Message Type Action Table Name\n0x01 Read Discrete Output Coils\n0x02 Read Discrete Input Contacts\n0x03 Read Analog Output Holding Registers\n0x04 Read Analog Input Registers\n0x05 Write single Discrete Output Coil\n0x06 Write single Analog Output Holding Register\n0x10 Write multiple Analog Output Holding Registers\n0x0F Write multiple Discrete Output Coils<\/code><\/pre>\n\n\n\nData Types<\/h4>\n\n\n\n
A register is 16 bits and can store the following:<\/p>\n\n\n\n
A 16-bit unsigned integer
A 16-bit signed integer
A two character ASCII string
A discrete on\/off value<\/p>\n\n\n\n
To store larger values, for instance 32bit values, float values, ASCII strings, etc simply use successive registers.<\/p>\n\n\n\n
Byte Ordering<\/h5>\n\n\n\n
The Modbus specification doesn’t define exactly how the data is stored in the registers. Some manufacturers use HighByte|LowByte and others use LowByte|HighByte.<\/p>\n\n\n\n
Similarly when registers are combined to represent 32-bit data types some devices store the high 16 bits in the first register and the remaining low word in the second while others do the opposite.<\/p>\n\n\n\n
CRC (Cyclic Redundancy Check)<\/h4>\n\n\n\n
If a slave receives a packet with an invalid CRC no response should be sent.<\/p>\n\n\n\n
How to calculate the CRC:<\/p>\n\n\n\n
const UINT16 CRCTable[] = {\n 0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241,\n 0xC601, 0x06C0, 0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440,\n 0xCC01, 0x0CC0, 0x0D80, 0xCD41, 0x0F00, 0xCFC1, 0xCE81, 0x0E40,\n 0x0A00, 0xCAC1, 0xCB81, 0x0B40, 0xC901, 0x09C0, 0x0880, 0xC841,\n 0xD801, 0x18C0, 0x1980, 0xD941, 0x1B00, 0xDBC1, 0xDA81, 0x1A40,\n 0x1E00, 0xDEC1, 0xDF81, 0x1F40, 0xDD01, 0x1DC0, 0x1C80, 0xDC41,\n 0x1400, 0xD4C1, 0xD581, 0x1540, 0xD701, 0x17C0, 0x1680, 0xD641,\n 0xD201, 0x12C0, 0x1380, 0xD341, 0x1100, 0xD1C1, 0xD081, 0x1040,\n 0xF001, 0x30C0, 0x3180, 0xF141, 0x3300, 0xF3C1, 0xF281, 0x3240,\n 0x3600, 0xF6C1, 0xF781, 0x3740, 0xF501, 0x35C0, 0x3480, 0xF441,\n 0x3C00, 0xFCC1, 0xFD81, 0x3D40, 0xFF01, 0x3FC0, 0x3E80, 0xFE41,\n 0xFA01, 0x3AC0, 0x3B80, 0xFB41, 0x3900, 0xF9C1, 0xF881, 0x3840,\n 0x2800, 0xE8C1, 0xE981, 0x2940, 0xEB01, 0x2BC0, 0x2A80, 0xEA41,\n 0xEE01, 0x2EC0, 0x2F80, 0xEF41, 0x2D00, 0xEDC1, 0xEC81, 0x2C40,\n 0xE401, 0x24C0, 0x2580, 0xE541, 0x2700, 0xE7C1, 0xE681, 0x2640,\n 0x2200, 0xE2C1, 0xE381, 0x2340, 0xE101, 0x21C0, 0x2080, 0xE041,\n 0xA001, 0x60C0, 0x6180, 0xA141, 0x6300, 0xA3C1, 0xA281, 0x6240,\n 0x6600, 0xA6C1, 0xA781, 0x6740, 0xA501, 0x65C0, 0x6480, 0xA441,\n 0x6C00, 0xACC1, 0xAD81, 0x6D40, 0xAF01, 0x6FC0, 0x6E80, 0xAE41,\n 0xAA01, 0x6AC0, 0x6B80, 0xAB41, 0x6900, 0xA9C1, 0xA881, 0x6840,\n 0x7800, 0xB8C1, 0xB981, 0x7940, 0xBB01, 0x7BC0, 0x7A80, 0xBA41,\n 0xBE01, 0x7EC0, 0x7F80, 0xBF41, 0x7D00, 0xBDC1, 0xBC81, 0x7C40,\n 0xB401, 0x74C0, 0x7580, 0xB541, 0x7700, 0xB7C1, 0xB681, 0x7640,\n 0x7200, 0xB2C1, 0xB381, 0x7340, 0xB101, 0x71C0, 0x7080, 0xB041,\n 0x5000, 0x90C1, 0x9181, 0x5140, 0x9301, 0x53C0, 0x5280, 0x9241,\n 0x9601, 0x56C0, 0x5780, 0x9741, 0x5500, 0x95C1, 0x9481, 0x5440,\n 0x9C01, 0x5CC0, 0x5D80, 0x9D41, 0x5F00, 0x9FC1, 0x9E81, 0x5E40,\n 0x5A00, 0x9AC1, 0x9B81, 0x5B40, 0x9901, 0x59C0, 0x5880, 0x9841,\n 0x8801, 0x48C0, 0x4980, 0x8941, 0x4B00, 0x8BC1, 0x8A81, 0x4A40,\n 0x4E00, 0x8EC1, 0x8F81, 0x4F40, 0x8D01, 0x4DC0, 0x4C80, 0x8C41,\n 0x4400, 0x84C1, 0x8581, 0x4540, 0x8701, 0x47C0, 0x4680, 0x8641,\n 0x8201, 0x42C0, 0x4380, 0x8341, 0x4100, 0x81C1, 0x8081, 0x4040 };\n\nUINT16 ModbusCalculateCrc (BYTE *pTxBuffer, WORD Length)\n{\n\tunsigned char ucTemp;\n\tUINT16 CrcWord = 0xFFFF;\n\n\twhile (Length--)\n\t{\n\t\tucTemp = (*pTxBuffer++) ^ (BYTE)(CrcWord & 0x00ff);\n\t\tCrcWord >>= 8;\n\t\tCrcWord ^= CRCTable[ucTemp];\n\t}\n\treturn(CrcWord);\n}<\/code><\/pre>\n\n\n\n\u00a0Modbus RTU Exception Error Response<\/h4>\n\n\n\n
