Reading data with StreamDevice - order of input records processing for scalcout not respected

[Andrea Celentano via Tech-talk <tech-talk at aps.anl.gov>]

Dear colleagues,

I am struggling with a problem related to a waveform StreamDevice record.

Specifically, the device I want to interface with (Lecroy T3AFG200) has a remote interface based on a text protocol. When I send the string "C1:WaveBurst?", it replies with a string such as:

C1:BTWV STATE,ON,PRD,0.01S,STPS,0,TRSR,INT,TRMD,OFF,TIME,1,DLAY,1.38052e-06S

where the value of multiple parameters is reported. I want to read these values - in the above example, e.g., PRD is "0.01S", TRMD is "OFF", DLAY is "1.38052e-06S".

What makes this task not trivial is that the number of reported parameters, and their order, depends on the status of the device. Thus, the output string is not always the same.

I tried to work as follows.

• I read the data in a waveform record, where each element is a string, splitting the string by comma (and ignoring the initial C1:BTWV part)
  
• I process this data through an aSub record. The associated sub-routine takes as input the waveform record (INAA), and as also another string record reporting the field to be extracted (INBB). The two outputs are a double value and a string value containing, respectively, the double representation (OUTA) and the string representation (OUTB) of the element following INBB, if any.
  
• For example, if the waveform record is the one above, and the string identifier record is "TRSR", the routine will return "INT" in the string representation. If the string identifier record is "DLAY", the routine will return "1.38052e-06" in the double representation (I trim out the last "S" character)

I developed the following database records.

To read the data from the instrument and parse it, I have four records. The order of processing is dictated by the fact that, for a StreamDevice record, I cannot use it trivially as an input link with "Process Passive", since this will result in the OLD value to be reported before the protocol is executed, so I must go "the other way around".

record(waveform, "$(P):C1:Burst")
{
    field(DTYP, "stream")
    field(FLNK, "$(P):C1:BurstCalc")
    field(FTVL, "STRING")
    field(INP,  "@LecroyT3AFG.proto ChannelBurst($(P),1) T3AFG")
    field(NELM, "100")
    field(SCAN, "Passive")
}

record(stringin, "$(P):C1:BurstWord")
{
    field(FLNK, "$(P):C1:Burst")
    field(SCAN, "Passive")
}

record(aSub, "$(P):C1:BurstCalc")
{
    field(FTA,  "STRING")
    field(FTB,  "STRING")
    field(FTVA, "DOUBLE")
    field(FTVB, "STRING")
    field(INPA, "$(P):C1:Burst")
    field(INPB, "$(P):C1:BurstWord")
    field(LFLG, "IGNORE")
    field(NOA,  "100")
    field(OUTA, "$(P):C1:BurstCalcDouble")
    field(OUTB, "$(P):C1:BurstCalcString")
    field(SNAM, "LecroyT3AFG_asub")
}

record(ai, "$(P):C1:BurstCalcDouble")
{
}

record(stringin, "$(P):C1:BurstCalcString")
{
    field(VAL,  "INITIAL_VALUE")
}

To read the status, I have two specific records

record(scalcout, "$(P):C1:BurstStatus")
{
    field(CALC, "BB")
    field(INAA, "$(P):C1:BurstStatusS PP")
    field(INBB, "$(P):C1:BurstCalcString")
    field(SCAN, "Passive")
}

record(stringout, "$(P):C1:BurstStatusS")
{
    field(FLNK, "$(P):C1:BurstWord")
    field(OMSL, "closed_loop")
    field(OUT,  "$(P):C1:BurstWord")
    field(VAL,  "STATE")
}

My idea was the following: when I process "$(P):C1:BurstStatus", the following sequence should take place:

1. INAA is processed: $(P):C1:BurstStatusS -> $(P):C1:BurstWord -> $(P):C1:Burst ->$(P):C1:BurstCalc. The two records "$(P):C1:BurstCalcDouble" and "$(P):C1:BurstString" are populated.
2. INBB is processed $(P):C1:BurstCalcString

I thus assumed that the full chain triggered by INAA was processed before the INBB record was read. 

However, this is not the case. After triggering the process of $(P):C1:BurstStatus, I have $(P):C1:BurstStatus.SVAL equal to "INITIAL_VALUE", as if INBB is processed before (or during) INAA processing, and the order is not respected. But the value of $(P):C1:BurstCalcString is the correct one..

I am currently using epics version 7.0

Thanks,

Bests,

Andrea Celentano