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the contents of this document are not altered or modified.
The contents of this document may not be altered or modified
without the express written permission of Cisco Systems and
T3plus Networking. It is intended that this document will serve as
a high speed serial interface Specification and evolve into an
industry standard. With this intent, it is expected that this
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John T. Chapman
Senior Hardware Design Engineer
Cisco Systems, Inc.
375 East Tasman Drive
San Jose, CA 95134 firstname.lastname@example.org
TEL: (408) 526-7651 FAX: (408) 527-1709
Senior Hardware Design Engineer
T3plus Networking, Inc.
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Santa Clara, CA 95051 email@example.com
TEL: (408) 727-4545 FAX: (408) 727-5151
This document specifies the physical
layer interface that exists between a
DTE such as a high speed router or similar
data device and a DCE such as a
DS3 (44.736 Mbps) or SONET STS-1
(51.84 Mbps) DSU. Future extensions to
this specification may include support
for rates up to SONET STS-3 (155.52
This document is specification compatible with the HSSI Design Specification,
written by John T. Chapman and Mitri
Halabi, Revision 2.11, dated March 16,
1990 and Addendum Issue #1, dated
January 23, 1991.
HSSI is currently being ratified by the
American Standards Intitute. The physical
layer specification will be EIA/TIA-613
and the electrical layer specification
will be EIA/TIA-612. These specifications
should become available in mid
1993. Notation has been included here
where there are known differences
between the two specifications.
This section, Introduction, introduces HSSI and relates it to other specifications.
The (+side) of a given signal
will be at potential Vol while the (-side)
of the same signal will be at potential
Data Communications Channel
The transmission media and intervening
equipment involved in the transfer of
information between DCEs. In this specification,
the data communications
channel is assumed to be full duplex.
DCE: Data Communications Equipment
The devices and connections of a
communications network which connect
the data communications channel with
the end device (DTE). This will be used
to describe the CSU/DSU.
A loopback in either
direction that is associated with the DTE
port of a DCE piece of equipment.
DS3: Digital Signal level 3
as T3. Equivalent in bandwidth to 28
T1?s. The bit rate is 44.736 Mbps.
DSU: Data Service Unit. Provides a DTE
with access to digital telecommunications
DTE: Data Terminal Equipment
part of a data station that serves as a
data source, destination, or both and
that provides for the data communications
control function according to protocols.
This will be used to describe a
router or similar device.
A clock stream at a nominal
bit rate which may be missing clock
pulses at arbitrary intervals for arbitrary
lengths of time.
The optical signal that results
from an optical conversion of an STS-N
SONET: Synchronous Optical NETwork
An ANSI/CCITT standard for standardizing
the use of optical communication
STS-N: Synchronous Transport Signal level n, where n = 1,3,9,12,18,24,36,48
STS-1 is the basic logical building block
signal for SONET with a rate of 51.84
Mbps. STS-N are obtained by byte interleaving
N STS-1 signals together with a
rate of N times 51.84 Mbps.
RT is a gapped clock with a maximum
bit rate of 52 Mbps, and provides
receive signal element timing information
RD: Receive Datafrom DCE
The data signals generated by the DCE,
in response to data channel line signals
received from a remote data station, are
transferred on this circuit to the DTE.
RD is synchronous with RT.
ST: Send Timingfrom DCE
ST is a gapped clock with a maximum
bit rate of 52 Mbps, and provides transmit
signal element timing information to
TT: Terminal Timingto DCE
TT provides transmit signal element timing
information to the DCE. TT is the ST
signal echoed back to the DCE by the
DTE. TT should be buffered by the DTE
only, and not gated with any other signal.
SD: Send Datato DCE
The data signals originated by the DTE,
to be transmitted via the data channel to
a far end data station. SD is synchronous
TA: data Terminal equipment Availableto DCE
TA will be asserted by the DTE, independently
of CA, when the DTE is prepared
to both send and receive data to and
from the DCE. Valid data transmission
should not commence until CA has also
been asserted by the DCE.
If the data communications channel
requires a keep alive data pattern when
the DTE is disconnected, then the DCE
shall supply this pattern while TA is
CA: data Communications equipment Availablefrom DCE
CA will be asserted by the DCE, independently
of TA, when the DCE is prepared to both send and receive data to
and from the DTE. This indicates that
the DCE has obtained a valid data communications
channel. Data transmission
should not commence until TA has
also been asserted by the DTE.
Given that the data communications
channel is not valid unless both TA and
CA are asserted, then it may be good
implementation practice to gate the
incoming data stream with both TA and
CA on both the DTE and the DCE.
It should also be recognized that when
CA is deasserted by the DCE, the DCE is
in an unknown state, and that ST and
RT clocks may be absent and cannot be
considered by the DTE as valid.
LA: Loopback circuit Ato DCE
LB: Loopback circuit Bto DCE
LA and LB are asserted by the DTE to
cause the DCE and its associated data
communications channel to provide one
of three diagnostic loopback modes.
