_UNDERSTANDING THE GPIB_
by Don Morgan

[LISTING ONE]

/* Scope.c */
#include <stdio.h>
#include "decl.h"    /* a header file containing declares pertinent to the 
                        National Instruments library*/

/* Application program variables passed to GPIB functions */
char rd[512];   /* read data buffer*/
int  bd;        /* board number, in this case that representing controller  */

main()
{

/* Assign unique identifier to board 0 and store in variable bd. Check for 
error.*/
if ((bd = ibfind ("GPIB0")) < 0) error();
printf("\ninitializing controller");
getch();

/* Send the Interface Clear (IFC). */
if (ibsic (bd) & ERR) error();

/* Turn on Remote Enable (REN) signal so instrument can be programmed. */
if (ibsre (bd,1) & ERR) error();

/* Put scope in remote mode by addressing it to listen, send Device Clear 
(DCL) message to clear internal device functions, and address GPIB board to 
talk. */
ibcmd (bd,"?_@' ",4);
if (ibsta & ERR) error();

/* Write the function, range, and trigger source instructions to scope. */
ibwrt (bd,":bnc probe",10);
if (ibsta & ERR) error();

ibwrt (bd,":acquire:type normal",20);
if (ibsta & ERR) error();

ibwrt (bd,":timebase:range 5e-4",20);
if (ibsta & ERR) error();

ibwrt (bd,":timebase:delay 0",17);
if (ibsta & ERR) error();

ibwrt (bd,":timebase:reference center",26);
if (ibsta & ERR) error();

ibwrt (bd,":timebase:mode triggered",24);
if (ibsta & ERR) error();

ibwrt (bd,":channel1:probe 10",18);
if (ibsta & ERR) error();

ibwrt (bd,":channel:range 1.2",18);
if (ibsta & ERR) error();

ibwrt (bd,":trigger:mode edge",18);
if (ibsta & ERR) error();

ibwrt (bd,":trigger:slope positive",23);
if (ibsta & ERR) error();

ibwrt (bd,":trigger:level 300mv",20);
if (ibsta & ERR) error();

ibwrt (bd,":trigger:source channel1",24 );
if (ibsta & ERR) error();

/* Scope is now ready to go. Set up SRQ by first clearing internal 
data structures. */
ibwrt (bd,"*cls",4);
if (ibsta & ERR) error();

/* Clear the trigger by issuing the command to return the bit. */
ibwrt (bd,":ter?",5);
if (ibsta & ERR) error();

/* Then make the scope a talker. */
ibcmd (bd,"?_ G",4);

/* And become a listener. Read three bytes or stop when an EOI is received. */
ibrd(bd,rd,3);
if (ibsta & ERR) error();

/* When the data is in buffer issue an untalk and unlisten, then make the 
controller a talker again and scope a listener. */
ibcmd (bd,"?_@'",4);

/* Enable the SRQ bit within scope that will case an RQS on next trigger. */
ibwrt (bd,"*sre 1",6);
   if (ibsta & ERR) error();

/* Now wait for the trigger. */
if (ibwait (bd,SRQI) & (ERR)) error();

/* If we are here, scope must have been triggered and must have asserted SRQ 
command line. Do a serial poll. First unaddress bus devices and and send 
Serial Poll Enable (SPE) command, followed by scope's talk address, and GPIB
board's listen address. */
ibcmd (bd,"?_\x18G ",5);      /*UNL UNT SPE TAD MLA*/
if (ibsta & ERR) error();

/* Now read status byte. If it is 0x41, the scope has valid data to send; 
otherwise it has a fault condition to report. */
ibrd (bd,rd,1);
if (ibsta & ERR) error();
if ((rd[0] & 0xFF) != 0x41) error();
               
/* Note that if more than one device is attached to bus, each device must be 
checked to see which issued the SRQ. */

/* Complete serial poll by sending the Serial Poll Disable (SPD) message. */
if (ibcmd (bd,"\x19",1) & ERR) error();
               
/*Send scope untalk and unlisten; make controller talker and scope listener*/
ibcmd (bd,"?_@'");
               
/*Tell the scope to print the screen and associated data*/
ibwrt(bd,":hardcopy:page automatic",24);
ibwrt(bd,":print?",7);
               
/*Make scope a talker and printer a listener; have controller get out of 
the way for the transfer*/
ibcmd(bd,"?_@!G",5);
ibgts(bd,1);
               
/*Wait for the transfer to complete and reassert control over the bus*/
ibwait(END);

/*The program terminates with an interface clear*/
ibsic (bd);
}

/* Simple error routine that reads system variables and returns them. */
error()
 {
   printf("\nGPIB function call error:");
   printf("\nibsta=0x%x, iberr=0x%x,",ibsta,iberr);
   printf("\nibcnt=0x%x\n",ibcnt);
 }



Example 1: 

(a)

OUT "IFC";       /*Interface Clear*/

(b)

OUT "REN";      /*Remote Enable*/

(c)

OUT "?_@'\0X14";  /*? = Unlisten (UNL), _ = Utalk (UNT),  @ = My Talk Address
		  of controller (MTA), ' = My Listen address of scope (MLA), 
		  0X14 = Device clear*/

(d)

OUT "?_@'";    /*UNL, UNT, MTA of controller, MLA of scope*/
OUT ":ACQUIRE:TYPE NORMAL";
;              /*Now the device dependent configuration data*/

;
OUT ":TRIGGER:SOURCE CHANNEL1";
OUT ":SRQ 1";    /*Enable SRQ on trigger, bit 1 of SRQ mask register*/
OUT "?_";        /*UNL, UNT*/

(e)

OUT "?_\X18G ";     /*UNL, UNT, Serial Poll Enable  (SPE), Talk Address (TAD) 
 		    of scope, MLA of controller*/
IN &BUFFER;         /*Program puts Serial Poll Byte in a buffer*/
OUT "\0X19";        /*Issue Serial Poll Disable (SPD)*/

(f)

OUT "?_@'";       /*UNL, UNT, MTA of controller, MLA of scope*/
OUT "PRINT";      /*Tell scope to print*/
OUT "?_@!G";      /*UNL, UNT, MTA of controller, MLA of printer/plotter, 
		   MTA of scope*/
OUT "DATA";       /*Controller takes its own attention (ATN) line low to allow
		   scope to talk to printer/plotter*/
WAIT "END";       /*Controller watches command lines for end or identify (EOI),
		   which means that the transfer is complete*/