CIP / EtherNet/IP Protocol

Overview

CIP (Common Industrial Protocol) over EtherNet/IP is used to communicate with Allen-Bradley PLCs. tentacle-ethernetip handles both reading and writing tags.

CIP Type Codes

Type Code	Type	Size
0xC1	BOOL	1 byte
0xC2	SINT	1 byte
0xC3	INT	2 bytes
0xC4	DINT	4 bytes
0xC5	LINT	8 bytes
0xC6	USINT	1 byte
0xC7	UINT	2 bytes
0xC8	UDINT	4 bytes
0xC9	ULINT	8 bytes
0xCA	REAL	4 bytes
0xCB	LREAL	8 bytes

Reading Tags

// typeCodeToDatatype() converts CIP type codes to string names
const datatype = typeCodeToDatatype(0xCA); // "REAL"

Writing Tags

// datatypeToTypeCode() maps datatype string to CIP type code
const typeCode = datatypeToTypeCode("REAL"); // 0xCA

// encodeTagValue() encodes JS value to Uint8Array
const encoded = encodeTagValue(42.5, "REAL");
// Result: Float32 little-endian bytes

Encoding Examples

// BOOL
encodeTagValue(true, "BOOL");   // Uint8Array([1])
encodeTagValue(false, "BOOL");  // Uint8Array([0])

// INT (16-bit signed, little-endian)
encodeTagValue(1000, "INT");    // Uint8Array([0xE8, 0x03])

// DINT (32-bit signed, little-endian)
encodeTagValue(100000, "DINT"); // Uint8Array([0xA0, 0x86, 0x01, 0x00])

// REAL (32-bit float, little-endian)
encodeTagValue(3.14, "REAL");   // IEEE 754 float bytes

Connection Types

ForwardOpen (Connected): Establishes a connection for multiple operations
- May fail with status 0x1 on some PLCs
- Falls back to UCMM automatically
UCMM (Unconnected): Single request/response
- Works on all PLCs
- Slightly higher overhead per request

UDT (User Defined Types)

Template parsing extracts member names and types
Important: Trust actual PLC response typeCode over template
Alignment can cause issues in template parsing

Gotchas

Datatype trust hierarchy: PLC response typeCode > cached datatype > template parsing
Little-endian: All multi-byte values are little-endian
ForwardOpen failures: Normal, just falls back to UCMM
String values: Parse to number before encoding (except BOOL which accepts "true"/"false")