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Release 1.0,  29 March 1999   Andy Swales Schneider Electric aswales@modicon.com   Contents Contents............ 2 1. Status of this specification...... 3 2. Overview........ 3 2.1 Connection-oriented.............. 3 2.2 Data encoding........ 4 2.3 Interpretation of reference numbers......... 4 2.4 Implied length philosophy.... 5 3. Conformance class summary... 5 3.1 Class 0...... 5 3.2 Class 1...... 5 3.3 Class 2...... 6 3.4 Machine/vendor/network specific functions....... 7 4. Protocol structure............. 7 5. Protocol reference by conformance class.................... 8 5.1 Class 0 commands detail............... 9 5.1.1 Read multiple registers (FC 3)................ 9 5.1.2 Write multiple registers (FC 16).............. 9 5.2 Class 1 commands detail............. 10 5.2.1 Read coils (FC 1).................. 10 5.2.2 Read input discretes (FC 2).............. 10 5.2.3 Read input registers (FC 4).............. 11 5.2.4 Write coil (FC 5) 11 5.2.5 Write single register (FC 6).................. 12 5.2.6 Read exception status (FC 7).................. 12 5.3 Class 2 commands detail............. 13 5.3.1 Force multiple coils (FC 15)..... 13 5.3.2 Read general reference (FC 20)..... 14 5.3.3 Write general reference (FC 21)..... 15 5.3.4 Mask write register (FC 22)..... 16 5.3.5 Read/write registers (FC 23)............ 16 5.3.6 Read FIFO queue (FC 24)..... 17 6. Exception codes................ 17 Appendices..... 19 A. Client and Server Implementation Guidance.......... 19 A.1 Client design.......... 19 A.2 Server design.......... 20 A.2.1 Multithreaded server... 20 A.2.2 Single-threaded server 21 A.3 Required and expected performance 22 B. Data Encoding for non-word data........................... 23 B.1 Bit numbers within a word...................... 23 B.2 Multi-word quantities..... 24 B.2.1 984 Data Types.................. 24 B.2.2 IEC-1131 data types 25
  1. Status of this specification   Initial release 3 Sept 1997       Draft for public review.   Re-release 29 March 1999 at Revision 1.0.       No technical changes, clarifications only.     Added Appendices A and B in response to common implementation questions.     This specification of Modbus/TCP is being published by being publicly visible on the World Wide Web. It is intended for the benefit of developers wishing to use MODBUS/TCP as an interoperability standard in the field of industrial automation.   Since MODBUS and MODBUS/TCP are in reality  ‘de-facto’ standards, in that many vendors and products implement it already, this specification primarily explains the specific encoding of MODBUS messages over the TCP communication protocol universally available on the Internet.     2. Overview   MODBUS/TCP is a variant of the MODBUS family of simple, vendor-neutral communication protocols intended for supervision and control of automation equipment. Specifically, it covers the use of MODBUS messaging in an ‘Intranet’ or ‘Internet’ environment using the TCP/IP protocols. The most common use of the protocols at this time are for Ethernet attachment of PLC’s, I/O modules, and ‘gateways’ to other simple field buses or I/O networks.   The MODBUS/TCP protocol is being published as a  (‘de-facto’) automation standard. Since MODBUS is already widely known, there should be little information in this document which could not be obtained elsewhere. However, an attempt has been made to clarify which functions within MODBUS have value for interoperability of general automation equipment, and which parts are ‘baggage’ from the alternate use of MODBUS as a programming protocol for PLC’s.   This is done below by grouping supported message types into ‘conformance classes’ which differentiate between those messages which are universally implemented and those which are optional, particularly those specific to devices such as PLC’s.   2.1 Connection-oriented   In MODBUS, data transactions are traditionally stateless, making them highly resistant to disruption from noise and yet requiring minimal recovery information to be maintained at either end.   Programming operations, on the other hand, expect a connection-oriented approach. This was achieved on the simpler variants by an exclusive ‘login’ token, and on the Modbus Plus variant by explicit ‘Program Path’ capabilities which maintained a duplex association until explicitly broken down.   MODBUS/TCP handles both situations. A connection is easily recognized at the protocol level, and a single connection may carry multiple independent transactions. In addition, TCP allows a very large number of concurrent connections, so in most cases it is the choice of the initiator whether to reconnect as required or re-use a long-lived connection.   