Measurement Computing Personal488 rev.3.0 For DOS & Windows 3.Xi - Manual (Page 92)

II. SOFTWARE GUIDES - 8. Driver488/DRV 8H. Turbo C
Personal488 User’s Manual, Rev. 3.0 II-77
Interrupt Handling
The IEEE 488 bus is designed to be able to attend to asynchronous (unpredictable) events or
conditions. When such an event occurs, the bus device needing attention can assert the Service
Request (
SRQ ) line to signal that condition to the controller. Once the controller notices the SRQ, it can
interrogate the bus devices, using Parallel Poll ( PPOLL ) and/or Serial Poll ( SPOLL ) to determine the
source and cause of the SRQ , and take the appropriate action.
Parallel Poll is the fastest method of determining which device requires service. Parallel Poll is a very
short, simple IEEE 488 bus transaction that quickly returns the status from many devices. Each of the
eight IEEE 488 bus data bits can contain the Parallel Poll response from one or more devices. So, if
there are eight or fewer devices on the bus, then just the single Parallel Poll can determine which
requires service. Even if the bus is occupied by the full complement of 15 devices, then Parallel Poll
can narrow the possibilities down to a choice of no more than two.
Unfortunately, the utility of Parallel Poll is limited when working with actual devices. Some have no
Parallel Poll response capability. Others must be configured in hardware, usually with switches or
jumpers, to set their Parallel Poll response. If Parallel Poll is not available, or several devices share the
same Parallel Poll response bit, then Serial Polling is still required to determine which device is
requesting service.
Serial Poll, though it is not as fast as Parallel Poll, does offer three major advantages: it gives an
unambiguous response from a single bus device; it returns additional status information beyond the
simple request/no-request for service; and, most importantly, it is implemented on virtually all bus
devices.
The SRQ line can be monitored in two ways: it can be periodically polled by using the STATUS
command, or by checking the “light pen status.”
BASIC provides a method for detecting and servicing external interrupts: the ON PEN statement. The
ON PEN statement tells BASIC that, when an external interrupt is detected, a specific subroutine,
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;