Vital Interface Unit (Viu); Multiple Vius In A Large Interlocking; Message Time Stamp; Message Security - Siemens VIU-20 Installation And Instruction

WAYSIDE INTERFACE APPLICATION
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5.2.1 Vital Interface Unit (VIU)

The VIU is installed at a wayside location within an interlocking and monitors signal aspects and
switch positions at that location via its vital inputs. Each VIU can monitor up to 20 vital inputs.
The VIU compiles the switch and signal information into a vital Wayside Status Message (WSM)
and sends them to the ITCM via one of the Ethernet ports on the VIU top panel. These WSMs
are in Edge Message Protocol (EMP) format.
5.2.1.1

Multiple VIUs in a Large Interlocking

In larger interlockings, multiple auxiliary VIU modules may be necessary to report the status of
all switches and signals in the interlocking. The auxiliary VIUs monitor switches and signals via
their vital inputs and send the information to the main VIU in vital ATCS messages.
The main VIU creates a WSM for each WIU channel and broadcasts the messages periodically
as determined by a configurable broadcast time parameter.
5.2.1.2

Message Time Stamp

The VIU maintains clock synchronization from an external device with a one second resolution.
Two methods are supported by the WIU for this synchronization: EMP based time messages
transported over Class C or SNTP version 4 as per RFC 4330. Both time synchronization
methods are supported but only one is enabled in the WIU configuration at a time.
The 32-bit timestamp uses Greenwich Mean Time (GMT) and indicated Absolute Time, which is
UTC time expressed as the absolute number of seconds since midnight, January 1, 1970,
including leap seconds. A 32-bit timestamp is used by the system. The system uses GMT for
the timestamp when creating reports or saving configuration files.
5.2.1.3

Message Security

The WIU uses a keyed-Hash Message Authentication Code (HMAC) as a protection scheme for
the vital data portion of the message. The WIU uses a RC2 Decryption Key to decrypt the
encrypted HMAC Key to validate the message under PTC. The HMAC is stored in an encrypted
format in the WIU. The RC2 Decryption Key may be loaded in the field, but the Key values are
not viewable.
The non-encrypted HMAC key size is fixed at 20 bytes. The encrypted size is 24 hex bytes. The
RC2 key is 20 bytes. Trailing 0's in the entered 20-byte RC2 key shall be truncated, and the
truncated value is used in the computation. If a shorter HMAC string is entered, it shall be
padded with 0's to make 24 bytes.
The HMAC key is proved in TXT format for cut and paste or copy operation to the VIU.
SIG-00-07-11
Version No.: B.2
THE USER MUST ENSURE THAT EACH SITE IS GIVEN A
UNIQUE HMAC KEY.
WARNING
5-2
SEPTEMBER 2012 (REVISED OCTOBER 2019)