Measurement Computing Personal488 rev.3.0 For DOS & Windows 3.Xi - Manual (Page 96)

II. SOFTWARE GUIDES - 8. Driver488/DRV 8H. Turbo C
Personal488 User’s Manual, Rev. 3.0 II-81
CRITERR.ASM (Microsoft C & Turbo C)
Normally, when Driver488/DRV detects an error, perhaps due to a syntax error in a command, or due
to an IEEE 488 bus error (such as time out on data transfer), it responds with an I/O error to DOS.
When this happens, DOS normally issues an
ABORT , RETRY or IGNORE message and waits for a
response from the keyboard. There is no way for the user’s program to detect such an error, determine
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.
Sample Program
#include “ieeeio.h”
#include .h
void main (void) {
char response[256];
float voltage;
int i;
float sum;
char hundred[1700];
ieee=open(“ieee”,O_RDWR | O_BINARY);
ieee=open(“ieee”,O_rdwr);
rawmode(ieee);
ioctl_wt(ieee,"break",5);
ieeewt(“reset\r\n”);
ieeewt(“eol out lf\r\n”);
ieeewt(“eol in $0\n”);
ieeewt(“fill error\n”);
if (ieeeinit() == -1) {
printf(“Cannot initialize IEEE system.\n”);
exit(1);