National Instruments GPIB-SBX User manual

© Copyright 1983, 1996 National Instruments Corporation.
All Rights Reserved.
GPIB-SBX
User Manual
January 1996 Edition
Part Number 320015-01

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Limited Warranty
The GPIB-SBX is warranted against defects in materials and workmanship for a period of two years from the date of
shipment, as evidenced by receipts or other documentation. National Instruments will, at its option, repair or replace
equipment that proves to be defective during the warranty period. This warranty includes parts and labor.
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shipping costs of returning to the owner parts which are covered by warranty.
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© National Instruments Corporation v GPIB-SBX User Manual
Preface
This manual contains information you need to install, configure, and use the GPIB-SBX
interface board. Included in this manual are descriptions of the interface registers, programming
considerations, diagnostic tests, and sample programs.
Organization of the GPIB-SBX User Manual
The GPIB-SBX User Manual is organized as follows:
• Chapter 1, Introduction, contains a brief description of the GPIB-SBX kit, a list of equipment
supplied, and a list of optional equipment.
• Chapter 2, General Description, contains the physical and electrical specifications for the
GPIB-SBX and describes the characteristics of key interface board components.
• Chapter 3, Configuration and Installation, describes the steps needed to configure and install
the GPIB-SBX hardware.
• Chapter 4, Register Descriptions, contains detailed descriptions of the interface registers as
well as summary tables for easy reference.
• Chapter 5, Programming Considerations, explains important considerations for persons
programming the GPIB-SBX.
• Chapter 6, Theory of Operation, contains a detailed technical discussion of the major
elements of the GPIB-SBX.
• Chapter 7, Diagnostic and Troubleshooting Test Procedures, contains test procedures for
determining if the GPIB-SBX is installed properly and operating correctly.
• Appendix A, Parts List and Schematic Diagram, includes a parts list and detailed schematic
diagram.
• Appendix B, Sample Programs, provides sample programs in 8080 and 8088 Assembly
language code for implementing the most commonly used GPIB functions. Line-by-line
comments provide an explanation of each function.
• Appendix C, Multiline Interface Messages, contains a listing of the multiline GPIB interface
messages.
• Appendix D, Mnemonics Key, contains an alphabetical listing of all mnemonics used in this
manual and indicates whether the mnemonic represents a bit, register, function, remote
message, local message, or a state.
• The Index contains an alphabetical list of key terms and topics used in this manual, including
the page where each one can be found.

Preface
GPIB-SBX User Manual vi © National Instruments Corporation
Terminology
The term set is used throughout this manual to mean writing a value of 1 to a memory element.
The term clear is used to mean writing a value of 0 to a memory element. The terms set to zero,
set false, preset, and reset are avoided in preference to the use of set and clear. Bit signatures are
written in upper case letters. An asterisk (*) after a bit signature indicates the signal is active
low. An asterisk is equivalent to an overbar. For example, if bit ATN*=0, the GPIB ATN signal
line is asserted. After the mnemonic of a register name is introduced and the name written out,
the mnemonic is used thereafter.
Where it is necessary to specify a particular bit of a register the bit position appears as a decimal
number in square brackets after the mnemonic (for example, ISR1[2] indicates the ERR bit of
Interrupt Status Register 1).
Uppercase mnemonics are used for control, status, data registers, register contents, and interface
functions, as well as GPIB remote messages, commands, and logic states as defined in the IEEE-
488 standard.
The term addressed means the interface has been configured to perform a function from the
GPIB side, while the term programmed means that it has been configured from the SBX bus
side. This distinction is important to make because many functions, such as making the interface
a Talker or Listener, can be activated from either side.
In logical expressions a minus sign (-) is used to indicate logical negation, an ampersand (&)
represents AND, and a plus sign (+) represents OR. In some cases, a plus sign is used to indicate
addition, such as I/O addresses for interface registers in Chapter 4.
All numbers are decimal unless specified otherwise. Register offsets are given in hexadecimal.
Abbreviations
The following abbreviations are used in the text of this manual.
≥greater than or equal to
< less than
A ampere
hex hexadecimal
in. inch
kbytes/sec 1,000 bytes per second
K 1,024 bytes of memory
m meter
µA microampere
µsec microsecond
mA milliampere
MHz megahertz
nsec nanosecond
V volt
VDC volts direct current