Specifically, LB = 0, LA = 0: no loopback
LB = 1, LA = 1: local DTE loopback LB =
0, LA = 1: local line loopback LB = 1, LA
= 0: remote line loopback
A 1 represents assertion, and a 0 represents
deassertion. All loopbacks are
payload loopbacks. Therefore, if the
HSSI data stream is multiplexed on to
only part of the data communications
channel, then, as a minimum, only that
part of the data communications channel
needs to be loopbacked.
A local DTE (?digital?) loopback occurs
at the DTE port of the DCE, and is used
to test the link between the DTE and
DCE. A local line (?analog?) loopback
occurs at the line side port of the DCE,
and is used to test the DCE functionality.
A remote line (?analog?) loopback
occurs at the line port of the remote
DCE, and is used to test the functionality
of the data communications channel.
These three loopbacks are initiated in
this sequence. The remote DCE is tested
by remotely commanding its local loopbacks.
Note that LA and LB are direct
supersets of the EIA signals LL (Local
Loopback) and RL (Remote Loopback).
The local DCE continues to assert CA
during all three loopback modes. If the
local DCE is unable to support a particular
loopback mode, then it may elect
to deassert CA while LA or LB are
asserted by the DTE, The remote DCE
will deassert CA when remote loopback
is in effect. If the remote DCE can detect
a local loopback at the local DCE, then
the remote DCE will deassert its CA;
otherwise the remote DCE will assert its
CA when there is a local loopback at the
The DCE implements the loopback
towards the commanding DTE only.
Receive data from the data communications
channel is ignored. Send data to
the data communications channel is
filled with either the commanding DTE?s
send data stream, or with a keep alive
data pattern, depending upon the data
communications channel?s specific
There is no explicit hardware status signal
to indicate that the DCE has entered
a loopback mode. The DTE waits for an
appropriate amount of time after asserting
LA and LB before assuming the loopback
to be valid. The appropriate
amount of time is application dependent,
and is not a part of this specification.
The loopback mode applies to both timing
and data signals. Thus, on the DTE -DCE
link, the same timing signal could
traverse the link three times, first as ST,
then as TT, and finally as RT.
LC: Loopback circuit Cfrom DCE
LC is an optional loopback request signal
from the DCE to DTE, to request
that the DTE provide a loopback path to
the DCE. More specifically, the DTE
would set TT=RT and SD=RD. ST would
not be used, and could not be relied
upon as a valid clock source under
This would then allow the DCE/DSU
network management diagnostics to test
the DCE/DTE interface independent of
the DTE. This follows the HSSI philosophy
that both the DCE and the DTE are
intelligent independent peers, and that
the DCE is capable of and responsible
for maintaining its own data communications
In the event that both the DTE and DCE
asserted loopback requests, the DTE will
be given preference.
Note that LC is optional and has not
been included in the ANSI standard.
TM: Test Modefrom DCE
Test Mode is asserted by the DCE when
it is in a test mode caused by either local
or remote loopbacks. This signal is
optional. TM has been added by ANSI
and was not part of the original HSSI
SG: Signal Ground
SG is connect?s to circuit ground at both
ends. SG ensures that the transmit signal
levels stay within the common mode
input range of the receivers.
SH: Shield Direction
The shield encapsulates the cable for
EMI purposes, and is not implicitly
intended to carry signal return currents.
The shield is connected to DTE frame
ground directly, and may choose one of
two options at the DCE frame ground.
The first option is to connect the shield
to DCE frame ground directly.
The second option is to connect the
shield to DCE frameground through a
parallel combination of a 470 ohm, +/-10%,
1/2 wattresistor, 0.1 uF, +/- 10%,
50 volt, monolithic ceramic capacitor,
and a 0.01 uF, +/- 10%, 50 volt, monolithic
The R-C-C network should be located as
close to the shield/chassis junction as
possible. Because the shield is terminated
directly to the DTE and DCE
chassis, the shield is not given a pin
assignment within the connector. Shield
continuity between connecting cables is
maintained by the connector housing.
In practice, the first option is usually
All signals are balanced, differentially
driven, and received at standard ECL
levels. The ECL negative supply voltage,
Vee, may be either -5.2 Vdc +/- 10% or -5.0
Vdc +/- 10% at either end. Rise
times and fall times are measured from
20% to 80% threshold levels.
Electrical characteristics of the HSSI
transmitter and receiver are given in the HSSI Receiver table and the HSSI Transmitter table, both of which are presented below.
In addition to the 10KH ECL electrical
characteristics listed in this specification,
interoperation with 100K ECL is
also possible and will be allowed for in
the ANSI specification.
Fail Safe Operation
In the event that the interface cable is
not present, the differential ECL receivers
must default to a known state. To
guarantee this, it is necessary when
using the 10H115 or 10H116 to add a
1.5 kohm, 1%, pull-up resistor to the (-side)
of the receiver, and a 1.5 kohm,
1%, pull-down resistor to the (+side) of
This allows the proper 150 mvolts minimum
to be developed across the 110
ohm resistors and will create a longitudinal
termination of 750 ohms. The
default state of all interface signals is
It is not necessary to use external resistors
when using the 10H125 since it has
an internal bias network which will force
an output low state when the inputs are
The interface must not be damaged by
an open circuit or short circuit connection
on any combination of pins.
