Schneider Electric VW3 A3 303 user manual Modbus protocol, Principle, Slave Slave j Slave k

Page 25

8. Modbus protocol

8. 1. Principle

The Modbus protocol is a master-slave protocol.

	Only one device can transmit on the line at any one time.
	The master manages the exchanges and only it can take the initiative.
	It interrogates each of the slaves in succession.
Master	No slave can send a message unless it is invited to do so.
	In the event of an error during data exchange, the master repeats the question and declares the interrogated
	slave absent if no response is received within a given time period.
	If a slave does not understand a message, it sends an exception response to the master.
	The master may or may not repeat the request.

Slave i

Slave j	Slave k

Direct slave-to-slave communications are not possible.

For slave-to-slave communication, the master’s application software must therefore be designed to interrogate one slave and send back data received to the other slave.

Two types of dialog are possible between master and slaves:

•The master sends a request to a slave and waits for it to respond.

•The master sends a request to all slaves without waiting for them to respond (broadcasting principle).

Note: The Modbus server on the "Controller Inside" card cannot be accessed via the Modbus card.

25

Image 25

Contents Altivar Contents Electric shock will result in death or serious injury Before you beginDocumentation structure Installation ManualProgramming Manual Communication Parameters ManualPresentation IntroductionNotation Installing the card in the drive Hardware setupReceipt Hardware descriptionChoosing polarization Switch codingCoding the address 1234 AddressWiring recommendations Connecting to the busModbus RS485 standard Connection accessories Connection via RJ45 wiring systemConnection cables Type of master Master interface Description Catalog number Connection cables for the masterTWD XCA RJ030 TSX SCP CMDescription Catalog Number Subscriber socket Connection via junction boxesRS485 double shielded twisted pair CablesTSX P ACC TSX CSA500Communication parameters ConfigurationControl via Modbus in I/O profile Control CommandControl via Modbus or the terminals in I/O profile Parameter Value CommentControl via Modbus or the terminals in Drivecom profile Control via Modbus in Drivecom profileConfiguration Communication scanner Example of configuration via the graphic display terminalRDY NET COM. Scanner InputCOM. Fault Management Communication faultsMonitored parameters CONFIG. COMM. MAPFormat Format Range Terminal displayChecking the address DiagnosticsLEDs Green RUN Red ERR Meaning Corrective actionCommunication MAP RUN NETCOM. Scanner Input MAP COM Scan Output MAP CMD. Word ImageOutput variable Scanner parameter Input variable Scanner parameterCOM Scan Output MAP Card fault Communication faultPrinciple Modbus protocolSlave Slave j Slave k Ascii mode RTU modeModes Slave address Function code DataRead Holding/Input Registers 3/4 Modbus functions availableCode Modbus name Description Broadcast Size of data Write Single Register Diagnostics Write Multiple Registers 16 = 16#10 Get Comm Event Counter 11 = 16#0BRead/Write Multiple Registers 23 = 16#17 Machine Read Device Identification 43 = 16#2BMEI Example of positive responseInput variables Logic address Default assignment Output variables Logic address Default assignmentCode Modbus name Description Size of data Parameter Logic Read Input variable address Logic address Default valueParameter Logic Parameter assigned Output controlInput monitoring Value Parameter assignedException responses Code Error codeResponse Logic address Parameter Value Read non-existent or protected parametersRS485 standard AppendixMaster Modbus 2-wire standard schematicModbus 4-wire standard schematic Uni-Telway schematic Type of trunk cable Cable with 1 shielded twisted pair Wire Jbus schematicDifferent scenarios Creating a Modbus bus using non-standard equipmentRecommendations Example Calculating the polarizationCard connector pinout RS485 schematic for the cardContact no. Signal SUB-DAtv71ModbusJbusENV1 2005-05