Preface
© National Instruments Corporation vii GPIB-SBX User Manual
Related Documents
The following documents contains information that you may find helpful as you read this
manual:
•IEEE Standard Digital Interface for Programmable Instrumentation, ANSI/IEEE Standard
488.1-1987
• µ PD7210 GPIB-IFC User's Manual, NEC Electronics USA, Inc., One Natick Executive
Park, Natick, MA 01760
• µ PD7210 Intelligent GPIB Interface Controller, Engineering Data Sheet, NEC Electronics
USA, Inc., Microcomputer Division
•How to Interface a Microcomputer System to a GPIB, (& The NEC µPD7210 TLC), NEC
Electronics USA, Inc.
•Intel iSBX Bus Specification, (manual order number 142686-002) Intel Corporation, 3065
Bowers Ave., Santa Clara, CA 95051
•Designing iSBX Multimodule Boards, Stephen Grubb, OEM Microcomputer Systems
Applications, Intel Corporation
Customer Communication
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hours of 8:00 a.m. and 5:30 p.m. (central time) to reach the National Instruments applications
engineering department:
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(800) IEEE-488 (toll-free U.S. and Canada)

© National Instruments Corporation viii GPIB-SBX User Manual
Contents
Chapter 1
Introduction .........................................................................................................................1-1
GPIB-SBX Interface Kit Description ...........................................................................1-1
What Your Kit Should Contain .....................................................................................1-2
Optional Equipment ......................................................................................................1-2
Chapter 2
General Description...........................................................................................................2-1
GPIB-SBX Specifications..............................................................................................2-1
Physical Specifications......................................................................................2-1
Electrical Specifications.....................................................................................2-1
SBX Interface Bus Signals.............................................................................................2-3
Control Lines......................................................................................................2-5
Command Lines (IORD*, IOWRT*).....................................................2-5
DMA Lines (MDRQT, MDACK*, TDMA)..........................................2-5
Initialize Line (MRESET)......................................................................2-5
Clock Line (MCLK)...............................................................................2-5
System Control Lines (MWAIT*, MPST*)...........................................2-5
Address and Chip Select Lines (MA2-MA0, MCS1*-MCS0*)........................2-5
Data Lines (MD7-MD0)....................................................................................2-6
Interrupt Line (MINTR1-MINTR0)...................................................................2-6
Power and Ground Lines (+5 V, +-12 V, GND)................................................2-6
Data Transfer Modes and Performance.........................................................................2-6
Description of the General Purpose Interface Bus (GPIB)............................................2-7
GPIB Operation..................................................................................................2-7
GPIB Signal Lines.............................................................................................2-8
GPIB Physical Characteristics...........................................................................2-9
GPIB-SBX Functional Description................................................................................2-10
Chapter 3
Configuration and Installation.......................................................................................3-1
Configuration.................................................................................................................3-1
Installation......................................................................................................................3-1
Cabling...........................................................................................................................3-3
Verification Testing.......................................................................................................3-4
Chapter 4
Register Descriptions.........................................................................................................4-1
Introduction....................................................................................................................4-1
Terminology...................................................................................................................4-1
Interface Registers..........................................................................................................4-2
Data In Register (DIR).......................................................................................4-5
Command/Data Out Register (CDOR)..............................................................4-6
Interrupt Status Register 1 (ISR1)......................................................................4-7
Interrupt Mask Register 1 (IMR1).....................................................................4-7

