GPIB Messages

There are two types of messages on the General Purpose Interface Bus (GPIB): command messages and data messages. Every GPIB Controller and every GPIB instrument has a unique identity on the bus; this identity is known as its address. GPIB Controllers use command messages to tell instruments when they can talk to provide information to the bus, and when they can listen to information on the bus. This information is passed as data messages.

Controllers, Talkers, and Listeners

In the world of GPIB, there are Controllers, Talkers, and Listeners:

• Controllers govern the flow of information on the bus by issuing Talker and Listener assignments to other devices on the bus. They respond to service requests from instruments, and they can pass control of the bus to other Controllers. There can be only one Controller-In-Charge (CIC or System Controller) per bus, which is responsible for overall management of the bus.
• Talkers place information on the data lines of the bus, but only when a Controller instructs them to do so. Only one device can talk at a time.
• Listeners retrieve information from the data lines of the bus, but only when a Controller instructs them to do so. Any number of devices can listen at the same time.

  Command Messages vs. Data Messages
  

GPIB Controllers use command messages to tell devices (instruments or other Controllers) when they can talk to provide information to the bus and when they can listen to information on the bus. The Talker and Listener assignments are sent as command messages, while the information is passed as data messages.

The major difference between a command message and a data message is the state of the Attention (ATN) line, which is one of the bus management lines. If the ATN line is asserted, any messages sent on the data lines are heard by all devices, and they are understood to be command messages. If the ATN line is not asserted, only the devices that were addressed to listen may receive the messages on the data lines.

The GPIB Addressing Protocol

The GPIB has eight data lines, which are used to send information one byte (8 bits) at a time. Command messages use seven of the eight bits, as shown in Figure 1, below:

Bits 0 through 4 indicate the primary address of the device, for which the Talker/Listener assignment is intended. If bit 5 is high, the device should listen. If bit 6 is high, the device should talk. Bit 7 is a "don't care" bit. Its value is ignored, so it is interpreted as a value of zero in command messages.

Each device on the bus must have a unique address. This address consists of a primary address (PAD) and a secondary address (SAD). As you can see from Figure 1, five of the data lines are used to indicate the GPIB primary address. This means that you could have a value from 0 to 31, for a total of 32 (2 to the power of 5) addresses; however, PAD 31 is never used as a primary address, because it is used for special command messages. This leaves a total of 31 possible primary addresses. The Controller-In-Charge (CIC) for a bus is almost always at PAD 0, so the instruments on a bus can have primary addresses from 1 to 30. A common mistake in working with the GPIB is assigning the same address to the controller board and the instrument, which will result in an EADR error (addressing error).

The GPIB secondary address is identical in its range of 0 to 30, which allows for a total of 961 (31 x 31) possible GPIB addresses, but the secondary address is very rarely used (the SAD is typically set to zero). Talker/Listener assignments are part of the primary addressing information, so with PADs you use either bit 6 or bit 5 when you send a command message. This might prompt you to ask, "How do I send the SAD information?" For secondary addresses, you set both bits 6 and 5 high when you send a command message. If you need to communicate with a device that has a secondary address, you need to indicate its primary address first and then immediately indicate its secondary address.

The easiest way to play around with primary and secondary addressing, command messages, and data messages is to use the Interactive Control (IBIC). For example, if you have a board at PAD 0 and an instrument with PAD 2 and SAD 4, and you want the board to talk and the instrument to listen, you would send the following command message in IBIC:

ibcmd "\x40\x22\x64"

The \x40 stands for hex 40, which represents the binary pattern of 0 1 0 0 0 0 0 0. This means that bit 6 is high (talk) and the primary address is zero. The \x22 stands for hex 22, which represents the binary pattern of 0 0 1 0 0 0 1 0. This means that bit 5 is high (listen) and the primary address is 2. The \x64 stands for hex 64, which represents the binary pattern of 0 1 1 0 0 1 0 0. This means that bits 6 and 5 are high -- so, treat the following address as a secondary address -- and the address is 4.

