the delay mechanism, which clock types are allowed. A profile fixes those choices for an industry so devices from different makers interoperate. The profile matters more than the version number when you are matching devices.

For Time-Sensitive Networking, 802.1AS is the profile that every other TSN feature depends on; our guide to time sensitive networking explains how scheduling builds on it. In substations, the power utility profile supports IEC 61850 sampled values. Several of the Antaira managed switches we supply list the 1588 default, G.8265.1 and G.8275.1 profiles, which shows how a single switch may need to serve more than one.

Where a timestamp is taken decides how accurate it can be. A timestamp taken by the operating system includes interrupt latency, driver queues and scheduling delay, all of which vary. A timestamp taken by the network controller or PHY as the frame crosses the wire does not.

On Linux, the network controller exposes a PTP hardware clock, ptp4l runs the protocol against it and phc2sys copies that time to the system clock; ethtool shows whether a port supports hardware timestamping. A PTP-capable network interface card can add hardware timestamping to a computer whose onboard ports lack it. Figures in the table are typical; real accuracy depends on the network, the number of hops and the clock quality.

Machine vision systems increasingly use several cameras, lights, encoders and other sensors that must act together. Traditionally that meant hardware trigger lines from a controller to every camera. PTP lets the network carry the timing instead: every device shares a clock to within a microsecond or better, so a trigger can be scheduled for a moment in time rather than wired to an edge on a cable.

GigE Vision cameras are the most common place to meet PTP in vision. Shutter type matters as well: a global shutter sensor exposes every pixel at once, so the PTP timestamp describes the whole frame. In stereo cameras and multi-sensor rigs that combine cameras with LiDAR, shared time is what allows sensor fusion to line data up correctly.

Worked example: timing error on a conveyor

Consider an inspection line moving at 2 m/s, with three cameras inspecting the same part from different sides and a reject gate downstream. Any difference in when each camera fires turns directly into a position error on the part.

The same arithmetic applies to the reject gate: if the gate controller and the cameras share PTP time, the gate can act on the timestamp of the frame plus a known travel time, instead of relying on how quickly a message crosses the network. Figures are typical, not guaranteed; the camera’s own trigger latency and exposure time also count.

Machine vision is the ability of computers and robots to interpret visual information. These are some of the sectors where it is used and where shared timing helps.

Our spot spraying case study shows a multi-camera GigE vision system on NVIDIA Jetson AGX Orin in agriculture, the kind of moving-machine application where camera timing and position have to line up.

Both synchronise clocks over a network, but they are built for different accuracies. NTP is designed for computers across the internet and is accurate enough for logs and file times. PTP is designed for measurement and control on a local network and relies on hardware support to reach sub-microsecond accuracy.

Many systems use both: PTP for the devices that need tight timing and NTP for everything else. Some switches act as an NTP server and a PTP clock at the same time.

IEEE 1588 vs IEEE 802.1AS

IEEE 802.1AS, generalised PTP, is a profile of IEEE 1588 written for bridged Ethernet. It runs only at Layer 2, uses the peer-to-peer delay mechanism, requires every bridge in the path to be time-aware and fixes many settings that 1588 leaves open. That makes it simpler to deploy and easier to guarantee, which is why Time-Sensitive Networking builds on it. A device that supports 802.1AS supports a specific form of PTP, but not every IEEE 1588 device supports 802.1AS.

PTP support on industrial PCs varies with the hardware and software. Some industrial PCs offer it on their onboard Ethernet ports, others need a PTP-capable card. When selecting a computer for a machine vision or other time-sensitive application, check that the network controller supports hardware timestamping, and that the operating system and the application software can use it.

PTP event messages are multicast by default, so on a managed network check that IGMP snooping and VLAN settings let them reach every clock. Our guides to managed vs unmanaged switches and the types of Ethernet switches explain where a managed switch is needed.

Products from our catalogue that support IEEE 1588, from network cards to switches. Figures are from each product’s page.

Antaira designs and builds its switches and we supply and specify them, pre-configuring IP addressing, VLANs and timing settings where you ask. Diamond Systems builds its switch boards and rugged switches in full, and we supply them; browse our Ethernet switch boards and managed industrial switches for more. The PCIe-AE304bp also appears in our guide to single pair Ethernet, where vehicle sensors use automotive Ethernet.