Allow usage of asynOctet interface for ADST_STRING

[AndreFavotto]

It seems to be possible to read a parameter of type ADST_STRING only if the underlying linked record has DTYP set to asynInt8ArrayIn/asynInt8ArrayOut, as seen below:

twincat-ads/adsApp/src/adsAsynPortDriver.cpp

Lines 4182 to 4193 in 852a4bd

	case ADST_STRING:
	switch(paramInfo->asynType){
	case asynParamInt8Array:
	// handled in fireCallbacks()
	ret=asynSuccess;
	break;
	default:
	asynPrint(pasynUserSelf, ASYN_TRACE_ERROR, "%s:%s: Type combination not supported. PLC type = %s, ASYN type= %s\n", driverName, functionName,adsTypeToString(paramInfo->plcDataType),asynTypeToString(paramInfo->asynType));
	return asynError;
	break;
	}
	break;

That way, it is possible to retrieve the string value only explicitly formatting the waveform output value, i.e, caget -S <pvname>.
Instantiating an asynOctetRead fails with:

adsAsynPortDriver:adsUpdateParameter: Type combination not supported. PLC type = ADST_STRING, ASYN type= asynUnknownType

Feature Request: allow usage of asynOctetRead/asynOctetWrite.

Expected behaviour:

When trying something like

record(lsi, "$(P)$(R):MyLongString") {
    field(DESC, "my lsi")
    field(DTYP, "asynOctetRead")
    field(SIZV, "50")
    field(SCAN, "I/O Intr")
    field(INP, "@asyn($(PORT),0,1)ADSPORT=$(ADSPORT)/MAIN.text1?")
}
#or
record(stringin, "$(P)$(R):MyShortString") {
    field(DESC, "my stringin")
    field(DTYP, "asynOctetRead")
    field(SCAN, "I/O Intr")
    field(INP, "@asyn($(PORT),0,1)ADSPORT=$(ADSPORT)/MAIN.text2?")
}

we should be able to get the string values directly without output formatting.

[AndreFavotto]

I can work on a pull request for this if that's ok.

[tboegi]

@AndreFavotto : Feel free to go ahead with a fix