Measurement Computing Personal488 rev.3.0 For DOS & Windows 3.Xi - Manual (Page 121)

8L. Language-Specific Information II. SOFTWARE GUIDES - 8. Driver488/DRV
II-106 Personal488 User’s Manual, Rev. 3.0
the cause, and take appropriate action. However, DOS does provide a method of redefining the action
to be taken on such a “critical error”.
CRITERR.ASM contains a critical error handler that, when
invoked, makes it appear to the calling program that some less-critical error has occurred. The critical
error handler is installed by
CRIT_ON() and removed by CRIT_OFF() . The critical error handler is
also automatically removed by DOS when the program exits.
The following program fragment demonstrates the use of the critical error handler:
#include “criterr.h”
crit_on(ieee);
if (ieeewt(“output 16;F0X”) == -1) {
printf(“Error writing F0X to device 16, \n”);
crit_off();
ioctl_wt(ieee,"break",5);
ieeewt(“eol out lf\r\n”);
ieeewt(“status\n”);
ieeerd(response);
printf(“status = %s\n”,response);
crit_on(ieee);
}
We must first #include the header file with the definitions of the critical error routines. We then
enable critical error trapping with CRIT_ON which takes as a parameter the handle of the file for which
critical error trapping is to be enabled. Only read and write commands to that handle are trapped.
Errors caused by other actions, or associated with other files are not trapped. Error trapping may only
be enabled for one file at a time.
Now, if IEEEWT signals an error by returning a -1 , we can check what happened. We first PRINTF an
error message, then we turn critical error trapping off with CRIT_OFF so that, if another critical error
occurs, we get the ABORT , RETRY or IGNORE message and know a catastrophic double error has
occurred. We then IOCTL_WT(_BREAK_) to force Driver488/DRV to listen to our next command.
The IOCTL_WT also resets the EOL OUT terminator so we can be sure that Driver488/DRV detects the
end of our commands. We next reset the EOL OUT terminator to our preferred line feed only and ask
Driver488/DRV for its status. On receiving the response, we could interpret the status and take
whatever action is appropriate. However, in this example, we just display the status. Finally, we re-
enable the critical error handler and continue with the program.
GW-BASIC (for GW-BASIC or Interpreted BASIC)
Use of Direct DOS I/O Devices
Once Driver488/DRV has been installed in your system, it is ready to begin controlling IEEE 488 bus
devices. To show how this is done, we develop a short program in BASIC to control a Keithley
Instruments Model 195 digital multimeter. The complete sample program is provided on the
Driver488/DRV disk as
195DEMO.BAS in the \MSBASIC directory.
BASIC VARPTR & SADDR
The BASIC VARPTR and SADDR functions must be used with caution. The first time a variable such as
I or ST$ is encountered, or an array such as R%() is dimensioned, space is made for it in BASIC’s data
space. The other variable or arrays may be moved to make room for the new item. If the memory
location of an item must be fixed, then BASIC cannot be allowed to encounter any new variables or
arrays. For example, in the
ENTER statement shown above, Driver488/DRV is told the memory address
of R$ (for GW-BASIC, R%(0) ). Then, while the transfer is going on, the Driver488/DRV status is read
into the string variable ST$ . If ST$ has not been used previously then BASIC would have to create a
new ST$ and might move R$ . Of course, Driver488/DRV would have no way of knowing that R$ has
been moved, and the data would not be placed correctly into R$ .