Information processing -- Processor system bus interface (Eurobus A)

Provides a processor system bus interface known as Eurobus A being one of a family of interfaces for modular data acquisition, processing communication and control systems for military, industrial and other applications.

Traitement de l'information -- Bus d'interfaces entre processeurs (Eurobus A)

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ISO 6951:1986 - Information processing -- Processor system bus interface (Eurobus A)
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ISO (the International Organization for Standardization) is a worldwide federation of

national Standards bodies (ISO member bodies). The work of preparing International

Standards is normally carried out through ISO technical committees. Esch member

body interested in a subject for which a technical committee has been established has

the right to be represented on that committee. International organizations, govern-

mental and non-governmental, in liaison with ISO, also take part in the work.

Draft International Standards adopted by the technical committees are circulated to

the member bodies for approval before their acceptance as International Standards by

the ISO Council. They are approved in accordance with ISO procedures requiring at

least 75 % approval by the member bodies voting.
International Standard ISO 6951 was prepared by Technical Committee ISO/TC 97,
Information processing s ystems.

Users should note that all’ International Standards undergo revision from time to time

and that any reference made herein to any other International Standard implies its

latest edition, unless othetwise stated.
International Organkation for Standardkation, 1986
Printed in Switzerland
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ISO 6951-1986 (El
Contents Page
0 Introduction .........................................................
1 Scope and field of application ..........................................
2 Definitions.. ........................................................ 3
3 Designation of a particular Eurobus .....................................
4 Compliance .........................................................
5 Protocols for Eurobus A ...............................................
6 Electrical and timing requirements ...................................... 11
................................... 20
A Eurobus lO/A logical implementation
................................... 20
B Eurobus 18/A logical implementation
C Eurobus 26/A logical implementation
D Eurobus 34/A logical implementation
E Connector allocation
F Examples of application of protocol rules
G Method of address allocation for mixed data widths
H Example of Eurobus backplane construction
........ 43
: Forced air convection cooled double Eurocard
J Mechanical Option 1
K Extenderpanel
L Examples of the application of Eurobus A timing requirements
M Bus receiver a.c. noise rejection
N Test circuit and waveform for determination of transient sink current
P Publications referred to
. . .
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ISO 6951-1986 (E)
Information processing - Processor System bus interface
(Eurobus A)
The full addressing capability of the bus enables devices
0. lntroduction
to address any 8-bit byte of any word in a normal address
0.1 General. This Standard specifies the set of Signal lines
space defined by both of the following.
that constitute the bus itself, and the interfacing of devices
connected to the bus. (a) The addressing range determined by the number
of data/address bits.
This Standard specifies protocols for the allocation of bus

time to devices wishing to make transfers and for the (b) A two-bit extension to the foregoing (a). The full

transfer of data between devices. The Standard does two-bit extension is available on buses with non-shared

not, however, specify priority rules, these being left to width, but on shared-width buses the use of these bits

be formulated individually for each System. is restricted.

This Standard specifies a full set of signalling rules to be In addition, any complete word tan be addressed in a

followed by the device responsible for bus allocation and second address space of equal magnitude to the first,

by devices conducting transfers. Annex F gives illustrative designated the pseudo address space.

examples of each of the possible types of transfer.
0.3 Devices. Free choice is available to the System designer

The’set of electrical and Signal timing requirements as to the devices connected to a Eurobus and the Order in

