gpib
DESCRIPTION
TRANSCRIPT
SECTION 2AN OVERVIEW OF THE I
2.1 INTRODUCTION
The IEEE-488 bus is an instrumentation data bus standard-ized by the Institute of Electronic and Electrical Engineers in1975= The most recent revision of bus standards was made in1978; hence the complete description for current bus stan-dards is the IEEE-488-1978 designation.
This section gives a brief description of the general bus struc-ture along with an outline of bus commands. The informationpresented here is not intended to be an in-depth descriptionof what is a very complex standard. More complete informa-tion on the lEEE-488 bus, which is also frequently referred toas the GPIB (General Purpose Interface Bus), is availablefrom the IEEE and a variety of other sources.
2.2 BUS DESCRIPTION
The IEEE bus was designed as a parallel data transfer mediumto optimize data transfer without using an excessive numberof bus lines. In keeping with this goal, the bus has only eightdata lines that are used for both data and some commands.Five bus management lines and three handshake lines roundout the complement of signal lines. Since the bus is of paralleldesign, all devices connected to the bus have the same infor-mation available simultaneously. Exactly what is done withthe information by each device depends on many factors, in-cluding device capabilties.
A typical bus configuration for controlled operation is shownin Figure 2-1. The typical system will have one controller andone or more instruments to which commands are given and,in most cases, from which data is received. Generally, thereare three categories that describe device operation. Thesedesignations include: controller; talker; listener.
The controller does what its name implies: it controls otherdevices on the bus. A talker sends data, while a listenerreceives data. Depending on the instrument, a particulardevice may be a talker only, or both a talker and a listener.The Model 485, through the 4853 interface, is capable of be-ing both a talker and a listener, but it does not have controllercapability.
Any given system can have only one controller (control maybe passed to an appropriate device through a special com-mand), but any number of talkers or listeners may be presentup to the hardware contraints of the bus. Generally, the busis limited to 15 devices, but this number may be reduced ifhigher than normal data transfer rates are required or if longerthan normal cables are used.
Several devices may be commanded to listen at once, butonly one device may be a talker at any given time* Otherwise,communications would be scrambled much like an individualtrying to pick out a single conversation in a large crowd.
Before a device can talk or listen, it must be appropriately ad-dressed. Devices are selected on the basis of their primary ad-dress. To avoid confusion, the addressed device is sent a talkor listen command derived from its primary address. Normal-ly, each device on the bus has a unique primary address sothat each may be addressed individually. The primary addressof the Model 4853 interface Is set to 22 at the factory, but itmay be changed to any value between 0 and 30 as describedin Section 3.
Once the device is addressed to talk or listen, the appropriatebus transactions will take place. For example, if the Model4853 is properly addressed to talk, it will normally place itsdata string on the bus one byte at a time. The controller willthen read this information, and the appropriate software canthen be used to channel the information to the desired loca-tion. Other bus functions and instrumentation may be con-trolled by special bus commands as described in paragraph2.3.
23 lEEE-488 BUS LINES
The signal lines on the lEEE-488 bus are grouped into threegeneral categories. The data lines handle bus information,while the handshake and bus management lines ensure thatproper data transfer and bus operation takes place. Each ofthe bus lines are inverted so that low is true. The followingparagraphs describe the purpose of these lines, which areshown in Figure 2-1.
2.3.1 Bus Management Lines
The bus management group is made up of five signal linesthat send certain single-line bus commands and ensure anorderly transfer of data. These lines are used to send thesingle-Sine commands described in paragraph 2.4.1. ATN (Attention)—The attention line is one of the more im-
portant management lines. The state of the ATN linedetermines whether information on the data bus is to beconsidered data or a command as described in paragraph2.4.
2. IFC (Interface Clear! — Setting the »FC line true flow)causes the bus to go to a known state by sending the IFCcommand.
3. REN (Remote Enable) —Setting the REN line sends theREN command. This sets up instruments on the bus forremote operation.
4. EOI (End or Identify)—The EOI Sine is used to terminate amults-byte transfer sequence.
5. SRQ (Service Request)—The SRQ line is set low by a busdevice when it requires service from the controller.
TO OTHER DEVICES
DEVICE 1ABLE TO
TALK, LISTEN.AND CONTROL{COMPUTER?