Developers familiar with MODBUS may wonder why the connection-oriented TCP protocol is used rather than the datagram-oriented UDP. The main reason is to keep control of an individual ‘transaction’ by enclosing it in a connection which can be identified, supervised, and canceled without requiring specific action on the part of the client and server applications. This gives the mechanism a wide tolerance to network performance changes, and allows security features such as firewalls and proxies to be easily added.   Similar reasoning was used by the original developers of the World Wide Web when they chose to implement a minimal Web query as a single transaction using TCP on well-known port 80.   2.2 Data encoding   MODBUS uses a ‘big-endian’ representation for addresses and data items. This means that when a numerical quantity larger than a single byte is transmitted, the MOST significant byte is sent first. So for example            16 - bits              0x1234                   would be          0x12  0x34          32 - bits              0x12345678L         would be          0x12  0x34         0x56  0x78   2.3 Interpretation of reference numbers   MODBUS bases its data model on a series of tables which have distinguishing characteristics. The four primary tables are       input discretes      single bit, provided by an I/O system, read-only     output discretes      single bit, alterable by an application program, read-write     input registers      16-bit quantity, provided by an I/O system, read-only     output registers      16-bit quantity, alterable by an application program, read-write   The distinction between inputs and outputs, and between bit-addressable and word-addressable data items, do not imply any application behavior. It is perfectly acceptable, and very common, to regard all four tables as overlaying one another, if this is the most natural interpretation on the target machine in question.   For each of the primary tables, the protocol allows individual selection of 65536 data items, and the operations of read or write of those items are designed to span multiple consecutive data items up to a data size limit which is dependent on the transaction function code.   There is no assumption that the data items represent a true contiguous array of data, although that is the interpretation used by most simple PLC’s   The ‘read and write general reference’ function codes are defined to carry a 32 bit reference number, and could be used to allow direct access to data items within a VERY large space. Today there are no PLC devices which take advantage of that.   One potential source of confusion is the relationship between the reference numbers used in MODBUS functions, and the ‘register numbers’ used in Modicon PLC’s. For historical reasons, user reference numbers were expressed as decimal numbers with a starting offset of 1. However MODBUS uses the more natural software interpretation of an unsigned integer index starting at zero.   So a modbus message requesting the read of a register at offset 0 would return the value known to the application programmer as found in register 4:00001 (memory type 4 = output register, reference 00001)   2.4 Implied length philosophy   All MODBUS requests and responses are designed in such a way that the recipient can verify that a message is complete. For function codes where the request and response are of fixed length, the function code alone is sufficient. For function codes carrying a variable amount of data in the request or response, the data portion will be preceded by a byte count.   When Modbus is carried over TCP, additional length information is carried in the prefix to allow the recipient to recognize message boundaries even if the message had to be split into multiple packets for transmission. The existence of explicit and implicit length rules, and use of a CRC-32 error check code (on Ethernet) results in an infinitesimal chance of undetected corruption to a request or response message.   3. Conformance class summary   When defining a new protocol from scratch, it is possible to enforce consistency of numbering and interpretation. MODBUS by its nature is implemented already in many places, and disruption to existing implementations must be avoided.   Therefore the existing set of transaction types have been classified into conformance classes where level 0 represents functions which are universally implemented and totally consistent, and level 2 represents useful functions but with some idiosyncrasies. Those functions of the present set which are NOT suitable for interoperability are also identified.   It must be noted that future extensions to this standard may define additional function codes to handle situations where the existing de-facto standard is deficient. However, it would be misleading for details of such proposed extensions to appear in this document. It will always be possible to determine if a particular target device supports a particular function code by sending it ‘speculatively’ and checking for the type of exception response if any, and this approach will guarantee the continued interoperability of current MODBUS devices with the introduction of any such extensions. Indeed, this is the philosophy which has led to the current function code classification.   