specified in clause 6 uniquely defines the interface that
which they are connected. However, each bus needs to
is Eurobus A. Certain mechanical requirements are specified include both:
in clause 6, namely those that directly affect the electrical
(a) an arbiter, the purpose of which is to control the
characteristics (e.g. the physical length of the bus,
timedivision multiplexing of transfers on the bus;
the spacing of device connectors on the bus, the pin pitch
(b) if communication with other buses is required, a
on connectors and the Signal disposition on the connectors),
bus link to each of the other buses.
but this Standard does not further specify the mechanical
Figure 1 Shows an example of a bus with a number of
implementation. An example of a possible mechanical
typical devices including an arbiter and a bus link.
implementation of Eurobus A is given in annex J.
0.4 Bus allocation. Information is transferred between
Implernentations of Eurobus A are possible with 8, 16,24,
devices on a master-and-Slave basis. A device bids for
-bit data widths and devices having different data
control of the bus by means of its starred Request line
widths tan operate on the same bus. Logical implementa-
and becomes the master device for that transfer after
tion summaries for the first four of the possible data
the arbiter has allocated the bus to it. This Standard
widths are given in annexes A to D. Annex E specifies the
specifies the protocols by which devices bid for use of
connector allocation.
the bus and by which the arbiter allocates the use of the
The group of Signal iines constituting an assembled
bus to one of them. The Standard does not, however,
Eurobus A provides the means for the transfer of binary
specify the algorithm used in making the selection,
digital information between up to 20 devices plugged into
thus the System designer is given the choice of an
the backplane of a Single equipment shelf. Devices share
allocation algorithm in Order to optimize System
the bus on a timedivision multiplex basis. The length of
the backplane is limited to a maximum of 460 mm.
The Signal lines form an asynchronous unbalanced voltage
The protocol whereby a master device may flag an
interface capable of operating at transfer rates of up to
interrupt to the arbiter is also specified, but the
6,5 x IO6 words or bytes per second.
subsequent action by the arbiter is left to the System
designer to define.
0.5 Bus transfers. In addition to specifying the protocols
0.2 Data width and addressing capability. The data/
for the execution of Read cycles (in which the master
address width of any device using the bus is theoretically
addresses a device as Slave and reads data from it) and
unconstrained. However, the asynchronous protocols
Write cycles (in which the master transfers data to the
and addressing facilities of Eurobus A permit devices of
addressed Slave), this Standard also specifies the protocol
8, 16, 24 and 32-bit data widths to share the Same bus,

and when the bus is so shared, the maximum data width for a Vector cycle in which an address, without data,

is transferred from master to Slave.
is that of the widest device.
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ISO 6951-1986 (E)
Normaily operates as As no one processor is
Master device, but tan act deemed to be the Master

as Slave when in receipt device on the bus, several Controls the transfer of data

procossors may be
of interrupts. to or from a user peripheral,
connected to the Same bus. performs DMA bus transfers
as a Master device and
receives control words as a
1 J
Allocates use of the bus to I
requesting Master devices. 1
Blocks of memory are PERIPHERAL INTERFACE Permits information to be
normally Siave devices, i.e.
CONTROLLER transferred between the two
they respond to transfers This Slave device controls
buses to which it is
initiated by other (Master) the activity of a user
devices. peripheral.
Figure 1. Eurobus with some typical devices
(c) the required characteristics of the bus transmitters
The bus allocation protocols permit a master to hold the
and receivers;
bus for repeated use without the need to make a fresh
bid for every transfer, while also giving the arbiter the
(d) the required characteristics of the spurs to be
abiiity to instruct any master to release the bus for
connected to the bus.
reallocation. A master is also permitted to retain the bus
The specified set of eiectricai characteristics presupposes
for an indivisible sequence of cycles, such as a Read-
certain bus settiing times for the transitions on the Signal
Modify-Write sequence. An additional protocoi is defined
iines. Arising from these, certain timing constraints are
whereby the arbiter may abort a cycle that is deemed to
specified. These constraints ensure that the relevant
have failed.
Signal iines will have settled to the appropriate state before
0.6 Interbus transfers. The protocols for Read, Write and
an associated control Signal transition is issued.
Vector cycles permit a master on bus A, for exampie,
0.8 Commercial and military Versions. Two Versions of
wishing to effect a transfer with a Slave on bus B,
Eurobus A are specified in this Standard, a version for a
to address a bus linker on bus A as Slave. The bus linker
commerciai temperature range (0 *C to 70 “C) and a
then bids for use of bus B as master and addresses the
Version for a military temperature range (-55 *C to
required Slave on bus B. Should master devices on both
125 “C). Where the requirements are different they are
buses attempt simultaneous transfers, the bus link cannot
separateiy specif ied for each Version.
become master on either bus and a condition of deadly
embrace ensues. The Eurobus protocois permit the
embrace to be broken simply.
The protocols used by bus links to perform interbus
1. Scope and field of application
addressing and data transfer are not within the scope
This International Standard specifies a processor System
of this Standard.
bus interface known as Eurobus A (referred to in the
0.7 Electrical requirements. The Standard specif ies the
following text as “the bus”) that is one of a family of in-
electrical and timing requirements that need to be obeyed
terfaces for use in modular data acquisition, processing
by Eurobus A devices. Aspects covered within the electrical
communication and control Systems for military, industrial
requirements include:
and other applications.
NOTE 1. More detailed information about the requirements
(a) the voltage levels of the active and quiescent logic
specified in this international Standard, including the data width
states on the bus;
and addressing capability, devices connected to the bus, bus
(b) the required characteristics of the termination
allocation, bus transfers, interbus transfers and electrical re-
networks; quirements, and background information are given in clause 0.
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2.19 indivisible Operation. A sequence of bus cycles for
NOTE 2. In this International Standard, upper case letters are

