Measurement Computing Personal488 rev.3.0 For DOS & Windows 3.Xi - Manual (Page 109)

8J. Spreadsheets II. SOFTWARE GUIDES - 8. Driver488/DRV
II-94 Personal488 User’s Manual, Rev. 3.0
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, Lotus 1-2-3 does not provide an automatic method of checking for light pen status.
Therefore, a subroutine is needed to check for the interrupt. This subroutine uses the
SPOLL command
to check for SRQ :
CheckSRQ:
{DEFINE Signal:VALUE}
{WRITELN SPOLL}
{READLN SP}
{LET SP,@VALUE(@MID(SP,0,@LENGTH(SP)-1))}
{IF SP=0}{BLANK ST195}{RETURN}
The CheckSRQ takes a numeric parameter, Signal , which can be used to note where in the program
the interrupt occurred. The subroutine begins by reading the response to the SPOLL command and
converting that response to a numeric value, SP . If SP is zero ( 0 ), then no SRQ is pending and we clear
the 195 status cell, ST195 , and then return. If SP is non-zero, we know that an SRQ is pending.
The 195 can be set to request service on any of several different internal conditions. In particular, the
M2 command causes an SRQ upon the detection of any invalid command or command option by the
195:
{WRITELN “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 check for service requests by calling CheckSRQ at appropriate places in the program. The only
place CheckSRQ should not be used, is between a command that requests a response, such as STATUS
or ENTER , and the statement(s) that read that response. The CheckSRQ parameter, Signal can be used
to identify where the interrupt was detected. A typical sequence might be:
{WRITELN “STATUS”}
{READLN Status}
{CheckSRQ 10}
{WRITELN ENTER 16}
{READLN Reading}
{CHECKSRQ 20}
Once CheckSRQ has determined, with a Serial Poll, that a service request is indeed pending, it then
checks the 195 to determine if it is the source of the interrupt. If there were other devices on the bus
that could be generating the
SRQ , each of them would be have to be checked in turn.
{WRITELN SPOLL 16}
{READLN ST195}
{LET ST195,@VALUE(@MID(ST195,0,@LENGTH(ST195)-1))}~
The tilde (~) at the end of the LET statement forces evaluation of the spreadsheet. In particular, it
causes the values of the cells DIO8 through DIO1 to be set to the values of the bits of ST195 . These
cells can then be examined to inspect the 195’s status:
{IF DIO7}{BRANCH 195SRQ}
{BEEP}{GETLABEL “Non-195 SRQ detected! Press Return.”,TypeHere}
{RESTART}{RETURN}
Bit DIO7 , is returned as true ( 1 ) in the Serial Poll response of those devices requesting service. In our
simple example, we expect that the 195 is the only possible cause of an SRQ , and if it is not, there must
be some error. If DIO7 is set, we BRANCH to 195SRQ and continue with the subroutine. Otherwise we
BEEP , display an error message, and terminate macro execution. TypeHere is a blank cell that holds
anything that is typed by the user before the return is pressed.