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16 Programming the Status System Chapter 2
• Define an SRQ handler to do the following:
-- Read the Status Byte using ireadstb
. ireadstb returns the RQS
(request for service) bit in bit 6 of the status byte. After issuing a
ireadstb, RQS is cleared indicating that the Service Request is being
acknowledged. A new SRQ will not be issued unless RQS is
cleared. Using *STB? will return the Master State Summary in bit 6
and does not affect RQS.
-- Check if the MAV bit (bit 4) is set to indicate that a message is
available. If the MAV bit is set, then a message is available and the
SRQ handler can process the message. In this example, the output
queue is read using iscanf
.
• Enable SRQ Handler in SICL with ionsrq.
• Enable MAV bit (Message Available Bit) in the Status Byte Enable
Register (e.g. *SRE 16). This will cause an SRQ to arrive when there
is a message in the output queue (i.e. data is available to be read)
Example Program
/* status1.c *
* The following program provides an interactive command line interface
* to send SCPI commands to SCPI compatible instruments.
* This utilizes the MAV bit of the Status Byte in order to determine if
* the instrument is returning any output. */
#include <sicl.h>
#include <stdio.h>
/* Theses are Masks for the Status Byte */
/* all bits start at bit 0 */
#define MAV_MASK 0x10 /* MAV - bit 4 */
/* This is the SRQ handler to check for Message Available (MAV) */
void srq_hdlr( INST id) {
unsigned char stb;
char buf[255];
int esr;
int errnum;
char errmsg[100];
/* read the status byte to determine what caused the SRQ.
* Note: use ireadstb instead of *STB? because you want to
* clear RQS instead of reading the MSS bit in the status byte.*/
ireadstb(id, &stb);
/* check if MAV caused the SRQ */
if( MAV_MASK == (stb & MAV_MASK))
{
/* message is available so read in the result. */
iscanf( id, "%t", buf);
printf("%s", buf);
}
}