Eaton MTL838B-MBF Analogue Multiplexer Receiver

Modbus message framing

Modbus messages must be structured (or 'framed') so that the different Modbus

components can detect the start, content structure and end point of a message. It also

allows any errors to be detected.

The framing used depends on the transmission mode chosen - ASCII  or RTU.

ASCII message framing

In ASCII mode, messages start with a 'colon' (:), which in hex is '3A'. The message

end is shown by 'carriage return/line feed' (CRLF) or 'OD OA' in hex .

The allowable characters for all other fields are hexadecimal 0-9. A-F. Networked

components monitor the bus continuously for the 'colon' character and when one is

received, they decode the next field (the address field) to find out if the address is for

that slave. If the address is for another slave, then no action is taken, and the slave

returns to monitoring for the 'colon' character. If  the field following the colon is the

address of the slave in question, then the slave continues to read the message and to

act on it's contents.

Intervals of up to one second can elapse between characters within the message, but

if an interval is greater than this, then the device assumes that an error has occurred.

If the delay occurs in the 'query' to a slave, then the addressed slave will discard the

message received up to that point and wait till the next message (marked by the colon

start character) is received.

RTU message framing

In RTU mode, the message begins with a gap in transmission of at least 3.5 character

periods. Network components monitor the bus continuously and when a 'silent' period

of more than 3.5 character periods is detected, the first character following the

transmission gap is translated to determine if it corresponds to the device's own

address.

The end of the transmitted message is marked by a further interval of at least 3.5

character periods duration. An new message can only begin after this interval.

The entire message field must be transmitted as a continuous stream. If an interval of

more than 1.5 character periods is detected during transmission of the message, then

the message is assumed to be incomplete and the device returns to waiting for the

next device address. The action taken on receipt  of an incomplete message is as for

receipt of an incorrect message, and it is ignored.

If a new message begins within 3.5 characters periods of the end of the previous

frame, the device again ignores the message.

The message fields

The address field

Slave addresses may be in the range 1 to 247 with Modbus (1 to 255 with JBUS). A

slave is addressed by the master placing the relevant address in the address field of

the query message. When the slave sends its response, it places its own address in

the message field to indicate to the master that the correct slave is replying.

Address '0' is used for 'broadcast' messages. All suitable slaves read them, but do not

provide responses to such query messages.

The function field

Function codes may be in the range 1 - 255. though not all functions will be supported

by all devices. When a message is sent from a master to a slave, the function code

defines the action that is required from the addressed slave. Examples of action

requested by the various function codes include: read input status; read register

content; change a status within the slave; etc..

When the slave sends its response to the master, it will repeat the function code

received, to indicate that the slave has understood the query and acted accordingly. If

the query instruction could not be carried out by the slave, an 'exception response' is

generated and the function code and data fields are used to inform the master of the

reason for the exception.

The exception response is generated by returning the original function code from the

master, but with its most significant bit set to '1'. Further information regarding the

exception response is passed to the master via the data field of the response

message. This tells the master what kind of error occurred and allows it to take the