The function Code byte value is returned with bit 7 set to indicate exception.<\/p>\n\n\n\n
Following this a exception code is sent:<\/p>\n\n\n\n
0x01 Illegal Function (The function code received in the query is not an allowable action for the slave)\n0x02 Illegal Data Address (The data address received in the query is not an allowable address for the slave)\n0x03 Illegal Data Value (A value contained in the query data field is not an allowable value for the slave)\n0x04 Slave Device Failure (An unrecoverable error occurred while the slave was attempting to perform the requested action)\n0x05 Acknowledge\n0x06 Slave Device Busy\n0x07 Negative Acknowledge (the slave cannot perform the program function received in the query)\n0x08 Memory Parity Error\n0x0A Gateway Path Unavailable\n0x0B Gateway Target Device Failed to Respond<\/code><\/pre>\n\n\n\nFor example for a failed read of a word (16 bits) from a register the following is sent:<\/p>\n\n\n\n
Byte 1 Modbus Device Address (1 to 247)\nByte 2 Function Code \/ Message Type (0x03)\nByte 3 Register Number (high byte)\nByte 4 Register Number (low byte)\nByte 5 Data Length (high byte, specified in Words)\nByte 6 Data Length (low byte)\nByte 7 CRC of bytes 1 to 6 (high byte)\nByte 8 CRC of bytes 1 to 6 (low byte)<\/code><\/pre>\n\n\n\nAnd this is returned<\/p>\n\n\n\n
Byte 1 Modbus Device Address\nByte 2 Function Code \/ Message Type (0x83)\nByte 3 The Exception Code\nByte 4 CRC of bytes 1 to 5 (high byte)\nByte 5 CRC of bytes 1 to 5 (low byte)<\/code><\/pre>\n","protected":false},"excerpt":{"rendered":"Resources modbus.org Modbus Application Protocol Describes function codes modbus.org Modbus over Serial Line Specification and Implementation Guide Register Addressing The Modbus register number should really be thought of as an offset rather than an address, as different vendors can use different register addressing schemes. Firstly registers may start from 0 for some devices and 1 for others. The register addressing […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[78],"tags":[],"class_list":["post-1298","post","type-post","status-publish","format-standard","hentry","category-modbus"],"_links":{"self":[{"href":"https:\/\/ibex.tech\/resources\/wp-json\/wp\/v2\/posts\/1298","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ibex.tech\/resources\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ibex.tech\/resources\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ibex.tech\/resources\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/ibex.tech\/resources\/wp-json\/wp\/v2\/comments?post=1298"}],"version-history":[{"count":21,"href":"https:\/\/ibex.tech\/resources\/wp-json\/wp\/v2\/posts\/1298\/revisions"}],"predecessor-version":[{"id":2452,"href":"https:\/\/ibex.tech\/resources\/wp-json\/wp\/v2\/posts\/1298\/revisions\/2452"}],"wp:attachment":[{"href":"https:\/\/ibex.tech\/resources\/wp-json\/wp\/v2\/media?parent=1298"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ibex.tech\/resources\/wp-json\/wp\/v2\/categories?post=1298"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ibex.tech\/resources\/wp-json\/wp\/v2\/tags?post=1298"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}