I t l t t f f f
DEVICE 2ABLE TO
TALK ANDLISTEN
DEVICE 3ONLY ABLETO LISTEN(PRINTER)
N-
DEVICE 4ONLY ABLE
TO TALK
DATA BUS
DATA BYTETRANSFERCONTROL
GENERALINTERFACE
MANAGEMENT
DIGITS DATA18 LINES)
^ HANDSHAKE
-IFC )-ATN /-SRQ v BUS MANAGEMENT
i-EOI
Figure 2-1. IEEE Bus Configuration
2.3.2 Handshake Lines
The bus uses three handshake lines that operate in an in-terlocked sequence. This method ensures reliable data
transfer regardless of the transfer rate. Generally, datatransfer will occur at a rate determined by the slowest activedevice on the bus.
One of the handshake lines is controlled by the data source,while the remaining two lines are controlled by acceptingdevices. The three bus handshake lines are:1. DAV (Data Valid)—The source controls the state of the
DAV line.2. NRFD (Not Ready For Data)—The acceptor controls the
state of the NRFD line*3. NDAC (Not Data Accepted)—The acceptor also controls
the NDAC line.
The complete handshake sequence for one data byte isshown in Figure 2-2. Once data is high, indicating that alldevices on the bus are ready for data. At the same timeNDAC should be low from the previous byte transfer. If theseconditions are not met, the source must then wait until NRFDand NDAC lines have the correct status* Because of thepossibility of a bus hang up, some controllers have time-outroutines to display error messages if the handshake sequencestops for any reason.
Once the NRFD and NDAC lines are properly set, the sourcesets the DAV line low, indicating that data on the bus is nowvalid. The NRFD line then goes low; the NDAC line then goeshigh once all devices on the bus have accepted the data.Each device will release the NDAC line at its own rate, but theNDAC line will not go high until the slowest device has ac-cepted the data.
Once the NRFD and NDAC lines are properly set, the sourcesets the DAV line low, indicating that data on the bus is nowvalid. The NRFD line then goes low; the NDAC line then goeshigh once all devices on the bus have accepted the data.Each device will release the NDAC line at its own rate, but theNDAC line will not go high until the slowest device has ac-cepted the data.
After the NDAC line goes high, the source then sets the DAVline high to indicate that the data on the bus is no longervalid. At this point, the NDAC line returns to its low state*Finally, the NRFD Sine is released by each of the devices attheir own rates, until the NRFD Sine finally goes high whenthe slowest device is ready, and the bus is set to repeat thesequence with the next byte of data.
The sequence just described is used to transfer both data andmultiline commands* The state of the ATN lines determineswhether the data bus contains data or commands as de-scribed in paragraph 2.4,
2-2
DATA
DAV
NRFD
NDAC
V Y SOURCE
DATRAM
SEC
TASFEI
VALID
iDATA
1 TRANSFEREND
SOURCE
ACCEPTOR
ACCEPTOR
Figure 2-2. Handshake Sequence
2.3.3 Data Lines
The IEEE-488 bus uses the eight data lines that allow data tobe transmitted and received in a bit-parallel, byte-serial man-ner. These eight lines use the convention DIO1 through DIO8instead on the usual DO through D7 binary terminology. Thedata lines are bidirectional and, as with the remaining bussignal lines, low is true.
2.4 BUS COMMANDS
While the hardware aspects of the bus are important, the in-terface would be worthless without appropriate commandsto control communications between the various instrumentson the bus. This section will briefly describe the purpose of
the bus commands which are grouped into the followingthree general categories:1, Uniline commands: Sent by setting the associatd bus line
low.2, Multiline commands: General bus commands which are
sent with the ATN Sine low.3, Device-Dependent commands: Special commands that
depend on device configurations; sent with ATN high.
These commands are summarized In Table 2-1, Only com-mands that affect Model 485 operation are covered in thissection.
2.4=1 Uniiine Commands
Uniline commands are sent by setting the associated bus linelow. The ATN, IFC and REN commands are sent only by thesystem controller. The SRQ command is asserted by an ex-ternal device. The EOI command may be sent by either thecontroller or an external device. The following ss a briefdescription of each command:1. REN (Remote Enable)—When the controller sends the
REN command, the instrument will be set up for remoteoperation before attempting to program over the bus.