used for the first letter of names of bus cycles. which the correct System function tan only be guaranteed

if no other bus cycles occur within that sequence,
NOTE 3. The titles of the publications referred to in this lnter-
national Standard are listed in annex P. e.g. a Read-Modify-Wri te sequence.
2.20 interb us transfe r. A transfe r of inf ormation between
2. Definitions
devices that uses two 0 r more bu ses and one or m ore bus
For the purposes of this International Standard the follow-
irig definitions apply.
2.21 interrupt. A flag passed to the arbiter by a device
2.1 address. The location of a data word, or the value on
in Order to initiate a predetermined system-dependent
the highway during the addressing phase of any Read,
Write or Vector cycle.
2.22 master. The device that initiates the transfer in
2.2 arbiter. The device that performs the function of
arbitration for the bus and is also responsible for servicing
2.23 normal address space. An addressing space whose
interrupts and for timing-out failed cycles and aborting
size is determined by the number of lines in the highway
and that is addressable as words or bytes.
2.3 arbitration. The means whereby use of the bus is
2.24 protocol. The signalling ruies used to convey
allocated to one of the bidding devices which then becomes
information or commands between devices connected
the master.
to the bus.
2.4 backplane. The assembly of the bus with connectors
2.25 pseudo address space. A second, independent
into which spur cards may be plugged.
addressing space whose size is determined by the number

2.5 bidding device. A device that wishes to initiate a cycle of lines in the highway and that is addressable as words