3.1 Class 0   This is the minimum useful set of functions, for both a MASTER and a SL****E.       read multiple registers (fc 3)       write multiple registers (fc 16)   3.2 Class 1   This is the additional set of functions which is commonly implemented and interoperable. As explained before, many slaves choose to treat input, output, discrete and register as equivalent.       read coils (fc 1)       read input discretes (fc 2)       read input registers (fc 4)       write coil (fc 5)       write single register (fc 6)       read exception status (fc 7)           This function typically has a different meaning for each slave family   3.3 Class 2   These are the data transfer functions needed for routine operations such as HMI and supervision       force multiple coils (fc 15)       read general reference (fc 20)           This function has the ability to handle multiple simultaneous requests, and can accept a reference number of 32 bits. Current 584 and 984 PLC’s only use this function to accept references of type 6 (extended register files).           This function would be the most appropriate to extend to handle large register spaces and data items which currently lack reference numbers such as ‘unlocated’ variables.   write general reference (fc 21)           This function has the ability to handle multiple simultaneous requests, and can accept a reference number of 32 bits. Current 584 and 984 PLC’s only use this function to accept references of type 6 (extended register files).           This function would be the most appropriate to extend to handle large register spaces and data items which currently lack reference numbers such as ‘unlocated’ variables.   mask write register (fc 22)   read/write registers (fc 23)           This function allows the input of a range of registers and the output of a range of registers as a single transaction. It is the most efficient way, using MODBUS, to perform a regular exchange of a state image such as with an I/O module.           Thus a high performance but versatile data collection device might choose to implement functions 3, 16 and 23 to combine rapid regular exchange of data (23) with the ability to perform on-demand interrogations or updates of particular data items (3 and 16)   read FIFO queue (fc 24)           A somewhat specialized function, intended to allow the transfer of data from a table structured as a FIFO (for use with the FIN and FOUT function blocks on the 584/984) to a host computer. Useful in certain types of event logging applications   3.4 Machine/vendor/network specific functions   All of the following functions, although mentioned in the MODBUS protocol manuals, are not appropriate for interoperability purposes because they are too machine-dependent.       diagnostics (fc 8)       program (484) (fc 9)       poll (484) (fc 10)       get comm event counters (Modbus) (fc 11)       get comm event log (Modbus) (fc 12)       program (584/984) (fc 13)       poll (584/984) (fc 14)       report slave ID (fc 17)       program (884/u84) (fc 18)       reset comm link (884/u84) (fc 19)       program (ConCept) (fc 40)       firmware replacement (fc 125)       program (584/984) (fc 126)       report local address (Modbus) (fc 127)   4. Protocol structure   This section describes the general form of encapsulation of a MODBUS request or response when carried on the MODBUS/TCP network. It is important to note that the structure of the request and response body, from the function code to the end of the data portion, have EXACTLY the same layout and meaning as in the other MODBUS variants, such as            MODBUS serial port - ASCII encoding          MODBUS serial port - RTU (binary) encoding          MODBUS PLUS network - data path   The only differences in these other cases are the form of any ‘framing’ sequence, error check pattern, and address interpretation.     All requests are sent via TCP on registered port 502.   Requests are normally sent in half-duplex fashion on a given connection. That is, there is no benefit in sending additional requests on a single connection while a response is outstanding. Devices which wish to obtain high peak transfer rates are instead encouraged to establish multiple TCP connections to the same target  However some existing client devices are known to attempt to ‘pipeline’ requests. Design techniques which allow a server to accommodate this behavior are described in Appendix A.   The MODBUS ‘slave address’ field is replaced by a single byte ‘Unit Identifier’ which may be used to communicate via devices such as bridges and gateways which use a single IP address to support multiple independent end units.   