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MODBUS Protocol
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1. Introduction
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MODBUS MCCB Protocol
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2. Physical Layer
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⊙ Communication port : RS485
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⊙ Asynchronous format : Each character is composed of 10 bit.
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(1 start bit + 8 data bits + 1 stop bit)
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⊙ Baud rate : 19200 bps
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⊙ Data Bits : 8 bits
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⊙ Parity : No parity
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⊙ Stop Bits : 1 bits
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⊙ Master-Slave method: Only master is qualified for requesting,
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while slave delivers the requested data and response to the required performance according to inquiry.
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3. Data Link Layer
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⊙ if master delivers the request frame, slave does the response frame.
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Each frame is differenciated by dead time.
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General form to send and receive frames is as follows:
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DESCRIPTIONSIZE
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SLAVE ADDRESS1 byte
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FUNCTION CODE1 byte
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DATAN byte
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CRC2 byte
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DEAD TIME3.5 bytes transmission time
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MasterRequestResponseSlave
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Slave addressDevice AddressDevice AddressSelf-address
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Definition of slave action Function CodeFunction CodeEcho or MSB=1
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Additional information to perform the requested action DataDataRequested data or
exception code
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CRCError checkError checkCRC
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⊙ SLAVE ADDRESS
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Valid slave device address range : 0~247 decimal
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Slave device address range in practice : 1~32 decimal
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If the range of slave device address of the requested frame by master is 0, it means that
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master device bradcasts to all slaves.
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In case that master requests to slave, fill the relative slave address in address field and send it.
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If slave responses to master, fill its own adress in address field and send it.
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⊙ FUNCTION CODE
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Valid range : 1~255
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normal : 1~127, error : 129 ~ 255(normal + 0x80)
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It defines slave's requested action by master.
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Slave enters the following.
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In case of normal response: Echo the function code requested, as it stands.
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In case of exceptjonal response: Set to 1 as MSB of the function code requested and fill it in.
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⊙ DATA
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Register address
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Amount of items to be handled
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The number of byte of actual data
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⊙ CRC
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Error checking method.
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CRC-16
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CRC Generation Function
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unsigned short CRC16(puchMsg, usDataLen)
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unsigned char *puchMsg ; /* message to calculate CRC upon */
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unsigned short usDataLen ; /* quantity of bytes in message */
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{
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unsigned char uchCRCHi = 0xFF ; /* high byte of CRC initialized */
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unsigned char uchCRCLo = 0xFF ; /* low byte of CRC initialized */
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unsigned uIndex ; /* will index into CRC lookup table */
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while (usDataLen––) /* pass through message buffer */
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 {
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uIndex = uchCRCHi ^ *puchMsgg++ ; /* calculate the CRC */
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uchCRCHi = uchCRCLo ^ auchCRCHi[uIndex} ;
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uchCRCLo = auchCRCLo[uIndex] ;
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 }
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return (uchCRCHi << 8 | uchCRCLo) ;
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}
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⊙ DEAD TIME
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Frame is completed only when it has silent interval more than 3.5 character time after receiving the last character.
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1 character time is approximately 0.5ms since BPS is 19200
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(700ms to1000ms is recommended for the interval of master's request.)
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4. MODBUS Exception Codes
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codeName
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0x01ILLEGAL FUNCTION
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0x02ILLEGAL DATA ADDRESS
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0x03ILLEGAL DATA VALUE
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0x04SLAVE DEVICE FAILURE
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0x10No data for event/fault record
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0x11SBO TIME OUT
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0x12ILLEGAL ADU LENGTH