I'm going to paste in some LabWindows CVI code, then I'll explain what the program does.
float read_position (void)
{
fillnaf (2,0,0);
pos_counts = camac_read(crate, naf);
position = (pos_counts + 32768) / 314.0;
SetCtrlVal (panelHandle, PANEL_CURRENT_POSITION, position);
if (!slewFlag) SetCtrlVal (panelHandle, PANEL_SLEWPOSITION, position);
ProcessDrawEvents ();
return position;
}
int move_it (int delta)
{
moveFlag = 0;
fillnaf (7,0,16);
camac_write (crate, naf, delta);
fillnaf (7,12,1);
SetCtrlAttribute (panelHandle, PANEL_STEPMSG, ATTR_VISIBLE, 1);
while (moveFlag == 0) {
moveFlag = camac_read (crate, naf);
ProcessSystemEvents ();
}
SetCtrlAttribute (panelHandle, PANEL_STEPMSG, ATTR_VISIBLE, 0);
if (moveFlag > 1) end_limit (moveFlag);
read_position ();
return moveFlag;
}
void camac_command(int crate, char naf[])
{
ibwrt(crate, naf, 3);
if (ibsta & 0x8000) {
iberr_msg = gpib_error ();
on_error (iberr_msg);
}
}
void camac_write(int crate, char naf[], int instruct)
{
unsigned char message[3];
char *ip;
ip = (char *) &instruct;
message[0] = *(ip+2);
message[1] = *(ip+1);
message[2] = *ip;
camac_command(crate, naf);
ibwrt(crate, message, 3);
if (ibsta & 0x8000) {
iberr_msg = gpib_error ();
on_error (iberr_msg);
}
}
int camac_read(int crate, char naf[])
{
int answer = 0 ;
unsigned char message[3];
char *ip;
camac_command(crate, naf);
ibrd(crate, message, 3);
if (ibsta & 0x8000) {
iberr_msg = gpib_error ();
on_error (iberr_msg);
}
ip = (char *) &answer;
*(ip+2)=message[0];
*(ip+1)=message[1];
*ip =message[2];
return(answer);
}
The purpose of this code is to operate a stepper motor and read its position. The stepper motor controller and the counter are modules in a CAMAC crate. The naf commands tell the crate which module is to be addressed, and what the specific command to that module is. The camac_write in the move_it function starts the motor moving. The stepper motor controller has a status byte which is then read by the camac_read in the loop. The returned byte can be zero, meaning the motor is still moving; 1, which means the move is complete; or 2 or 4, which mean the motor has hit either end of its travel limits. The problem I'm having with the PCI-GPIB card (which doesn't happen with the ISA AT-GPIB/TNT card) is that during the move the card will
intermittently pick up a garbage value (typically a seven digit integer) which kicks out of the loop and causes the position to be read before the end of the move. This happens during any iteration of the loop (I can watch with NISpy to see how many times the status byte is read before the loop is exited). I don't claim that this code is the most elegant way to perform these operations, but the question is, why does this happen with the PCI card and not the ISA? In both cases, the GPIB is simply reading a status byte. It doesn't seem that timing should be an issue.
Thank you for your help.
Stuart Van Deusen
(This is also posted on the CVI forum)