Multiline Interface Message Table

As we saw in the previous section, each command message corresponds to a particular bit pattern, which can be represented in hexadecimal format. All GPIB command messages can also be mapped to ASCII characters (because ASCII is a 7-bit character set), so ASCII is the "language" of the GPIB. The Multiline Interface Table (Table 1) lists the correspondence of ASCII characters to command messages, as well as their hexadecimal, octagonal, and decimal values. The Multiline Interface Message Table is in the appendix in most National Instruments GPIB-related technical manuals.

Hex	Oct	Dec	ASCII	Msg	Hex	Oct	Dec	ASCII	Msg
00	000	0	NUL		20	040	32	SP	MLA0
01	001	1	SOH	GTL	21	041	33	!	MLA1
02	002	2	STX		22	042	34	"	MLA2
03	003	3	ETX		23	043	35	#	MLA3
04	004	4	EOT	SDC	24	044	36	$	MLA4
05	005	5	ENQ	PPC	25	045	37	%	MLA5
06	006	6	ACK		26	046	38	&	MLA6
07	007	7	BEL		27	047	39	'	MLA7
08	010	8	BS	GET	28	050	40	(	MLA8
09	011	9	HT	TCT	29	051	41	)	MLA9
0A	012	10	LF		2A	052	42	*	MLA10
0B	013	11	VT		2B	053	43	+	MLA11
0C	014	12	FF		2C	054	44	,	MLA12
0D	015	13	CR		2D	055	45	-	MLA13
0E	016	14	SO		2E	056	46	.	MLA14
0F	017	15	SI		2F	057	47	/	MLA15
10	020	16	DLE		30	060	48	0	MLA16
11	021	17	DC1	LLO	31	061	49	1	MLA17
12	022	18	DC2		32	062	50	2	MLA18
13	023	19	DC3		33	063	51	3	MLA19
14	024	20	DC4	DCL	34	064	52	4	MLA20
15	025	21	NAK	PPU	35	065	53	5	MLA21
16	026	22	SYN		36	066	54	6	MLA22
17	027	23	ETB		37	067	55	7	MLA23
18	030	24	CAN	SPE	38	070	56	8	MLA24
19	031	25	EM	SPD	39	071	57	9	MLA25
1A	032	26	SUB		3A	072	58	:	MLA26
1B	033	27	ESC		3B	073	59	;	MLA27
1C	034	28	FS		3C	074	60	<	MLA28
1D	035	29	GS		3D	075	61	=	MLA29
1E	036	30	RS		3E	076	62	>	MLA30
1F	037	31	US		3F	077	63	?	UNL

Message Definitions
DCL	Device Clear		MSA	My Secondary Address
GET	Group Execute Trigger		MTA	My Talk Address
GTL	Go To Local		PPC	Parallel Poll Configure
LLO	Local Lockout		PPD	Parallel Poll Disable
MLA	My Listen Address

Table 1 (Part 2). Multiline Interface Messages

Hex	Oct	Dec	ASCII	Msg	Hex	Oct	Dec	ASCII	Msg
40	100	64	@	MTA0	60	140	96	`	MSA0,PPE
41	101	65	A	MTA1	61	141	97	a	MSA1,PPE
42	102	66	B	MTA2	62	142	98	b	MSA2,PPE
43	103	67	C	MTA3	63	143	99	c	MSA3,PPE
44	104	68	D	MTA4	64	144	100	d	MSA4,PPE
45	105	69	E	MTA5	65	145	101	e	MSA5,PPE
46	106	70	F	MTA6	66	146	102	f	MSA6,PPE
47	107	71	G	MTA7	67	147	103	g	MSA7,PPE
48	110	72	H	MTA8	68	150	104	h	MSA8,PPE
49	111	73	I	MTA9	69	151	105	i	MSA9,PPE
4A	112	74	J	MTA10	6A	152	106	j	MSA10,PPE
4B	113	75	K	MTA11	6B	153	107	k	MSA11,PPE
4C	114	76	L	MTA12	6C	154	108	l	MSA12,PPE
4D	115	77	M	MTA13	6D	155	109	m	MSA13,PPE
4E	116	78	N	MTA14	6E	156	110	n	MSA14,PPE
4F	117	79	O	MTA15	6F	157	111	o	MSA15,PPE
50	120	80	P	MTA16	70	160	112	p	MSA16,PPD
51	121	81	Q	MTA17	71	161	113	q	MSA17,PPD
52	122	82	R	MTA18	72	162	114	r	MSA18,PPD
53	123	83	S	MTA19	73	163	115	s	MSA19,PPD
54	124	84	T	MTA20	74	164	116	t	MSA20,PPD
55	125	85	U	MTA21	75	165	117	u	MSA21,PPD
56	126	86	V	MTA22	76	166	118	v	MSA22,PPD
57	127	87	W	MTA23	77	167	119	w	MSA23,PPD
58	130	88	X	MTA24	78	170	120	x	MSA24,PPD
59	131	89	Y	MTA25	79	171	121	y	MSA25,PPD
5A	132	90	Z	MTA26	7A	172	122	z	MSA26,PPD
5B	133	91		MTA27	7B	173	123	{	MSA27,PPD
5C	134	92	\	MTA28	7C	174	124	|	MSA28,PPD
5D	135	93		MTA29	7D	175	125	}	MSA29,PPD
5E	136	94	^	MTA30	7E	176	126	~	MSA30,PPD
5F	137	95	_	UNT	7F	177	127	DEL

