Common External Message Interface (cEMI)

KNX defines legacy External Message Interfaces (EMI1, EMI2) that have frame formats specific to each individual media (twisted pair, powerline, radio frequency, etc.).

The goal of Common External Message Interface (Common EMI, or cEMI) is to define a single frame format that can be used across all media.

Common EMI frames are defined only for KNX Data Link Layer primitives in KNX 1.1.

Common EMI Frame Format

Common EMI frame has a variable size but does follow this generic structure:

 +--------+--------+-------- ... --------+---------------- ... ----------------+
 |Msg Code| AI Len |      Add. Info      |           Service Info              |
 +--------+--------+-------- ... --------+---------------- ... ----------------+
   1 byte   1 byte      [0..n] bytes                     n bytes

Where:

Msg Code	Message code matching a data link layer primitive (see details below)
AI Len	Additional Info Length, zero value can be used if no additional info is required
Add. Info	Additional Info – optional if zero additional info length is set.
Service Info	This structure changes depending on the data link layer primitive chosen with message code parameter. See the sample Common EMI frames below.

Each Common EMI frame contains a message code as its first byte. A message code is defined for each data link primitive as shown in the tables below.

Common EMI Message Codes for Data Link Layer Primitives

From Network Layer to Data Link Layer

Data Link Layer Primitive	Message Code	Data Link Layer Service	Service Description	Common EMI Frame
L_Raw.req	0x10
L_Data.req	0x11	Data Service	Primitive used for transmitting a data frame	Sample Common EMI frame
L_Poll_Data.req	0x13	Poll Data Service

From Data Link Layer to Network Layer

Data Link Layer Primitive	Message Code	Data Link Layer Service	Service Description
L_Poll_Data.con	0x25	Poll Data Service
L_Data.ind	0x29	Data Service	Primitive used for receiving a data frame
L_Busmon.ind	0x2B	Bus Monitor Service
L_Raw.ind	0x2D
L_Data.con	0x2E	Data Service	Primitive used for local confirmation that a frame was sent (does not indicate a successful receive though)
L_Raw.con	0x2F

From Common EMI Management Client to Common EMI Management Server

Data Link Layer Primitive
M_PropRead.req
M_PropWrite.req
M_Reset.req

From Common EMI Management Server to Common EMI Management Client

Data Link Layer Primitive
M_PropRead.con
M_PropWrite.con
M_PropInfo.ind
M_Reset.ind