or group of cycles on a bus and that requests use of the
2.26 Read cycle. A bus cycle in which the master obtains
2.6 bus. complete set of bus lines used by a particular
The a word or byte from the Slave.
implerne ntati on of Eurobus.
2.27 reset. The operat io n whereby each dev ice con nected
2.7 bus cycle. A closed group of Signals on the bus that
to th e bus is put into a redetermined in itial condit ion.
convey information between devices connected to it.
2.28 Retain cycie. A bus cycle at the end of which the
This group consists of an addressing Phase, in which the
master keeps control of the bus in Order to complete an
master places an address on the highway for recognition
indivisible Operation.
by a Slave and, except in Vector cycles, a subsequent data
2.29 settiing time. The time taken for a bus line to settle
transfer Phase.
unambiguously into its new logical state from its previous
2.8 bus line. An electrical connection between two or
more devices.
2.30 shelf. The physical structure that supports the
2.9 bus linker. A device that plugs into two or more buses
backplane and the cards that plug into it.
thus providing a means whereby a master on one bus may
2.31 skew. On the assumption that two logical transitions
transfer information to or from a Slave on another bus.
are launched simultaneously on two bus lines, the time
2.10 bus voitage. The voltage on a bus line measured
differente betwecn the receipt of those transitions at a
relative to the bus zero voltage reference.
given pair of receivers on a card connected to the bus at
2.11 byte. A contiguous group of 8 bits.
the Point in question.
2.12 circuit card. A card on which various electronie
2.32 s lave. The device that r esponds to the address
components are mounted and that plugs into a Eurobus
on the bus by the mast er for the cycle in question.
backplane as a Spur.
2.33 Spur. Device connected to the bus at some Point
2.13 data. The information held at, written to, or read
between th
e two ends of the bus.
from an address.
2.34 state (of a bus line). 0ne of two conditions of a
2.14 deadiy embrace. The conditions when two interbus
bus line, namely active or quiescent.
transfers, using the same bus linker, have been commenced
2.35 Vector cycle. A bus cycle in which the purpose is
and neither transfer tan be completed.
to pass an address from a master to a Slave and in which
2.15 device. A functional block, occupying one or more
no data transfer takes place.
circuit cards, that communicates with other functional
2.36 word. A group of bits whose number corresponds
blocks by means of the bus or a subset of the bus.
to the maximum data width that tan be conveyed over
2.16 extender Panel. A circuit card that tan be inserted
the bus in a Single transfer.
between the bus and another circuit card to permit easy
2.37 0 V. The Signal return path and, as such, the reference
access to the latter while it is still connected to the bus.
for all voltage measurements.
2.17 Hold cycles. A sequence of cycles during which the
NOTE. 0 V is not a safety earth. Where a safety earth is referred to
master is not asked by the arbiter to release the bus for
in this Standard, it is specifically identified.
2.38 Write cycle. A bus cycle in which a master writes a
2.18 highway. Those bus lines used to convey data and
word or byte to a Slave.
addresses between devices on the Eurobus.
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ISO 6951-1986 (El
Statement of the limitations imposed on a System into
3. Designation of a particular Eurobus
which the device may be incorporated.
Esch member of the Eurobus A famity shall be designated
4.4 Mechanical compliance. Connector allocations for
using the following format.
data widths of 8, 16, 24 and 32 bits shall be as specified
in annex E.
Eurobus address width /A qualifying information
5. Protocols for Eurobus A
The designation entries shall be determined as follows:
5.1 Preliminary
(a) address width = 10 or 18 or 26 or 34 . . . etc.,
as appropriate (see note 1);
5.1.1 Genera/. The set of Signals constituting Eurobus A
shall be as specified in 5.2. The protocols, for use of those
(b) A designates the electrical characteristics specified
Signals for the allocation of the bus to potential users and
in clause 6;
thereafter for effecting transfers on the bus, shall be as
(c) qualifying information is additionai text stating the
specified in 5.3 and 5.4.
Version (see 0.8) and, optionally, text to enable users
NOTE 1. Annex F gives illustrative examples of the Operation
to identify a particular mechanical implementation.
of the bus protocols in accordance with these requirements.
NOTE 1. The address &dth is the number of highway bits,
NOTE 2. Any transfer using the Eurobus protocols generally
i.e. the number of data bits plus two.
involves three devices:
NOTE 2. It is recommended that sufficient qualifying information
(a) the arbiter which:
should be provided to enable users and potential users to identify
a particular mechanical implementation of Eurobus.
(1) grants use of the bus to a bidding device that then
becomes the bus master; or
NOTE 3. For example, an 18-bit address implementation of
Eurobus A (omitting optional qualifying information) is designated
(2) allows an existing master to continue using the bus;
‘Eurobus 18/A commercial’.
(b) the master device that initiates the transfer by addressing
another device;
4. Compliance
(c) the device that, having recognized the address, accepts the
transfer and so becomes the bus Slave.
4.1 Full compliance of devices. Devices that are Said,
without qualification, to comply with this International
5.1.2 Basic bus cycles. There shall be three basic types of
Standard, shall comply with :
bus cycle (see table ‘l) as follows.
(a) the logical requirements of clause 5;
(a) Read cycle in which data is read from a Slave
device by a master device.
(b) the electrical requirements of clause 6;
(b) Write cycle in which data is written to a Slave
(c) the requirements for connector allocation (see 4.4).
device by a master device.
4.2 Logical compliance
(c) Vector cycle in which an address is transferred
4.2.1 Devices said to be logically compliant shall comply
from the master device to the Slave device.
with the protocol requirements of clause 5 or with an
NOTE. The address used by the master device to identify one
appropriate subset of those requirements.
of a set of locations recognized by a Slave device is the only
NOTE. For example, a slaveonly device need not be capable
information transferred over the bus in a Vector cycle.
of acting as a bidding device.
5.1.3 Bus cycle variants. The number of possible variants
4.2.2 In an implementation said to be logically compliant,
of each basic type shall be two, as follows:
(a) Read and Hold;
(a) the bus lines shall be used only for the purposes
(b) Read and Retain;
specified in this International Standard; or
(c) Write and Hold;
(b) if one or more of the bus lines is used for purposes
(d) Write and Retain;
not so specified:
(e) Vector and Hold;
(1) normal Signals on the bus between devices that
operate according to the specified protocols shall
(f) Vector and Retain.
not Cause any malfunction in the implementation
NOTE 1. The main purpose of a Hold cycle is to allow devices
that have a requirement for numerous bus cycles, e.g. processors,
to access the bus repeatedly without having to bid for each
(2) Signals generated within that implementation
individual cycle. Because the use of such a cycle tan delay the
shall not Cause malfunction in devices, connected
reallocation of the bus to another device, such cycles are only
recommended where there is a high probability that the device
to the bus, that operate according to the protocols
concerned will make use of the next bus cycle.
specified in this International Standard.
The main purpose of Retain cycles is to enable indivisible
4.3 Electrical compliance. Devices said to be electrically
.operations to be performed, e.g. Read-Modify-Write.
compliant shall eitber:
NOTE 2. The differentes between these variants and the basic
cycles concern the time at which the bus is released by the device
(a) comply with 6.1 and 6.2, relating to the electrical
for reallocation by the arbiter.
requirements of devices; or
NOTE 3. The bus allocation protocol is designed so that:
(b) when incorporated into a bus, not prevent that bus
(a) the minimum avoidable deiay is experienced when
from complying with 6.3, relating to the electrical
allocating an idle bus;
requirements of buses.
(b) wherever possible, bus allocation is overlapped with bus
If a device is togically compliant (see 4.2) and is electricat-
cycles in Order to reduce delays;
ly comptiant in accordance with item (b) (i.e. not in ac-
(c) a device that requires numerous bus cycles, such as a
cordante with item (a)), alt descriptions of the device that
processor, does not necessarily have to make a fresh request
for each cycle;
refer to this International Standard shall inctude an explicit
---------------------- Page: 7 ----------------------
ISO 6951-1986 (El
Such Codes shall be ailocated by data width in Order
(d) a device tan perform indivisible cycles (this provides the
facility necessary, for example, for the construction of a
from the smallest widths to the largest width, and any
Ready-Modify-Write cycle from a Read cycle followed by a
code that has then been aliocated shall not be available
Write cycle);
for ailocation to another data width as follows.
(e) a device tan, as an alternative to performing a Read,
(1) If the second, third or fourth block is already
Write or Vector cycle, Signal an Interrupt to the arbiter which

