PTX Interface Specification

This repository hosts the neutral, multi-vendor PTX Interface Specification between C-ITS On-Board Units (OBUs) and ITCS On-Board Computers (IBIS) used in public transport.

PTX stands for "Public Transport to Everything", inspired by the term "V2X"

What is the PTX Interface Specification?

The PTX Interface Specification defines an open, vendor-neutral interface between C-ITS On-Board Units ("OBU") and ITCS On-Board Computers ("IBIS") used in public transport vehicles. It enables interoperable exchange of vehicle position, operation status, service identifiers, and C-ITS events between independent onboard systems. The protocol is based on MQTT to allow easy integration.

PTX improves the public transport technology ecosystem by:

• providing a standardized communication interface between OBU and IBIS
• reducing vendor lock-in and enabling multi-vendor competition
• simplifying tendering processes, in particular for OBUs
• ensuring long-term interoperability and lifecycle flexibility
• supporting future extensions through semantic versioning
• enabling modular upgrades and independent replacement of hardware components

The specification is openly published to support operators, authorities, vendors, and integrators in building interoperable, future-proof public transport fleets.

PTX is especially relevant for:

• Public transport operators (PTOs) and fleet owners
• Public transport authorities (PTAs) and regulators
• C-ITS OBU vendors
• ITCS or AVL system vendors
• Integrators, engineering consultancies

The goal of PTX is to define a stable, interoperable interface so that:

• OBUs and IBISs from different vendors are interoperable
• Implementations share a common, versioned set of messages and semantics
• Changes to the specification are discussed openly and applied consistently

For a summary of expected benefits to operators and procurement, see BENEFITS.md.

Scope

This repository contains:

• The human-readable specification documents
• The Protobuf schemas of PTX messages, from which JSON is generated
• The authoritative JSON Schemas for PTX messages exchanged on the wire
• Supporting information, and possibly tools and scripts

Governance

The specification is edited and maintained by a group of maintainers from the participating companies. The maintainers are listed under "Contact Persons" within the specification.

Evolution

The PTX Interface Specification uses semantic versioning (major.minor.patch):

• Patch releases (e.g. 2.0.1) are documentation-only updates and do
  not modify the wire format. Protobuf and JSON Schema remain unchanged.

• Minor releases (e.g. 2.1.0) will introduce new messages or fields.
  In such cases, a new schema version appears under dedicated directories such as schemas/proto/v2.1/.

• Major releases (e.g. 3.0.0) will break compatibility.
  The express goal is not having to create any major releases in the future.

A PTX specification version 2.1.0 is planned for release by the end of 2026. It will be fully compatible with version 2.0.x and will introduce enhancements with a particular focus on safety-related functionality.

License

Unless otherwise noted, the contents of this repository are licensed under the Apache License 2.0.
The PDFs are provided under the CC BY 4.0 licence.