CIRCUTOR computer SMART III-Fast Series - Manual (Page 59)

4�9�2� PROTOCOL
TheModbusprotocolisanindustrycommunicationstandardwhichenablesnetworkingof
multipleunits,withonemasterandseveralslaves�Itallowsindividualmaster-slavedialogue
andalsoenablescommandsinbroadcastformat�
IntheModbusprotocol,the Computer SMART III FASTunitusestheRTU(RemoteTerminal
Unit)mode�
IntheRTUmode,themessagestartandendaredetectedwithsilencesofatleast3�5
characters,andthe16-bitCRCerror-detectionmethodisused�
TheModbus functionsimplementedintheunitareasfollows:
Function 01�Readingthestatusoftherelays�
Functions 03 and 04�Readinglogs�
Function 05�Writingarelay�
Function 0F�Writingmultiplerelays�
Function 10�Writingmultiplelogs�
Exception codes
Ifthebitwithgreatestweightofthebytecorrespondingtothefunctioninthereplyoftheunit
is1,thisindicatesthatthenextbyteisanexceptioncode�
Table 11: Exception codes, Modbus communications�
Exception code Description
01 Incorrectfunction�Thefunctionnumberisnotimplemented�
02 Incorrectaddressornumberoflogsoutoflimits
03 Dataerror�ACRCerrorhasoccurred
04 Peripheralerror�Anerroroccurredwhenaccessingaperipheral(EE-
PROM,card,etc�)
06 SlaveerrororSlavebusy�Retrysending�
Example:
Address Function
Exception
code
CRC
0A 84 01 XXXX
Address: 0A,Peripheralnumber:10indecimal�
Function: 84,Readingfunction04withbitno�7at1�
Exception code: 01,see Table 9�
CRC:16-bitCRC�
Forreasonsofoperationalsecurityoftheunit,communicationframesof
morethan80bytesarenotaccepted(sentorreceived)�
59 Instruction Manual
Computer SMART III FAST