2e EDI—The EOI command is transmitted by setting the EO!iEnd Or Identify) line low during the last byte of a multi-byte transfer sequence.
3. IFC (Interface Clear)—The IFC command is sent by settingthe IFC line low; it sets the bus to a known state,
4, ATN (Attention)—The controller sets ATN low whensending multiline commands. Device-dependent com-mands are sent with ATN high. The ATN line must remainhigh while a device transmits its data stringa
Table 2-1. IEEE-488 Bus Command Summary
Command TypeUniline
MultilineUniversal
Addressed
Unaddress
Device-dependent**
CommandREN (Remote Enable)EOI (End or Identify)SFC (Interface Clear)ATN (Attention)SRQ (Service Request)
DCL (Device Clear)SPE (Serial Poll Enable)SPD (Serial Poll Disable)SDC (Selective Device Clear)GTL (Go to Local)GET (Group Execute Trigger)UNL (Unlisten)UNT (Utalk)
State ofATN Line*
XXX
LowX
LowLowLowLowLowLowLowLowHigh
CommentsSet up for remote operation.Sent by setting EOI low.Clears InterfaceDefines data bus contents.Controlled by external device. J
Returns device to default conditions.Enables serial polling.Disables serial polling.Returns unit to default conditions.Returns to local controlTriggers device for reading.Removes all listeners from bus.Removes all talkers from bus*
Programs Model 485 for various modes.
*X = Don't Care*See Section 3 for complete description.
5. SRQ (Service Request)™ I he SRQ line is set low by an ex-ternal device when it requires service from the controller. Aserial polling sequence, as described in paragraph 3.4.7,must be used to determine which device has requestedservice.
2.4.2 Universal Commands (Multiline)
Universal commands are multiline commands that require noaddressing. All instrumentation equipped to implement thecommand will do so when the command is transmitted overthe bus. As with all multiline commands, the universal com-mands are sent with ATM low.1. DCL (Device Clear)—After a DCL is sent, instrumentation
equipped to implemented the command will revert to someknown state.
2. SPE (Serial Poll Enable)-The SPE command is the firststep in the serial polling sequence, which is used to deter-mine which instrument has requested service.
3. SPD (Serial Poll Disable)-The SPD command is sent bythe controller to remove all instrumentation on the busfrom the serial poll mode. The Model 485 will no longerplace its status byte on the bus when addressed to talkafter the SPD command is sent.
2.4.3 Addressed Commands
Each of these commands must be preceded by a listen com-mand derived from the device's primary address before theinstrument will respond. Only the addressed device will re-spond to each of these commands:1. SDC (Selective Device Clear)—The SDC command per-
forms essentially the same function as the DCL commandexcept that only the addressed device will respond.
2. GTL (Go To Local)—The GTL command is used to remove-instruments from the remote mode of-operation.
3. GET (Group Execute Trigger) —The GET command is usedto trigger devices to perform some action that depends ondevice configuration.
2=4.4 Unaddressed Commands
The two unaddressed commands are used by the controllerto remove all talkers and listeners from the bus simultaneous-ly. No addressing is required to implement these commands.1. UNL (Unlisten) —All listeners are removed from the bus at
once when the UNL command is placed on the bus.2. UNT (Untalk)—The controller sends the UNT command to
clear the bus of any talkers.
2.4.5 Dewice=Bependent Commands
The meaning of the device-dependent commands is deter-mined by instrument configuration. Generally, these com-mands are sent as one or more ASCII characters that tell thedevice to perform a specific function. For complete informa-tion on using these commands with the Model 485, refer toSection 3. The IEEE-488 bus treats device-dependent com-mands as data in that the ATN line is high when the com-mands are transmitted.
2.5 COMMAND CODES
Each bus command gives a unique code that is transmittedover the bus as 7 bit ASCII data. This section will briefly ex-plain the code groups which are summarized in Figure 2-3,Every command is sent with ATN low.1. Addressed Command Group (AC6I—Addressed com-
mands are listed in column 0(B) in the table. Column 0(A)lists the corresponding ASCII codes.
