Measurement Computing Personal488 rev.3.0 For DOS & Windows 3.Xi - Manual (Page 74)

II. SOFTWARE GUIDES - 8. Driver488/DRV 8E. Microsoft C
Personal488 User’s Manual, Rev. 3.0 II-59
known as the interrupt service routine (ISR), is to be executed. Normally, the interrupt detected by
ON PEN is the light pen interrupt. However, Driver488/DRV redefines this “light pen interrupt” to
signal when an IEEE 488 bus related interrupt (such as SRQ ) has occurred.
Unlike BASIC, C does not provide an automatic method of checking for light pen interrupts.
Therefore, a function is needed to check for the interrupt. The function could use the
STATUS
command, but it is much faster to check the interrupt status directly using a BIOS interrupt. The
CKLPINT (check light pen interrupt) function provided in IEEEIO.C uses the BIOS to check for
Driver488/DRV interrupts and returns true ( 1 ) if one is pending. Interrupts are checked automatically
by the IEEEWT routine before sending any data to Driver488/DRV. However, IEEEWT does not call
CKLPINT directly. Instead, it calls the routine that is pointed to by IEEE_CKI ( IEEE check interrupt).
If IEEE_CKI points to CKLPINT , then IEEEWT checks for Driver488/DRV interrupts, but if
IEEE_CKI points to _false_ , a function that always returns 0 , then interrupt checking is disabled.
Initially, IEEE_CKI does point to _false_ , and so interrupt checking is disabled. To enable interrupt
checking IEEE_CKI must be redirected to CKLPINT :
int cklpint();
ieee_cki = cklpint;
Once an interrupt has been detected, an interrupt service routine must be invoked to handle the
interrupting condition. When
IEEEWT detects an interrupt, it calls the interrupt service routine (ISR).
Just as IEEEWT does not call the check-for-interrupt routine directly, it does not call the ISR directly,
either. Instead, it calls the routine pointed to by IEEE_ISR ( IEEE interrupt service routine). If
IEEE_ISR is set to point to some specific ISR, then that ISR is executed when IEEEWT detects an
interrupt. Initially, IEEE_ISR points to no_op , a function that does nothing. So, unless IEEE_ISR is
redirected to another routine, nothing is done when an interrupt is detected. In the 195DEMO example
program an interrupt service routine, called isr , has been provided. So, IEEE_ISR must be set to
point this routine for interrupts to be handled properly:
ieee_isr = isr;
Once we have enabled interrupt checking by setting IEEE_CKI to point to CKLPINT , and specified the
interrupt service routine by setting IEEE_ISR to point to isr , then we can specify which conditions are
to cause an interrupt. The ARM command specifies those conditions. In this example we want the
interrupt to occur on the detection of a Service Request ( SRQ ):
ieeewt(“arm srq\n”);
The 195 can be set to request service on any of several different internal conditions. In particular, the
M2 command causes an SRQ on the detection of any invalid command or command option by the 195:
ieeewt(“output 16;M2X”);
This OUTPUT command is placed early in the program so that all subsequent commands to the 195
cause an SRQ , if they are invalid.
Now that interrupt detection is enabled, and the interrupt service routine is specified, we must specify
the actions to take to service the interrupt. We first display a message indicating that an interrupt was
detected, and then turn off interrupt checking:
void isr()
{ int _false_();
printf(“Interrupt detected...”);
ieee_cki = _false_;
We next check the Driver488/DRV Serial Poll Status to determine if an SRQ actually caused the
interrupt:
int sp;
ieeewt(“spoll\n”);
ieeescnf(“%d”,&sp);
if (sp==0) {
printf(“Non-SRQ Interrupt!\n”);
exit(1);
}