Contents
GPIB-SBX User Manual ix© National Instruments Corporation
Interrupt Status Register 2 (ISR2)......................................................................4-13
Interrupt Mask Register 2 (IMR2).....................................................................4-13
Serial Poll Status Register (SPSR).....................................................................4-17
Serial Poll Mode Register (SPMR)....................................................................4-17
Address Status Register (ADSR).......................................................................4-18
Address Mode Register (ADMR)......................................................................4-20
Command Pass Through Register (CPTR)........................................................4-23
Auxiliary Mode Register (AUXMR).................................................................4-24
Hidden Registers................................................................................................4-31
Internal Counter Register (ICR).............................................................4-31
Parallel Poll Register (PPR)...................................................................4-32
Auxiliary Register A (AUXRA)............................................................4-34
Auxiliary Register B (AUXRB).............................................................4-36
Auxiliary Register E (AUXRE).............................................................4-38
Address Register 0 (ADR0)...............................................................................4-39
Address Register (ADR)....................................................................................4-40
Address Register 1 (ADR1)...............................................................................4-41
End Of String Register (EOSR).........................................................................4-43
Chapter 5
Programming Considerations........................................................................................5-1
Initialization...................................................................................................................5-1
The GPIB-SBX as GPIB Controller..............................................................................5-2
Becoming Controller-In-Charge (CIC) and Active Controller..........................5-2
Sending Remote Multiline Messages (Commands)...........................................5-3
Going from Active to Standby Controller..........................................................5-3
Going from Standby to Active Controller..........................................................5-4
Going from Active to Idle Controller................................................................5-4
The GPIB-SBX as GPIB Talker and Listener...............................................................5-5
Programmed Implementation of Talker and Listener........................................5-5
Addressed Implementation of Talker and Listener............................................5-5
Address Mode 1.....................................................................................5-5
Address Mode 2.....................................................................................5-5
Address Mode 3.....................................................................................5-6
Sending/Receiving Messages.........................................................................................5-7
Sending and Receiving Data (Programmed I/O Mode).....................................5-7
Sending and Receiving Data (DMA Mode).......................................................5-7
Sending END or EOS........................................................................................5-8
Terminating on END or EOS.............................................................................5-8
Serial Polls.....................................................................................................................5-8
Conducting Serial Polls......................................................................................5-8
Responding to a Serial Poll................................................................................5-8
Parallel Polls..................................................................................................................5-9
Conducting a Parallel Poll..................................................................................5-9
Responding to a Parallel Poll.............................................................................5-10
Interrupts........................................................................................................................5-11

Contents
GPIB-SBX User Manual x© National Instruments Corporation
Chapter 6
Theory of Operation..........................................................................................................6-1
SBX Bus Interface.........................................................................................................6-1
GPIB Talker/Listener/Controller (TLC)........................................................................6-2
GPIB Interface...............................................................................................................6-3
Chapter 7
Diagnostic and Troubleshooting Test Procedures..................................................7-1
Interpreting Test Procedures..........................................................................................7-1
GPIB-SBX Hardware Installation Tests........................................................................7-2
Appendix A
Parts Listand SchematicDiagram .............................................................................A-1
Appendix B
SamplePrograms................................................................................................................B-1
Appendix C
MultilineInterfaceMessages..........................................................................................C-1
Appendix D
MnemonicsKey...................................................................................................................D-1
Index..................................................................................................................................Index-1

Contents
GPIB-SBX User Manual xi© National Instruments Corporation
Figures
Figure1-1. GPIB-SBX Interface Board .............................................................................1-1
Figure2-1. GPIB-SBX with a Multibus Computer............................................................2-11
Figure2-2. GPIB-SBX in a Multiprocessor Application....................................................2-12
Figure2-3. GPIB-SBX Block Diagram..............................................................................2-13
Figure3-1. GPIB Cable Shield Grounding.........................................................................3-1
Figure3-2. GPIB-SBX Installed on Host Board.................................................................3-2
Figure3-3. GPIB Cable Connector.....................................................................................3-3
Figure3-4. GPIB-SBX Cable-to-Interface Board Connection...........................................3-4
Figure4-1. GPIB-SBX Interface Registers.........................................................................4-3
Figure4-2. Writing to the Hidden Registers.......................................................................4-4
Tables
Table2-1. Electrical Connections of the GPIB-SBX Interface.........................................2-1
Table2-2. Loading on Incoming SBX Signal Lines.........................................................2-2
Table2-3. Drive Capability of Output SBX Signal Lines.................................................2-2
Table2-4. GPIB-SBX Connector Pin Assignments..........................................................2-3
Table2-5. GPIB-SBX Internal Registers..........................................................................2-6
Table2-6. GPIB-SBX IEEE-488 Interface Capabilities...................................................2-14
Table4-1. Clues to Understanding Mnemonics................................................................4-1
Table4-2. Auxiliary Command Summary........................................................................4-25
Table4-3. Auxiliary Commands: Detail Description........................................................4-26
Table4-4. Examples for Configuring the PPR..................................................................4-33

