DoCAN
Diagnostics & Protocols · 7 specs
Direct Answer
Diagnostics on CAN (DoCAN, standardized under ISO 15765-2 and ISO 15765-4) is the network and transport layer protocol that enables diagnostic protocols (such as UDS ISO 14229 and OBD-II ISO 15031) to transmit multi-byte messages (up to 4095 bytes) over 8-byte CAN bus frames. Managing packet segmentation, Flow Control handshakes, and block sizes.
Source: ISO 15765-2:2016 (Transport protocol and network layer services) & ISO 15765-4 (Requirements for emissions-related systems) · Last reviewed Aug 28, 2026
Specs
| Standardization | ISO 15765-2 (ISO-TP Network Layer) / ISO 15765-4 (Emissions CAN) |
| Maximum Message Length | Up to 4,095 bytes (segmented across multiple sequential 8-byte CAN frames) |
| Single Frame (SF–PCI 0x0_) | Used when total payload fits in a single frame (up to 7 bytes in Classic CAN; byte 0 indicates data length) |
| First Frame (FF–PCI 0x1_) | First frame of a segmented message; bytes 0-1 encode the total 12-bit message length (e.g. 0x10 0x80 = 128 bytes) |
| Flow Control Frame (FC–PCI 0x3_) | Receiver sends Flow Status (Clear to Send), Block Size (BS), and Separation Time (STmin) pacing inter-frame delays |
| Consecutive Frame (CF–PCI 0x2_) | Transmits remaining data with a 4-bit rolling Sequence Number (0 to 15) to detect dropped packets |
| Emissions OBD Request IDs | Standard 11-bit functional broadcast ID: 0x7DF | Standard response IDs: 0x7E8 (Engine ECM), 0x7E9 (TCM) |
ISO-TP Segmentation and Flow Control
Because a classical CAN frame holds only 8 bytes of data, requesting a 200-byte fault log requires ISO-TP (ISO 15765-2). The sender transmits a First Frame (FF) with the total length; the scanner answers with a Flow Control (FC) packet specifying STmin=10ms (delay between frames); the sender then streams Consecutive Frames (CF) indexed with rolling counter numbers until all bytes are delivered.