The request and response are prefixed by six bytes as follows   byte 0:         transaction identifier - copied by server - usually 0 byte 1:         transaction identifier - copied by server - usually 0 byte 2:         protocol identifier = 0 byte 3:         protocol identifier = 0 byte 4:       length field (upper byte) = 0 (since all messages are smaller than 256) byte 5:      length field (lower byte) = number of bytes following   byte 6:      unit identifier (previously ‘slave address’) byte 7:         MODBUS function code byte 8 on: data as needed   So an example transaction ‘read 1 register at offset 4 from UI 9’ returning a value of 5 would be   request:    00 00  00  00  00  06  09  03  00  04  00  01   response:   00  00  00  00  00  05  09  03  02  00  05   See later section for examples of the use of each of the function codes in conformance classes 0-2   Designers familiar with MODBUS should note that the ‘CRC-16’ or ‘LRC’ check fields are NOT needed in MODBUS/TCP. The TCP/IP and link layer (eg. Ethernet) checksum mechanisms instead are used to verify accurate delivery of the packet. 5. Protocol reference by conformance class   Note that in the examples, the request and response are listed from the function code byte onwards. As said before, there will be a transport - dependent prefix which in the case of MODBUS/TCP comprises the seven bytes            ref  ref  00  00  00  len  unit   The ‘ref ref’ above is two bytes of ‘transaction reference’ number which have no value at the server but are copied verbatim from request to response for the convenience of the client. Simple clients usually choose to leave the values at zero.   In the examples, the format for a request and response is given like this (the example is for a ‘read register’ request, see detail in later section)            03 00 00 00 01  =>   03 02 12 34   This represents a hexadecimal series of bytes to be appended to the prefix, so the full message on the TCP connection would be (assume unit identifier 09 again)            request:   00 00 00 00 00 06 09 03 00 00 00 01          response: 00 00 00 00 00 05 09 03 02 12 34   (All of these requests and responses were verified by using an automatic tool, querying a current specification Modicon Quantum PLC)   5.1 Class 0 commands detail 5.1.1 Read multiple registers (FC 3)   Request   Byte 0:               FC = 03 Byte 1-2:         Reference number Byte 3-4:   Word count (1-125)   Response   Byte 0:               FC = 03 Byte 1:               Byte count of response (B=2 x word count) Byte 2-(B+1):         Register values   Exceptions   Byte 0:               FC = 83 (hex) Byte 1:               exception code = 01 or 02   Example   Read 1 register at reference 0 (40001 in Modicon 984) resulting in value 1234 hex   03 00 00 00 01  =>   03 02 12 34   5.1.2 Write multiple registers (FC 16)   Request   Byte 0:               FC = 10 (hex) Byte 1-2:         Reference number Byte 3-4:   Word count (1-100) Byte 5:               Byte count (B=2 x word count) Byte 6-(B+5):         Register values   Response   Byte 0:               FC = 10 (hex) Byte 1-2:         Reference number Byte 3-4:   Word count   Exceptions   Byte 0:               FC = 90 (hex) Byte 1:               exception code = 01 or 02   Example   Write 1 register at reference 0 (40001 in Modicon 984) of value 1234 hex   10 00 00 00 01 02 12 34  =>   10 00 00 00 01   5.2 Class 1 commands detail 5.2.1 Read coils (FC 1)   Request   Byte 0:               FC = 01 Byte 1-2:         Reference number Byte 3-4:   Bit count (1-2000)   Response   Byte 0:               FC = 01 Byte 1:               Byte count of response (B=(bit count+7)/8) Byte 2-(B+1):         Bit values (least significant bit is first coil!)   Exceptions   Byte 0:               FC = 81 (hex) Byte 1:               exception code = 01 or 02   Example   Read 1 coil at reference 0 (00001 in Modicon 984) resulting in value 1   01 00 00 00 01  =>   01 01 01   Note that the format of the return data is not consistent with a big-endian architecture. Note also that this request can be very computation-intensive on the slave if the request calls for multiple words and they are not aligned on 16-bit boundaries.   5.2.2 Read input discretes (FC 2)   Request   Byte 0:               FC = 02 Byte 1-2:         Reference number Byte 3-4:   Bit count (1-2000)   Response   Byte 0:               FC = 02 Byte 1:               Byte count of response (B=(bit count+7)/8) Byte 2-(B+1):         Bit values (least significant bit is first coil!)   Exceptions   Byte 0:               FC = 82 (hex) Byte 1:               exception code = 01 or 02   Example   Read 1 discrete input at reference 0 (10001 in Modicon 984) resulting in value 1   02 00 00 00 01  =>   02 01 01   Note that the format of the return data is not consistent with a big-endian architecture. Note also that this request can be very computation-intensive on the slave if the request calls for multiple words and they are not aligned on 16-bit boundaries.   