Introduction Chapter 1
GPIB-SBX User Manual 1-2 © National Instruments Corporation
What Your Kit Should Contain
Your kit should contain the following components:
Component Part Number
GPIB-SBX Interface Board 179770-01
20-in. Interface Cable (chassis mount) 179753-01
GPIB-SBX User Manual 320015-01
Make sure each of these items is in your kit. If any item is missing, contact National
Instruments.
Optional Equipment
Equipment Part Number
Interface Cable Assembly:
2 m 179754-02
4 m 179754-04
Single-Shielded Cable:
GPIB Type X1 Cable - 1 m 763001-01
GPIB Type X1 Cable - 2 m 763001-02
GPIB Type X1 Cable - 3 m 763001-03

© National Instruments Corporation 2-1 GPIB-SBX User Manual
Chapter 2
General Description
This chapter contains the physical and electrical specifications for the GPIB-SBX and describes
the characteristics of key interface board components.
GPIB-SBX Specifications
The physical and electrical specifications are described in the following sections.
Physical Specifications
The GPIB-SBX measures 2.85 in. by 3.7 in. and is supplied with a 36-pin plug and spacing studs
for mounting on the host board. A 26-pin male header connects to the GPIB adapter cable.
Electrical Specifications
Three major components of the GPIB-SBX interface connect directly to the SBX bus and the
GPIB (via the cable supplied), as described in Table 2-1.
Table 2-1. Electrical Connections of the GPIB-SBX Interface
Component Purpose Connected Directly to
µPD7210 Talker/Listener Controller SBX
75160A Data Line Transceivers GPIB
75162A Bus Management and GPIB
Handshake Line Transceivers
The GPIB transceivers meet the requirements of the IEEE-488 standard. The GPIB-SBX
requires a regulated +5 VDC power from the SBX bus. Current load is typically 0.5 A (1 A
maximum).
All integrated circuits used on the GPIB-SBX meet the requirements of the SBX specification.
Table 2-2 shows the loading on incoming SBX signal lines.

General Description Chapter 2
GPIB-SBX User Manual 2-2 © National Instruments Corporation
Table 2-2. Loading on Incoming SBX Signal Lines
Signal Loading
MD7-MD0
MA2-MA0
MCS0 1 NMOS plus 1 or 2 ALSTTL
RESET Iih < 50 uA
Iil < -0.2 mA
IORD*
IOWRT*
MCLK
MDACK* 3 ASTTL plus 13K ohm pull-up
Table 2-3 shows the drive capability of output SBX signal lines.
Table 2-3. Drive Capability of Output SBX Signal Lines
Signal Loading
MD7-MD0
MINTR0 2 mA @ 0.45 V, -0.4 mA @ 2.4 V
MDRQT
MWAIT 8 mA @ 0.35 V, -0.4 mA @ 3.4 V
Additional circuitry permits the GPIB-SBX to automatically become System Controller or give
up control automatically when the GPIB-SBX receives the Take Control or Release Control
auxiliary commands. No special software support or hardware setting are necessary to make the
GPIB-SBX System Controller or not System Controller.

Chapter 2 General Description
© National Instruments Corporation 2-3 GPIB-SBX User Manual
SBX Interface Bus Signals
The 36 SBX interface bus signals are grouped into six functional classes as follows:
• 9 Control Lines
• 5 Address and Chip Select Lines
• 8 to 16 Data Lines
• 2 Interrupt Lines
• 2 Option Lines
• 2 Power Lines
The following paragraphs describe the signals used by the GPIB-SBX. The GPIB-SBX uses 29
of the available 36 SBX bus signals. Table 2-4 lists all 36 signals and indicates which ones are
used and which ones are not used.
Table 2-4. GPIB-SBX Connector Pin Assignments
Used/
Pin Mnemonic Description Not Used
1 +12V +12 Volts Not Used
2 -12V -12 Volts Not Used
3 GND Signal Ground Used
4 +5V +5 Volts Used
5 MRESET Reset Used
6 MCLK M Clock Used
7 MA2 M Address 2 Used
8 MPST* M Present Used
9 MA1 M Address 1 Used
10 Reserved Not Used
11 MA0 M Address 0 Used
12 MINTR1 M Interrupt 1 Grounded
13 IOWRT* I/O Write Command Used
(continues)