2. Universal Command Group (UCG) —Columns 1(A) andKB) list the Universal commands and the correspondingASCII codes.
3. Listen Addressed Group (LAGI™CoIumns 2SA) and 3(A)list the ASCI! codes corresponding to the primary addresslisted in columns 2(B) and 3(B). For example, if the primaryaddress of the instrument is set to eight, the LAG byte willcorrespond to the ASCII " ( " character.
4. Talk Address Group (TAGS—TAG primary address valuesand the corresponding ASCII characters are listed in col-umns 4(A) through 5(B).
The preceding address groups are all grouped together toform the Primary Command Group (PCG). The bus also hasanother group of commands, called the Secondary Com-mand Group (SCG). These are listed in Figure 2-3 for infor-mational purposes only; the Model 4853 does not respond tothese commands, but other devices may have secondary ad-dressing capability.
Commands are normally transmitted with the 7bit code listed in the table. For most devices, thecondition of D7 (DIO8) is unimportant, as shownby the "Don't Care" indication in the table.Some devices, however, may require that D7
assumes a specific logic state before the com-inands are recognized.
Hexadecimal and decimal values for each of the commandsor command groups are listed in Table 2-2e Each value in thetable assumes that D7 is set to 0.
2-4
Table 2-2* Hexadecimal and Decimal Command Codes
CommandGTLSDCGETDCLSPESPDLAGTAGUNLUNT
Hex Valye*01040814
• 1 819
2Q-3F4Q-5F
3F5F
Decimal Value
148
202425
32-6384-95
6395
^Values shown with Dj = 0
2.6 COMMAND SEQUENCE
The proper command sequence must be sent by the con-troller before an instrument will respond as intended. Theuniversal commands, such as DCL require only that ATN beset low before the command is sent Other commands re-quire that the device be addressed to listen first. This sectionwill briefly describe the bus sequence for several types ofcommands.
2.6.1 Addressed Command Sequence
Before a device will respond to one of these commands, itmust receive a LAG command derived from its primary ad-
dress* Table 2-3 shows a typical sequence for the SDC com-mand . The LAG command assumes that the Instrument is setat a primary address of 22.
Note that an UNL command is transmitted before the LAGSDC sequence. This is generally done to remove all otherlisteners from the bus first so that only the addressed deviceresponds.
2.6.2 Universal Command Sequence
The universal commands are sent by setting ATN low andthen placing the command on the bus. For example, thefollowing is placed on the bus to give the DCL command:
ATN*DCLNote that both the ATN and DCL commands are on the bussimultaneously. Also, addressing is not necessary.
2.6.3 Device-Dependent Command Sequence
The device-dependent commands are transmitted with ATNhigh. However, the device must be addressed to listen firstbefore the commands are transmitted. Table 2-4 shows thecommand sequence for the following:
D1XThis command, which sets the Model 485 to the LOG func-tion, is described in detail in Section 38
Table 2-3. Typical Addressed Command Sequence
Step
1234
Command
UNLLAG*SDC
ATN State
Set SowStays lowStays lowReturns high
Data BusASCII
?6
EOT
Hex
3F3604
Decimal
63544
* Assumes primary address = 22.
Table 2-4. Typical Device-Dependent Command Sequence
Step
12345
Command
UNLLAG*DataDataData
ATN State
Set lowStays lowSet highStays highStays high
Data BysASCII
?6F0X
Hex
3F36463058
Decimal
6354704888
* Assumes primary address = 22.
2-6
\D7
\D 6
Bits N J ^X \ D 4
D3
1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
D2
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
D1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
D0
1
0
1
0
1
0
0
0
1
0
1
0
1
0
1
0
1
COLUMN-
ROWi
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
X0
00
0 (A)
NUL
SOH
STX
ETX
EOT
ENQ
ACK
BEL
BS
HT
LF
VT
FF
CR
SO
SI
COMMAND
0 (B)
GTL
SDC
PPC4
GET
TCT*
X0
01
1 (A)
DLE
DC1
DC2
DC3
DC4
NAK
SYN
ETB
CAN
EM
SUB
ESC
FS
GS
RS
US
COMMAND
1 (B)
LLO
DCL
PPU*
SPE
SPD
X0
10
2 (A)
SP
!
tt
$
%
&
(
)
•
/
PRIMARYADDRESS
2 (B)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
X0
11
3 (A)
0
1
2
3
4
5
6
7
8
9
<
=
>
?