Source timing is defined as timing waveforms
generated at a transmitter. Destination
timing is defined as timing
waveforms incident at a receiver. Pulse
widths are measured between 50%
points of the final pulse amplitude. The
leading edge of the timing pulse shall be
defined as the boundary between deassertion
and assertion. The trailing edge
of the timing pulse shall be defined as
the boundary between assertion and
The HSSI link, from a specification and
implementation point of view, should be
considered as a ECL flip-flop to flip-flop
link. As data leaves the HSSI port, it
should be reclocked out of an ECL flip flop and directly into the line driver. At
the receiver, once passing through the
line receiver, the data should immediately
again be reclocked into an ECL flip
flop. Control signals do not require the
use of a flip-flop.
RT, TT, and ST minimum positive
source timing pulse width shall be 7.7
ns. This allows a source duty cycle tolerance
of +/- 10%. This value is obtained
Data will change to its new state within
+/- 3 ns of the leading edge of the
source timing pulse.
RT, TT, and ST minimum positive destination
timing pulse width shall be 6.7
ns. Data will change to its new state
within +/- 5 ns of the leading edge of the
destination timing pulse. These numbers
allow for transmission distortion
elements of 1.0 ns of pulse width distortion
and 2.0 ns of clock to data skew.
This leaves 1.7 ns for receiver setup
The data will be considered valid on the
trailing edge. Thus, transmitters clock
data out on the leading edge, and receivers
clock data in on the trailing edge.
This allows an acceptance window for
clock-data skew error.
The delay from the ST port to the TT port
within the DTE shall be less than 50 ns.
The DCE must be able to tolerate a
delay of at least 200 ns between its ST
port and its TT port. This allows for a
150 ns delay for 15 meters of cable
(round trip delay)
To facilitate various bit/byte/frame DCE
multiplexor implementations, RT and
ST may be gapped to allow the deletion of framing pulses and to allow bandwidth
limiting of the HSSI.
The maximum gapping interval is not
specified. However, the clock sources ST
and RT are expected to be generally continuous
when both TA and CA are
asserted. A gapping interval is measured
as the amount of time between two consecutive
clock edges of the same slope.
The instantaneous data transfer rate
must never exceed 52 Mbps.
The definition of valid data is application
dependent and not a subject of this
specification. This is consistent with
HSSI being a layer 1 specification, and
therefore having no knowledge of data
CA and TA are asynchronous of each
other. Upon assertion of CA, the signals
ST, RT, and RD will not be considered
valid for at least 40 ns. Upon the assertion
of TA, the signals TT and SD will
not be considered valid for at least 40
ns. This is intended to allow the receiving
end sufficient setup time.
TA should not be deasserted until at
least one clock pulse after the last valid
data bit on SD has been transmitted.
This does not apply to CA since the data
is transparent to the DCE.
The cable connecting the DCE and DTE
consists of 25 twisted pairs with an
overall foil/braid shield. The cable connectors
are both male connectors. The
DTE and DCE have female receptacles.
Dimensions are given in meters (m) and
Note that although the HSSI cable uses
the same connector as the SCSI-2 specification,
the cable impedances of HSSI
and SCSI-2 cables are different. SCSI-2
cables can be as low as 70 ohms,
whereas HSSI cables are specified at
110 ohms. As a result, cables made to
SCSI-2 specifications may not work correctly
with HSSI. Incompatibilities will be
more apparent with longer lengths of
The cable is completely described in the HSSI Cable Electrical Specification table, the HSSI Cable Physical Specification table, and the HSSI Connector Pinout table, all of which are presented below.
This appendix calculates the noise
immunity of this interface. The normal
specified 150 mvolts of noise immunity
for 10KH ECL is not applicable here
because the differential inputs do not
use the internal ECL bias Vbb.
The common mode (NMcm) and differential
mode (NMdiff) noise margins for the
10H115 and 10H116 differential line
NMcm- and NMdiff are the same for all
parts. To allow the use of all receivers,
the worst case common mode noise at
the receiver must be limited to 310
Interpret the common mode range,
Vcm_max to Vcm_min, as the maximum
range of absolute voltages that may be
applied to the receiver?s input, independent
of the applied differential voltage.
The signal voltage range, Voh_max to
Vol_min, represents the maximum range
of absolute voltages that the transmitter
will produce. The difference between
these two ranges represents the common
mode noise margins, NMcm+ and
NMcm-, with NMcm+ being the maximum
excursion for additive common
mode noise, and NMcm- being the maximum
excursion for subtractive common
With five 50 foot twisted pair grounds,
the amount of ground loop current
required to use up the common mode
noise margin is:
This amount of current should never be
present under normal operating conditions.
Common mode noise will have a negligible
effect on the differential noise margin,
Vdf_app. Rather, Vdf_app would be
affected by noise being introduced by
one side of the power rails at the transmitter.
ECL Vcc has a power supply
rejection ratio (PSRR) of 0 dB while ECL
Vee has a PSRR on the order of 38 dB.
Thus, to minimize differential noise, Vcc
is grounded and Vee is connected to a
negative power supply.