Experimental Physics and
Industrial Control System

Andrew Wagner <[email protected]> · Fri, 6 Aug 2010 16:56:03 -0700

Hey guys, 

I've got EPICS and ASYN running and have successfully followed one of Eric Norum's examples to talk to an Agilent 34970A MUX (over IP) and store the *IDN in a stringin record. I'm having problems however with reading a voltage measurement back reliably. I create device support with the devGpib template and the commands: 

  /* Param 0 -- Read identification string */
  {&DSET_SI, GPIBREAD, IB_Q_HIGH, "*IDN?\n++read eoi",NULL, 0, 200, NULL, 0, 0, NULL, NULL, NULL},

  /*Param 1 -- Read Measurement @ Channel */ 
  {&DSET_AI, GPIBREAD, IB_Q_HIGH, "MEAS:VOLT:DC? (@203)\n++read eoi", NULL, 0, 200, readMEAS, 0, 0, NULL, NULL, NULL} 

I'm interfacing with the GPIB device via the PROLOGIX GPIB-Ethernet controller, (Google them, they're pretty neat) so you can ignore the ++read eoi part of the command string. The conversion method is: 

static int readMEAS( struct gpibDpvt *pdpvt, int P1, int P2, char **P3){ 

  struct aiRecord *pai= (struct aiRecord *)pdpvt->precord;

  double meas; 
  sscanf(pdpvt->msg,"%lg",&meas); 
  printf("meas %f\n",meas); 
  printf("Messge Length %d. Msg %s\n",pdpvt->msgInputLen,pdpvt->msg);
  meas = (double)atof(pdpvt->msg);
  printf("meas %f\n",meas); 
  pai->val = meas;

  return 0;

}

I set:

asynSetTraceMask("L0",-1,0x9)
asynSetTraceIOMask("L0",-1,0x2)

in my st.cmd to do some debugging. When I force one of my voltage measurement records to process the first time I get: 

epics> dbtr AG34970:VOLT:Coil 
epics> 2010/08/06 15:39:30.848 172.25.100.85:1234 write 32
MEAS:VOLT:DC? (@203)\n++read eoi\n
2010/08/06 15:39:31.011 172.25.100.85:1234 read 17
+2.99865210E+00\n\003
meas 2.998652
Messge Length 15. Msg +2.99865210E+00
meas 2.998652

This is exactly what I expect since I'm supplying the MUX with 3 V at Channel 203. So far so good. When I print the record I get: 

epics> dbpr AG34970:VOLT:Coil
ASG:                DESC: Coil Voltage  DISA: 0             DISP: 0             
DISV: 1             NAME: AG34970:VOLT:Coil                 RVAL: 0             
SEVR: INVALID       STAT: UDF           SVAL: 0             TPRO: 0             
VAL: 2.9986521  

My first newbie question is why is SEVR: INVALID? The VAL is certainly what I want it to be. The problem occurs if I try to process the record more than once: 

epics> dbtr AG34970:VOLT:Coil
epics> 2010/08/06 15:39:49.785 172.25.100.85:1234 write 32
MEAS:VOLT:DC? (@203)\n++read eoi\n
2010/08/06 15:39:49.948 172.25.100.85:1234 read 17
+2.99867890E+00\n\003
meas 0.000000
Messge Length 16. Msg +2.99867890E+00
meas 0.000000

Hmmm........an ASCII string indicating an appropriate value is read but converted into a double of 0.0000 . Note the message length has increased by 1. Its as if a hidden character is appended somewhere that screws up the sscanf and atof functions. Perhaps there is a more error proof way to convert ASCII to doubles? Has anyone seen this before? I have no such problem with my stringin record. I can send *IDN? and read the device name reliably. 

Thanks very much for your help. I will happily make any device support I get working public. 

Cheers, 

Andrew Wagner

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System