PRIMARYADDRESS
3 (B)
16 I
17
18
19
20
21
22
23
24
25
26
27
28
29
30
UNL
X1
0
4 (A)
@
A
B
C
D
E
F
G
H
I
J
K
L
M
N
0
PRIMARYADDRESS
4(B)
0
1
2
4
5
6
7
8
9
10
11
12
13
14
15
X1
01
5 (A)
P
Q
R
S
T
uV
w
X
Y
z[
\
]
A
_
PRIMARYADDRESS
5(B)
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
UNT
X1
10
6 (A)
a
b
c
d
e
f
g
h
i
j
k
i
m
n
0
6(B)
x™ "
1
1
1
7 (A)
P
q
r
s
t
u
V
w
X
y
z
{
I
-DEL
7(B)
y vADDRESSEDCOMMAND
GROUP(ACG)
A /UNIVERSALCOMMAND
GROUP(UCG)
VLISTEN
ADDRESSGROUP(LAG)
VTALK
ADDRESSGROUP(TAG)
y
v y v y*PPC (Parallel Poll Configure), PPU (Parallel Pull Unconfigure),
LLO (Local Lockout), and TCT (Take Control) not implemented by Model 4853.
NOTE; Do - DIOL..D7 = DIO8X = Don't Care
PRIMARYCOMMAND
GROUP(PCG)
SECONDARYCOMMAND
GROUP(SDC)
Figure 2-3* Command Codes
SECTION 3
3.1 INTRODUCTION
The Model 4853 is designed to interface the Model 485 tothe iEEE-488 bus. The current ranges, and the zero check,LOG and REL features are controlled by programming com-mands over the SEEE-488 bus.
This section deals with important hardware and softwareaspects of bus operation and describes important program-ming functions in detail. Included are: general bus com-mands, device-dependent commands, status word andstatus byte, and other important operating information.
3.2 HARDWARE CONSIDERATIONS
Before the Model 485 can be used with the IEEE-488 bus, theinstrument must be connected to the bus using a suitableconnector. Also, the instrument must be set up for ad-dressable operation and the primary address must be properlyselected as described in this section.
NOTEAllow the Model 485/4853 to warm up (poweron) for one hour before using,
3.2.1 Typical Controlled Systems
The IEEE-488 bus is a parallel system. As a result, adding
more devices is simply a matter of using more cables to makethe desired connections. Because of this flexibility, systemcomplexity can range from the very simple to extremely com-plex.
Figure 3-1 shows two typical system configurations. Figure3-1 (a) shows the simplest possible controlled system* Thecontroller is used to send commands to the instrument,which sends data back to the controller.
The system becomes more complex in Figure 3-Kb), whereadditional instrumentation is added. Depending on program-ming, all data may be routed through the controller, or it maybe transmitted directly from one instrument to another.
For very complex applications, a much larger computer canbe used. Tape drives or disks can then be used to store data*
3.2.2 Bus Connections
The Model 485 is connected to the bus through an iEEE-488connector which is shown in Figure 3-2. This connector isdesigned to be stacked to allow a number of parallel connec-tions on one instrument
NOTETo avoid possible mechanical damage, it Isrecommended that no more than three connec-
MODEL 485
(A) SIMPLE SYSTESV!CONTROLLER
O O
CONTROLLER
INSTRUMENTINSTRUHiEWT
oI
o o 0I
IB! ADDITIONAL INSTRUMENTATION
tors be stacked on any one instrument. Other-wise, the resulting strain may cause internaldamage.
assignments for the various bus lines, while Figure 3-4 showscontact designations. Contact 18 through 24 are return linesfor the indicated signal lines, and the cable shield is con-nected to contact 12. Each ground line is connected to digitalcommon in the Model 485.
Table 3-1. f iEEE Contact Designations
Figure 3-2. IEEE-488 Connector
A typical connecting scheme for the bus is shown in Figure3-3. Each cable normally has the standard IEEE connector oneach end. The Keithley Model 7008-3 or 7008-6 cable is idealfor this purpose. Once the connections are made, the screwsshould be tightened securely. The connector is located on therear panel of the Model 485.