General Description Chapter 2
GPIB-SBX User Manual 2-4 © National Instruments Corporation
Table 2-4. GPIB-SBX Connector Pin Assignments (continued)
Used/
Pin Mnemonic Description Not Used
14 MINTR0 M Interrupt 0 Used
15 IORD* I/O Read Command Used
16 MWAIT* M Wait Used
17 GND Signal Ground Used
18 +5V +5 Volts Used
19 MD7 M Data Bit 7 Used
20 MCS1* M Chip Select 1 Not Used
21 MD6 M Data Bit 6 Used
22 MCS0* M Chip Select 0 Used
23 MD5 M Data Bit 5 Used
24 Reserved Not Used
25 MD4 M Data Bit 4 Used
26 TDMA Terminate DMA Grounded
27 MD3 M Data Bit 3 Used
28 OPT1 Option 1 Not Used
29 MD2 M Data Bit 2 Used
30 OPT0 Option 0 Not Used
31 MD1 M Data Bit 1 Used
32 MDACK* M DMA Acknowledge Used
33 MD0 M Data Bit 0 Used
34 MDRQT M Request Used
35 GND Signal Ground Used
36 +5V +5 Volts Used

Chapter 2 General Description
© National Instruments Corporation 2-5 GPIB-SBX User Manual
Control Lines
The control lines of the SBX bus determine when events occur and are described in the following
paragraphs.
Command Lines (IORD*, IOWRT*)
The command lines are active-low signals that determine the direction of data, that is, from the
GPIB-SBX to the host board (read), or from the host board to the GPIB-SBX (write). When
either of these is active, the address lines are valid.
DMA Lines (MDRQT, MDACK*, TDMA)
The DMA lines are used to coordinate DMA transfers using the DMA Controller function of the
host board. MDRQT is an active-high output signal used by the GPIB-SBX to request a DMA
cycle. MDACK* is an active-low signal from the host board to the GPIB-SBX acknowledging
that the DMA cycle has been granted. TDMA is an active-high signal from the GPIB-SBX to
the host board used to terminate a DMA cycle. TDMA is tied to ground so that it does not float
to a true state.
Initialize Line (MRESET)
MRESET is an active-high signal used by the host board to place the GPIB-SBX in a known
quiescent state.
Clock Line (MCLK)
MCLK is a 10-MHz timing signal provided by the host board to the GPIB-SBX.
System Control Lines (MWAIT*, MPST*)
The system control signal lines are output signals from the GPIB-SBX to synchronize the
transfer of data across the SBX bus and indicate its presence on the SBX bus.
Address and Chip Select Lines (MA2-MA0, MCS1*-MCS0*)
The host board generates the address and chip select signals. The signals are generally derived
from a subset of the address bus lines of the computer bus. The GPIB-SBX requires the three
address lines and MCS0* for selecting the eight interface registers.
The address lines select one of the eight interface registers of the GPIB-SBX.
The GPIB-SBX decodes eight consecutive I/O mapped interface register addresses, as shown in
Table 2-5.

General Description Chapter 2
GPIB-SBX User Manual 2-6 © National Instruments Corporation
Table 2-5. GPIB-SBX Internal Registers
Value of
MA2-MA0 Read Registers Write Registers
000 Data In (DIR) Control/Data Out (CDOR)
001 Interrupt Status 1 (ISR1) Interrupt Mask 1 (IMR1)
010 Interrupt Status 2 (ISR2) Interrupt Mask 2 (IMR2)
011 Serial Poll Status (SPSR) Serial Poll Mode (SPMR)
100 Address Status (ADSR) Address Mode (ADMR)
101 Command Pass Through (CPTR) Auxiliary Mode (AUXMR)
110 Address 0 (ADR0) Address (ADR)
111 Address 1 (ADR1) End of String (EOSR)
MCS0* is an active-low signal generated by the host board to enable communication with the
GPIB-SBX. MCS1* is not used.
Data Lines (MD7-MD0)
The MD signal lines are active high and are used to transmit data in both directions across the
SBX bus. MD7 is the most significant bit.
Interrupt Line (MINTR1-MINTR0)
MINTR0 is an active-high signal generated by the GPIB-SBX to send an interrupt to the host
board. MINTR1 is tied to ground so that it does not float to a true state.
Power and Ground Lines (+5 V, +-12 V, GND)
The GPIB-SBX requires a +5 volt power source and ground. Twelve volt power is not used.