Message Definitions
PPE	Parallel Poll Enable		SPE	Serial Poll Enable
PPU	Parallel Poll Unconfigure		TCT	Take Control
SDC	Selected Device Clear		UNL	Unlisten
SPD	Serial Poll Disable		UNT	Untalk

For example, the ASCII "@" character (hex 40) represents the message "MTA0", which assigns the device at primary address zero as the Talker. As another example, the ASCII "?" character (hex 3F) represents the message "UNL", which instructs all devices that had previously been assigned as Listeners to unlisten (that is, stop listening).

The Multiline Interface Message Table is very useful, if you are using the board-level GPIB function ibcmd. As a Controller, you would use the ibcmd function to send command messages. For example, if you were using the Interactive Control (IBIC) to assign the Controller (PAD 0) as the Talker and an instrument (PAD 5) as the Listener, you would use ibcmd "?@%" to instruct all devices to stop listening (?, UNL), the Controller to talk (@, MTA0), and the instrument to listen (%, MLA5).

The Controller would then send data to the instrument with the ibwrt function. (Note: Calling ibwrt will cause the ATN line to deassert, so that the data lines on the bus are used to send data messages, instead of command messages.) To continue with the previous example, you could use ibwrt "hello" to send the data message hello to the instrument. (Of course, the message you send should make sense to the instrument.)

Command Message Shortcuts

If you find yourself without easy access to the Multiline Interface Message Table, you can create the hexadecimal equivalents of the MLA (My Listen Address) command messages and the MTA (My Talk Address) command messages using these formulas: MLA = (hex 20) + (PAD in hex) and MTA = (hex 40) + (PAD in hex). For example, if you want to assign your Controller (PAD0) as the Listener and your instrument (PAD 25) as the Talker, you can use hex 20 (MLA0 = hex 20 + hex 00 = hex 20) and hex 59 (MTA25 = hex 40 + hex 19 = hex 59), respectively.

Unlisten (UNL) and Untalk (UNT) are formed by using PAD 31 (hex 1F) in the formulas for MLA and MTA, respectively. For example, UNL is hex 3F (hex 20 + hex 1F = hex 3F) and UNT is hex 5F (hex 40 + hex 1F = hex 5F).

Secondary addresses are formed by adding hex 60 to the SAD in hex: MSA (My Secondary Address) = (hex 60) + (SAD in hex). For example, if you have an instrument with SAD 20, you will use hex 74 (MSA20 = hex 60 + hex 14 = hex 74) to represent its secondary address.

Board-Level vs. Device-Level Functions

There are two types of GPIB functions: board-level functions and device-level functions. At the device level, you are limited to communicating with one device at a time, while at the board level, you can communicate with multiple devices (but only one device can talk at a time!). With respect to GPIB messages, board-level functions require you to perform Talker/Listener assignments manually with the ibcmd function; device-level functions take care of the Talker/Listener assignments for you.

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