rplc

Invoke RPC methods from TwinCAT PLC using ADS.

How?

Prerequisites

A RPC server (here another PLC on same machine LocalAmsNetId, but different ADS Port 852) provides following method signature:

NOTE
The method has to have the attribute TcRpcEnable in order to be invocable via RPC,
see Beckhoff InfoSys - Attribute 'TcRpcEnable'

{attribute 'TcRpcEnable'}
METHOD Greet : T_MaxString
VAR_INPUT
	sName : T_MaxString;
END_VAR
VAR_OUTPUT
	nLen : INT;
END_VAR
VAR_IN_OUT CONSTANT
	stLibVersion : ST_LibVersion;
END_VAR

Instantiate client

fbRpc : tchaxx.FB_RPC(sAmsNetId:= tchaxx.GCL.cLocalAmsNetId, 
                      nAmsPort:= 852, 
                      sInstancePath:= 'MAIN.fbRpcServer', 
                      sSymbol:= 'Greet');

// Arguments and RetVal
sArg  : T_MaxString := _AppInfo.AppName;
nLen  : INT;
stLibVersion : ST_LibVersion;
sRetVal : T_MaxString;
stRetVal : ST_AdsStatus;

Invoke RPC method

To invoke the RPC method, simply use fluent-api to describe it, e.g.

stRetVal := fbRpc.VarInput(sArg)
                 .VarOutput(nLen)
                 .VarInOut(stLibVersion)
                 .ReturnValue(sRetVal)
                 .Invoke();

NOTE
Unlike the TwinCAT ADS .NET library, this implementation does not check the datatypes of the arguments before invoking.
It only matches the total size of all arguments (inputs, var-in-out and return value).

Examples

See example projects RpcServer and RpcClient or unit-tests in TESTs/FB_RPC_Tests for more examples.

Publisher / Subscriber example

This examples in RpcServer/RpcClient project, showcases the use of FB_RPC_Dynamic.

• FB_Publisher in RpcServer

  A client can subscribe to events, by invoking Subscribe method, with following arguments:

  VAR_INPUT CONSTANT
      sAmsNetId		: T_AmsNetID;
      nPort			: T_AmsPort;
      sInstancePath	: STRING;
      sSymbol			: STRING;
  END_VAR
  

  These arguments are used to configure a FB_RPC_Dynamic instance in the array aSubscribers of type ST_Subscriber.

  Subscribe := S_PENDING;
  
  (* start of critical section *)
  IF fbCrititcalSection.Enter() THEN
      
    FOR i := 0 TO cSubsUpperBound DO	
      IF aSubscribers[i].nHandle > 0 THEN
        CONTINUE;
      END_IF
      nHandleCnt := nHandleCnt + 1;
      aSubscribers[i].nHandle := nHandleCnt;		
      aSubscribers[i].fbRPC.Configure().WithAmsNetId(sAmsNetId).WithAmsPort(nPort).WithSymbolPath(sInstancePath, sSymbol).Build();
      nHandle := nHandleCnt;
      Subscribe := S_OK;
      EXIT;
    END_FOR
    
      (* end of critical section *)
    fbCrititcalSection.Leave();
  END_IF
  

  The publisher invokes each registered subscriber:

  METHOD PRIVATE InvokeSubscribersCallback
  VAR_INPUT
    nTrigger : UDINT;
  END_VAR
  
  VAR
    i 	: UDINT;
    refSub : REFERENCE TO ST_Subscriber;
  END_VAR
  

  FOR i:= 0 TO cSubsUpperBound DO
    refSub REF= aSubscribers[i];
    
    IF refSub.nHandle = 0 THEN
    	CONTINUE;
    END_IF
    
    refSub.stAdsStatus := refSub.fbRPC.VarInput(nTrigger).NoReturnValue().Invoke();
  
  END_FOR
  

• FB_Subscriber in RpcClient
  In order to subscribe to events, a client must register with the publisher, which in this case is the FB_Publisher of the RpcServer:

  VAR
    fbRpcPublisherSubscribe 	: tchaxx.FB_RPC(sAmsNetId:= tchaxx.GCL.cLocalAmsNetId, nAmsPort:= 852, sInstancePath:= 'MAIN.fbPublisher', sSymbol:= 'Subscribe');
  END_VAR
  

  With the variable sSymbolCallback "pointing" to the callback-method PublisherEventCallback:

  VAR
    sSymbolCallback			: STRING := 'PublisherEventCallback';
  END_VAR
  

  This method is invoked by the publisher, in case of an event.

  IF NOT bSubscribed AND bSubscribe THEN
    stAdsStatus := fbRpcPublisherSubscribe.VarInput(anyArg:= sAmsNetId)
            .VarInput(anyArg:= _AppInfo.AdsPort)
            .VarInput(anyArg:= sInstancePath)
            .VarInput(anyArg:= sSymbolCallback)
            .VarOutput(anyArg:= nHandle)
            .ReturnValue(anyRetVal:= hr)
            .Invoke();
            
    bSubscribed := NOT stAdsStatus.bBusy AND NOT stAdsStatus.bError;
    IF bSubscribed THEN
      bSubscribe := FALSE;
    END_IF
  END_IF;
  

Call external RPC methods

• See TcHaxx/Snappy.

Acknowledgments

• TcUnit
  An unit testing framework for Beckhoff's TwinCAT 3