5.2.3 Read input registers (FC 4)   Request   Byte 0:               FC = 04 Byte 1-2:         Reference number Byte 3-4:   Word count (1-125)   Response   Byte 0:               FC = 04 Byte 1:               Byte count of response (B=2 x word count) Byte 2-(B+1):         Register values   Exceptions   Byte 0:               FC = 84 (hex) Byte 1:               exception code = 01 or 02   Example   Read 1 input register at reference 0 (30001 in Modicon 984) resulting in value 1234 hex   04 00 00 00 01  =>   04 02 12 34   5.2.4 Write coil (FC 5)   Request   Byte 0:               FC = 05 Byte 1-2:         Reference number Byte 3:                = FF to turn coil ON, =00 to turn coil OFF Byte 4:                = 00   Response   Byte 0:               FC = 05 Byte 1-2:         Reference number Byte 3:                = FF to turn coil ON, =00 to turn coil OFF (echoed) Byte 4:                = 00   Exceptions   Byte 0:               FC = 85 (hex) Byte 1:               exception code = 01 or 02   Example   Write 1 coil at reference 0 (00001 in Modicon 984) to the value 1   05 00 00 FF 00  =>   05 00 00 FF 00   5.2.5 Write single register (FC 6)   Request   Byte 0:               FC = 06 Byte 1-2:         Reference number Byte 3-4:         Register value   Response   Byte 0:               FC = 06 Byte 1-2:         Reference number Byte 3-4:         Register value   Exceptions   Byte 0:               FC = 86 (hex) Byte 1:               exception code = 01 or 02   Example   Write 1 register at reference 0 (40001 in Modicon 984) of value 1234 hex   06 00 00 12 34  =>   06 00 00 12 34   5.2.6 Read exception status (FC 7)   Note that ‘exception status’ has nothing to do with ‘exception response’. The ‘read exception status’ message was intended to allow maximum responsiveness in the early MODBUS polled multidrop networks using slow baud rates. PLC’s would typically map a range of 8 coils (output discretes) which would be interrogated using this message.   Request   Byte 0:               FC = 07   Response   Byte 0:               FC = 07 Byte 1:               Exception status (usually a predefined range of 8 coils)   Exceptions   Byte 0:               FC = 87 (hex) Byte 1:               exception code = 01 or 02   Example   Read exception status resulting in value 34 hex   07   =>   07 34 5.3 Class 2 commands detail 5.3.1 Force multiple coils (FC 15)   Request   Byte 0:               FC = 0F (hex) Byte 1-2:         Reference number Byte 3-4:   Bit count (1-800) Byte 5:               Byte count (B = (bit count + 7)/8) Byte 6-(B+5):         Data to be written (least significant bit = first coil)   Response   Byte 0:               FC = 0F (hex) Byte 1-2:         Reference number Byte 3-4:   Bit count   Exceptions   Byte 0:               FC = 8F (hex) Byte 1:               exception code = 01 or 02   Example   Write 3 coils at reference 0 (00001 in Modicon 984) to values 0,0,1   0F 00 00 00 03 01 04  =>   0F 00 00 00 03   Note that the format of the input data is not consistent with a big-endian architecture. Note also that this request can be very computation-intensive on the slave if the request calls for multiple words and they are not aligned on 16-bit boundaries.   5.3.2 Read general reference (FC 20)   Request   Byte 0:               FC = 14 (hex) Byte 1:               Byte count for remainder of request (=7 x number of groups) Byte 2:               Reference type for first group = 06 for 6xxxx extended register files Byte 3-6:         Reference number for first group                                   = file number:offset for 6xxxx files                             = 32 bit reference number for 4xxxx registers Byte 7-8:   Word count for first group Bytes 9-15:         (as for bytes 2-8, for 2nd group) . . .   Response   Byte 0:               FC = 14 (hex) Byte 1:               Overall byte count of response                              (=number of groups + sum of byte counts for groups) Byte 2:               Byte count for first group (B1=1 + (2 x word count)) Byte 3:               Reference type for first group Byte 4-(B1+2):         Register values for first group Byte (B1+3):         Byte count for second group (B2=1 + (2 x word count)) Byte (B1+4):         Reference type for second group Byte (B1+5)-(B1+B2+2): Register values for second group . . .   Exceptions   Byte 0:               FC = 94 (hex) Byte 1:               exception code = 01 or 02 or 03 or 04   Example   Read 1 extended register at reference 1:2 (File 1 offset 2 in Modicon 984) resulting in value 1234 hex   14 07 06 00 01 00 02 00 01    =>    14 04 03 06 12 34   (future)   Read 1 register at reference 0 returning 1234 hex, and 2 registers at reference 5 returning 5678 and 9abc hex   14 0E 04 00 00 00 00 00 01 04 00 00 00 05 00 02  =>  14 0A 03 04 12 34 05 04 56 78 9A BC   Note that the transfer size limits are difficult to define in a mathematical formula. Broadly, the message sizes for request and response are each limited to 256 bytes for buffer size reasons, and the aggregate size of the individual request and response data frames must be considered. Exception type 04 will be generated if the slave is unwilling to process the message because the response would be too large.   