B&B Electronics MODSCAN32 - Manual - Manual (Page 14)

14
MODBUS Message Types
ASCII Framing
Framing in ASCII Transmission mode is accomplished by the use of the unique colon, (:), character to
indicate the beginning of frame and carriage return/line feed, (CRLF), to delineate end of frame. The line
feed character also serves as a synchronizing character which indicates that the transmitting station is ready
to receive an immediate reply.
BEGIN
FRAME
ADDRESS FUNCTION DATA ERROR
CHECK EOF READY
TO
RECEIVE
: 2-CHAR 16-
BIT 2-CHAR 16-
BITS N X 4-CHAR
N X 16-BITS 2-CHAR
16-BITS CR LF
RTU Framing
Frame synchronization can be maintained in RTU transmission mode only by simulating a synchronous
message. The receiving device monitors the elapsed time between receipt of characters. If three and one-
half character times elapse without a new character or completion of the frame, then the device flushes the
frame and assumes that the next byte received will be an address.
T1,T2,T3
ADDRESS FUNCTION DATA CHECK T1,T2,T3
8-BITS 8-BITS N X 8-BITS 16-BITS
Address Field
The address field immediately follows the beginning of frame and consists of 8-bits, (RTU), or 2 characters,
(ASCII). These bits indicate the user assigned address of the slave device that is to receive the message
sent by the attached master.
Each slave must be assigned a unique address and only the addressed slave will respond to a query that
contains its address. When the slave sends a response, the slave address informs the master which slave is
communicating. In a broadcast message, an address of 0 is used. All slaves interpret this as an instruction
to read and take action on the message, but not to issue a response message.