GPIB Messages

Publish Date: Sep 06, 2006 | 52 Ratings | 4.13 out of 5 |  PDF

Overview

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.

Table of Contents

  1. Controllers, Talkers, and Listeners
  2. Command Messages vs. Data Messages
  3. The GPIB Addressing Protocol
  4. Multiline Interface Message Table
  5. Command Message Shortcuts
  6. Board-Level vs. Device-Level Functions

1. 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.

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    2. 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.

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3. 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:


Figure 1. GPIB Addressing Protocol


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.

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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.
Table 1 (Part 1). Multiline Interface Messages

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
DCLDevice ClearMSAMy Secondary Address
GETGroup Execute TriggerMTAMy Talk Address
GTLGo To LocalPPCParallel Poll Configure
LLOLocal LockoutPPDParallel Poll Disable
MLAMy 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
PPEParallel Poll EnableSPESerial Poll Enable
PPUParallel Poll UnconfigureTCTTake Control
SDCSelected Device ClearUNLUnlisten
SPDSerial Poll DisableUNTUntalk


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.)

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5. 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.

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6. 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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