Frame delivery problem while updating slave data

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.2.1 Class 0 This is the minimum useful set of functions, for both a MASTER and a multiple registers (fc 3) write multiple registers (fc 16)2.2 Class 1read 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 family2.3 Class 2 These are the data transfer functions needed for routine operations such as HMI and supervisionforce 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.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.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.

Today there are no PLC devices which take advantage of that.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. A connection is easily recognized at the protocol level, and a single connection may carry multiple independent transactions.In addition, TCP/IP 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.When MODBUS is carried over TCP/IP, 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.2.

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