NOTEThe IEEE-488 bus is limited to a maximum of 15devices, including the controller. Also, the maxi-mum cable length is 20 meters. Failure toobserve these limits will probably result in erraticbus operation.
Custom cables may be constructed using the information inTable 3-1 and Figure 3-4. Table 3-1. lists the contact
i ContactI Number
! 1I 2! 3! 4! 5i 6) 7i 8! 3! 10i 11\ 12I 13! 14I 15I 16I 17I 18
19( 20 !
21222324
IEEE-488Designat ion
D1O1DIO2
! DIO31 DIO41 EOS (24)*
DAVNRFDNDAC
IFCSRQATM
SHIELDDIO5DIO6DIO7DS08
REN (24)*Gnd, (6)*Gnd, (7)*Gnd, (8)*Gnd, (9)*
Gnd, (10)*Gnd, (11)*
Gnd, LOGIC
Type
DataDataDataData
j Management [Handshake |
I Handshake jHandshake j
ManagementManagement jManagement j
GroundData |DataData [Data
Management jGroundGroundGroundGroundGroundGroundGround
*Numbers in parentheses refer to signal groundreturn of reference contact number. EOI andREN signal lines return on contact 24=
INSTRUMENT INSTRUMENT 1NSTRUMEWT
Figure 3-3, IEEE-488 Connect ions
3-2
CONTACT 13 COWTACT,
CONTACT 1 CONTACT 12
Figure 3-4. Contact Assignments
CAUTIONThe voltage between IEEE common andground must not exceed 30V or damage tothe instrument may occur.
A typical signal line bus driver is shown in Figure 3-5. Withthe configuration shown, the driver has bidirectional capabili-ty. When the I/O control line is high, the line is configured asan output line. When the control Sine is Sow, the driver is setup for input operation, Note that not all signal lines havebidirectional capability. Some lines, such as ATM, will alwaysbe configured as an output line in the controller and as an in-put line for all other devices on the bus.
LINE
I/O >*CONTROL
• • <
c
" \
O OUT°UT
Figure 3-5, Typical IEEE-488 Bus Driver lone of 161
1.2.3 Addressable Mode Selection
"he Model 4853 must be set to the addressable mode whensed with an external controller. Mode selection is done withle TO/ADDRESSABLE mode selection switch located onie rear panel of the Model 485= This switch is grouped withlose that set the primary address; these switches are locatedn the rear panel of the Model 485* Figure 3-6 shows theADDRESSABLE position for the switch.
he TO/ADDRESSABLE switch is read only upon power-up,the mode is changed, the Model 486 must be turned off
and then powered-up again before it will recognize the newswitch position.
TALKER OWLY(TO) MODE
1 A
MODE
TO/ADDRESSABLE and Primary AddressSwitches {Factory Set Address 22 Shown)
Figure ;
3.2,4 Primary Address Selection
The Model 485 must receive a listen command before it willrespond to the addressed or device-dependent commandssent over the bus. Similarly, a talk command must be sent tothe Model 485 before it will transmit its data string, statusword, or status byte. Those talk and listen commands arederived from the primary address of the Instrumet Theprimary address of the Model 4353 is set to 22 at the factory,but it may be set to any value between 0 and 30 by placingthe primary address switches which are shown in Figure 3-6,in the desired position. The primary address specified in thecontroller's programming language must agree with theprimary address of the Model 4853=
NOTEThe primary address switch positions are readonly upon power-up. If the address is changedthe Model 485 must be turned off and thenpowered-up again before the new address canbe used*
Figure 3-6 shows the correct positions for the factory setvalue of 22, if a different address is required, the primary ad»dress may be changed as outlined in Table 3-2*
NOTEIf other instrumentation is also connected to thebus, be sure that each device has a differentprimary address. If this precaution is not ob-served, erratic bus operation may result.
The primary address switches are binary weighted; A1 is theleast significant bit, while A5 the most significant bit. For ex-ample, the binary value for the factory set primary address ofeight is 01000. Use the tip of a pen or pencil to operate theswitches.
NOTENo instrument on the bus (including the Model485) should be operated with a primary address