Serial Communications Protocol Specifications - Swissvacuum.com

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- 6 - Fig. 4 Decoding a data packet There are 2 types of errors which may occour: transmission errors and programming or operation errors. The Modbus system has specific methods for dealing with either type of error. COMMUNICATION ERRORS This kind of errors happens when a condition of FRAME or PARITY or OVERRUN error is detected (generally due to same problems of noise on the transmission lines) or when an invalid CRC (cyclical redundancy check) is found in the received data packet. When one or more of these errors is detected, the message (probably damaged) is not processed: the Slave device (microPLC) will not answer to the message. Operation / programming errors Are those which present incorrect data in a message. In this case the Slave device (MicroPLC) produces an “exception” depending on the error type; the exception error code is inserted in the answer data packet, so that it allows the master device (PC) to identify the error condition. To identify this exception, the most significant bit of the function code of the answer packet (coming from the microPLC to the PC) is set to 1. Exception codes Serial communication line MASTER SLAVE Data packet Any error ? No Decode the packet to get the answer from the slave. Noise Ye 01 Illegal function 02 Illegal data address Do not use the packet because corrupted Serial communications protocol vers. 2.02 Angelantoni Industrie Spa Massa Martana (PG) Italy
- 7 - 03 Illegal data value 04 Failure in associated device 05 Acknowledge: the slave is processing the request 06 Busy: busy device, rejected message NAK: It is impossible to execute the requested function 07 Memory parity error For example, let us say that we want to read data from the slave. Master prepares a data packet (query) with modbus function code 3 (function code 3 identifies a READ request as we will see later) and send the packet to the slave. In preparing the answer, the slave will return the same function code if no exception is present, otherwise it will return the exception code as reported below (note that only the first two bytes of the data packet are showed here: the detailed explanation of the entire frame will follow in the specific paragraph) 0 ADDRESS Chamber controller address 1 03 dec: 00000011 binary Modbus function code for read operation 2 …. 3 …. 4 …. 0 ADDRESS Chamber controller address 1 03 hex: 00000011 binary Modbus function code for read operation 2 …. 3 …. 4 …. 0 ADDRESS Chamber controller address 1 83 hex: 10000011 binary Modbus function code + exception 2 02 Exception number: illegal data address 3 Error check 4 Error check IDENTIFICATION OF A DATA PACKET Two different ways are available to the programmer to identify a data packet in binary mode: � FRAME SYNCHRONIZATION � LENGHT SYNCHRONIZATION. In the following section a short description of both will be done in order to have an overview on advantages and difficulties of both. FRAME SYNCHRONIZATION QUERY FROM THE MASTER TO THE SLAVE ANSWER FROM THE SLAVE, NO EXCEPTION ANSWER FROM THE SLAVE, EXCEPTION 02 Frame synchronization can be manteined in binary transmission mode (RTU) only by simulating a synchronous message. The receiving device monitors the elapsed time between receipt of characters. Modbus specifications requests that this time (time between the reception of a character and the following) is less or equal to 3.5 times the time of character transmission because the received data can be considered Serial communications protocol vers. 2.02 Angelantoni Industrie Spa Massa Martana (PG) Italy
Page 1 and 2: - 1 - ANGELANTONI Industrie S.P.A.
Page 3 and 4: - 3 - INDEX INTRODUCTION...........
Page 5: The entire task can be described as
Page 9 and 10: � Read Function: function code 3
Page 11 and 12: - 11 - READ MULTIPLE REGISTERS: FUN
Page 13 and 14: COMMUNICATION PARAMETERS - 13 - The
Page 15 and 16: C LANGUAGE - 15 - The following exa
Page 17 and 18: VISUAL BASIC - 17 - We assume you a
Page 19 and 20: - 19 - > Query sent: > 000: 017 001
Page 21 and 22: - 21 - > 012: 000 (0000) > 013: 000
Page 23 and 24: - 23 - > 152: 000 (0000) > 153: 000
Page 25 and 26: 0 193 129 64 1 192 128 65 0 193 129
Page 27 and 28: - 27 - ADDRESS MEANING 2032-2033 Re
Page 29 and 30: 2055.11 2055.12 2055.13 2055.14 205
Page 31 and 32: - 31 - REAL SETTINGS ADDRESS CHAMBE
Page 33 and 34: - 33 - TEST PROGRAM STATUS All info
Page 35 and 36: - 35 - 2328 Initial Segment Initial
Page 37: - 37 - APPENDIX A: CONNECTION OF A

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