Chapter 2 General Description
© National Instruments Corporation 2-7 GPIB-SBX User Manual
Data Transfer Modes and Performance
The GPIB-SBX is capable of operating in two modes of data transfer: Programmed I/O or Direct
Memory Access (DMA). In Programmed I/O mode, data is written to the CDOR or read from
the DIR with port output/input or memory write/read instructions, depending upon how the
address space of the GPIB-SBX is mapped. Performance in this mode is highly dependent upon
the structure of the driver software that manipulates the hardware. Using the sample code in
Appendix B with a 14-MHz Z80 microprocessor, data transfer rates up to about 30 kbytes/sec
can be achieved. If the driver is written in a high-level language and/or is embedded within an
operating system, the transfer rate usually drops significantly due to code inefficiencies and
system overhead. In DMA mode, data is transferred directly between memory and the
CDOR/DIR, using the facilities of a DMA Controller that is external to the GPIB-SBX (that is, it
is a function of the host board or the computer system). The data transfer in this mode can
exceed 500 kbytes/sec provided that the Controller, memory, and external GPIB devices are
capable of sustaining this rate.
Description of the General Purpose Interface Bus (GPIB)
The IEEE-488 GPIB provides a means for communications among a group of interconnected
devices. Two types of messages are carried by the bus:
• Interface messages, which are used for bus management, and
• Device-dependent messages, which are communicated between the various devices via the
interface bus, but are not used or processed by the bus.
The three types of devices that organize and manage the flow of information on the bus are a
Listener, a Talker, and (optionally) a Controller. A Listener has the capability of being
addressed by an interface message to receive device-dependent messages. A Talker has the
capability of being addressed by an interface message to send device-dependent messages. A
Controller can address devices to Listen or Talk. A Controller can also send interface messages
to command other specific actions within interfaced devices. A single bus can have one or more
Controllers. If more than one Controller is connected to the bus, one is designated as the System
Controller and can temporarily pass control to any other Controller.
The GPIB-SBX is capable of being a Listener, a Talker, a Controller, and, in particular, a System
Controller.
GPIB Operation
This section contains a simplified description of the operation of the GPIB. For more details,
refer to IEEE Standard Digital Interface for Programmable Instrumentation.

General Description Chapter 2
GPIB-SBX User Manual 2-8 © National Instruments Corporation
Bus operation is divided into two logical functions: interface functions (for bus management)
and device functions (for device control and communication). The interface functions, described
in this manual, are basically used to establish an environment in which device functions may be
performed. The device functions are unique to individual instruments and are beyond the
intended scope of this manual.
The Source Handshake (SH) and Acceptor Handshake (AH) interface functions are used to
transmit and receive bit-parallel, byte-serial messages (multiline messages) on the bus. These
messages are interpreted either as commands to the interface or as data bytes for the device,
depending upon the status of the bus signal Attention (ATN). ATN is asserted for interface
commands.
The Talker (T) or Extended Talker (TE) and Listener (L) or Extended Listener (LE) interface
functions are used to set up devices for data transfers. The transfers are made using the
handshake functions. When an interface has been addressed as a Talker, the applicable device
function is allowed to use the interface Source Handshake function to transmit data bytes. The
device functions of any interfaces that were addressed as Listeners must use the Acceptor
Handshake function to receive the data bytes. Only one device at a time can be the Talker,
although multiple Listeners can exist.
The Controller (C) function is the originator of all interface messages. Only one interface at a
time can be the Controller-In-Charge; protocol exists to pass the control capability among
interfaces. When the Controller-In-Charge is asserting the ATN line, it is in its active state and
is often referred to as the Active Controller. When not asserting ATN, it is in its standby state.
Any Controller that is not the Controller-In-Charge remains in its idle state. At any time, there is
at most one interface on the bus that has the capability to make itself the Controller-In-Charge. It
is referred to as the System Controller.
The Controller is responsible for addressing and unaddressing Talkers and Listeners, as well as
performing other bus management operations. These other operations include conducting a
Parallel Poll using the uniline message Identify (IDY); setting device functions in remote or local
mode using the uniline message Remote Enable (REN); and initializing the bus by asserting the
uniline message Interface Clear (IFC). The latter two operations are actually capabilities only of
the System Controller and are not transferable by the transfer of control protocol.
GPIB Signal Lines
A total of 24 signal lines are used to implement the bus. Of these lines, 16 are signal lines, one is
ground, one is the cable shield, and six are twisted pair common for six of the signal lines. The
16 signal lines are used to carry all information, interface messages, and device-dependent
messages among interconnected devices.
The 16 signal lines are organized into three sets, as follows:
• 8 data input/output lines
• 3 handshake lines
• 5 interface management signal lines