is then responsible for servicing the Interrupt. ailocated for recognition by B-bit devices, 16-bit

devices shall be allocated the next higher available
5.2 Signalling
5.2.1 Use of bus lines. All communication between the
(2) If the second, third or fourth block is already
arbiter, devices acting as master and devices acting as Slave
aliocated for recognition by B-bit or 16-bit devices,
shaii be over the Eurobus protocol lines as specified in
24-bit devices shall be allocated the next higher
table 1. The usage of these lines shali be such that any
available block.
device, or severai devices simuitaneously, tan Cause a

line to be active. If no device has caused a iine to be active, (3) if the se cond, third or fourth block is already

that line shall be quiescent. allocated for recognition by B-bit, 16-bit or 24-bit

devices, 32-bit devices shall be allocated the next
5.2.2 Bitnumbering. The bit number, (IV), of the most
higher available block.
significant data and address line shall be given by:
C Any unused b locks shall be available for extending
( 1
N=Bp- 1
the address range of an y of the devices.
5.4 Eurobus A protocol rules
p is any positive integer.
5.4.1 Preliminary. The rules for the use of the bus lines
The bit number (M) of the most significant byte address
specified and named in table 1 shall be as specified in 5.4.2
line shall be given by:
to 5.4.6, as foliows:
M = the largest positive integer that is
(a) for devices bidding for, and the arbiter granting,
less than log2 [UV + 1 )/81
use of the bus; 5.4.2, rules Al to AK?;
Ie, in Order to address one of two bytes in a
NOTE . For examp
word :
16-bit data
(b) for a device acting as master selecting and
N = 15and
communicating with a device acting as Slave; 5.4.3,
M < log, 16/18, i.e.M < 1
rules Ml to Mll;
* M = 0, i.e. only one byte address line is required.
. .
(c) for a device acting as Slave responding to the
master; 5.4.4, rules Sl to S6;
5.2.3 Byte mode address selection. The current bus master
shail specify, by coding the Byte Working and Byte Address
(d) for the arbiter aborting a bus cycle; 5.4.5,
Iines, as given in table 2, the selection of either full-word
rules Cl to C4;
working, or byte working. If full-word working is selected,
(e) for a device, being a bus linker, requesting
the coding shall further specify the selection of pseudo or
deallocation of the bus; 5.4.6, rules Dl to 02.
normal address space. If byte-working is selected,
NOTE. Within this Point-numbered text the rules and conditions
the coding shall further specify which byte is addressed
have been arranged and identified, in Order to aid understanding,
and pseudo address space shall then be unavailable.
by use of a code of upper case letters, lower case letters and
small roman numerals. Esch rule is designated by a letter and
5.3 Address recognition protocol
number (e.g. ‘A 9’). Within each rule the alternative (‘or’)
5.3.1 Data width. The fuil addressing capabiiity of the
conditions are distinguished by lower case letters while

highway, 2(N+3) bits provided by Ad, Ad, simultaneous (‘and’) conditions are distinguished by small

roman numerals.
H(N) to H(o), is available on a bus having oniy devices
all of equal data widths. The use of this capabiiity shall
5.4.2 Rules for bus a//ocation
be by the method given in table 3 for a data width of
Rule Al. When device n requires use of the bus it shall
8 bits.
bid for the bus by holding .m active. lt shall do this
For buses that include devices having unequal data widths,
if and only if:
the method of address aliocation shall be as specified in
i it is not locked (rules A7 and A12); and
( 1
annex G.
(ii) BusGr is quiescent; and
NOTE. The functioning of the bus is not dependent on this method.
(iii) Rs is quiescent.
5.3.2 Recognition of address modifier bits. Any Slave
Rute A2. The arbiter shall allocate the bus to one of the
device operating on a bus that it is sharing with other
bidding devices by also holding the appropriate Rq(n)
devices having different data width(s) from itself, shall in
line active. lt shail do this if and only if:
all instances recognize one coding and one coding only
i Rq(n) is already active; and
( 1
on the Address Modifier lines.
(ii) the arbiter is not aiready holding a Rq line
The specific Codes recognized by devices of a particular
active; and
data width shall, in any System, be aiiocated according to
(iii) Bu is quiescent; and
the following rules.
BusAcq is quiescent; and
under the heading first
(a) The Codes listed in table 4
V i? is quiescent; and
( )
first b e all ocated.
block (columns 3 and 4) shaii
Bus Deallocate is quiescent; and

(b) If further Codes are required, the Codes listed under (vii) CcAbort is quiescent; and

the headings second block, third block and fourth block
(viii) Rs is quiescent.
(columns 5 to 10) shall be aiiocated in that Order.
---------------------- Page: 8 ----------------------
ISO 6951-1986 (E)
Table 1. Eurobus A protocol lines
Number of Requirements
Signal name
High wa y lines
Time division multiplexed bi-directional data and address lines. WÖ) shall be
Data/Address N+l
associated with the least significant bit. The number of the most significant
(H(O)to ‘Ri)
bit, H(N), shall be determined as specified in 5.2.2
2 Address Modifier Iines. These are available and shall be used when it is
Address Modifier
required to increase the address range beyond that definable by H(O) to H(N),
Bits (0), (1)
and also for selection between devices of different data width that share the
(AdM(O), Aii)
same bus (see 5.3)
Byte mode/address space selection lines

Byte Working These lines shall be used to qualify the address on the highway in terms of the

(&-iiE) byte mode/address space selection coding (see 5.2.3). The number of the most

significant Byte Address bit, (M), shall be determined as specified in 5.2.2
Byte Address M-t1
If N=7 the Byte Working line shall remain quiescent
Bits (0) to
(BytAd(Oj to
Bus allocation pro tocol lines
Request(n) One Request Iine shal

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