5.3.3 Write general reference (FC 21)   Request   Byte 0:               FC = 15 (hex) Byte 1:               Byte count for remainder of request Byte 2:               Reference type for first group = 06 for 6xxxx extended register files Byte 3-6:         Reference number for first group                                   = file number:offset for 6xxxx files                             = 32 bit reference number for 4xxxx registers Byte 7-8:   Word count for first group (W1) Byte 9-(8 + 2 x W1): Register data for first group   (copy group data frame from byte 2 on for any other groups) . . .   Response   Response is a direct echo of the query   Byte 0:               FC = 15 (hex) Byte 1:               Byte count for remainder of request Byte 2:               Reference type for first group = 06 for 6xxxx extended register files Byte 3-6:         Reference number for first group                                   = file number:offset for 6xxxx files                             = 32 bit reference number for 4xxxx registers Byte 7-8:   Word count for first group (W1) Byte 9-(8 + 2 x W1): Register data for first group   (copy group data frame from byte 2 on for any other groups) . . .   Exceptions   Byte 0:               FC = 95 (hex) Byte 1:               exception code = 01 or 02 or 03 or 04   Example   Write 1 extended register at reference 1:2 (File 1 offset 2 in Modicon 984) to value 1234 hex   15 09 06 00 01 00 02 00 01 12 34   =>    15 09 06 00 01 00 02 00 01 12 34   (future)   Write 1 register at reference 0 to value 1234 hex, and 2 registers at reference 5 to values 5678 and 9abc hex   15 14 04 00 00 00 00 00 01 12 34 04 00 00 00 05 00 02 56 78 9A BC  ð      15 14 04 00 00 00 00 00 01 12 34 04 00 00 00 05 00 02 56 78 9A BC    Note that the transfer size limits are difficult to define in a mathematical formula. Broadly, the message sizes for request and response are each limited to 256 bytes for buffer size reasons, and the aggregate size of the individual request and response data frames must be considered. Exception type 04 will be generated if the slave is unwilling to process the message because the response would be too large.   5.3.4 Mask write register (FC 22)   Request   Byte 0:               FC = 16 (hex) Byte 1-2:         Reference number Byte 3-4:   AND mask to be applied to register Byte 5-6:   OR mask to be applied to register   Response   Byte 0:               FC = 16 (hex) Byte 1-2:         Reference number Byte 3-4:   AND mask to be applied to register Byte 5-6:   OR mask to be applied to register   Exceptions   Byte 0:               FC = 96 (hex) Byte 1:               exception code = 01 or 02   Example   Change the field in bits 0-3 of register at reference 0 (40001 in Modicon 984) to value 4 hex (AND with 000F, OR with 0004)   16 00 00 00 0F 00 04  =>   16 00 00 00 0F 00 04 5.3.5 Read/write registers (FC 23)   Request   Byte 0:               FC = 17 (hex) Byte 1-2:         Reference number for read Byte 3-4:   Word count for read (1-125) Byte 5-6:         Reference number for write Byte 7-8:   Word count for write (1-100) Byte 9:               Byte count (B = 2 x word count for write) Byte 10-(B+9):         Register values   Response   Byte 0:               FC = 17 (hex) Byte 1:               Byte count(B = 2 x word count for read) Byte 2-(B+1)         Register values   Exceptions   Byte 0:               FC = 97 (hex) Byte 1:               exception code = 01 or 02   Example   Write 1 register at reference 3 (40004 in Modicon 984) of value 0123 hex and read 2 registers at reference 0 returning values 0004 and 5678 hex   17 00 00 00 02 00 03 00 01 02 01 23  =>  17 04 00 04 56 78   Note that if the register ranges for writing and reading overlap, the results are undefined. Some devices implement the write before the read, but others implement the read before the write. 5.3.6 Read FIFO queue (FC 24)   Request   Byte 0:               FC = 18 (hex) Byte 1-2:         Reference number   Response   Byte 0:               FC = 18 (hex) Byte 1-2:   Byte count (B = 2 + word count) (maximum 64) Byte 3-4:   Word count (number of words accumulated in FIFO) (maximum 31) Byte 5-(B+2):         Register data from front of FIFO   Exceptions   Byte 0:               FC = 98 (hex) Byte 1:               exception code = 01 or 02 or 03   Example   Read contents of FIFO block starting at reference 0005 (40006 in Modicon 984) which contains 2 words of value 1234 and 5678 hex outstanding   18 00 05  =>   18 00 06 00 02 12 34 56 78   Note that this function as implemented on the 984 is very limited in versatility - the block of registers is assumed to consist of a count which can have values from 0 to 31, followed by up to 31 words of data. When the function completes, the count word is NOT reset to zero, as might have been expected from a FIFO operation.   All in all, this should be considered a limited subset of fn 16 - read multiple registers, since the latter can be used to perform all of the required functionality.