SSR-1U User’s Manual
Rev -, 27 Dec 2019
© 2019 Slerj, LLC
60
www.slerj.com
6
Time Tagged Archives
Often, it is important to know not only what serial data was transferred, but when it was
transferred. This is useful in both the analysis of communication systems and in the ability to
reconstruct streams as they originally occurred. The SSR-1U supports time tagged archives, in
which received bytes are tagged with the time they were received. When a channel is configured
to record time tagged archives, the received bytes are encapsulated in packets prior to being
written to the file system. Two types of packets are currently defined: the data packet and a time
correlation packet. All multi-byte words in the archive are big endian.
6.1
Data Packet
The data packet uses the system free running clock as the time stamp source. Bytes are grouped
into 2ms windows for stamping. Each data packet begins with a base time stamp that identifies
the whole second in which the data was collected. The base time stamp is followed by a series of
frames that are composed of an incremental time stamp (fractional second within the base time
stamp window) and a group of bytes that were received during the increment. The data packet is
terminated by a Fletcher checksum as defined in section 5.1 Message Format.
Table 14. Time Tagged Data Packet
Element
Bytes Description
Packet Header
0x82 0xA2
2
Packet start sequence.
Run Time
4
Current run time in seconds.
Frame repeated until an mSec_Count value of 0xFFFF is encountered.
mSec_Count
2
2
Fractional second and number of bytes for this frame.
bits 15-7: milliseconds / 2
bits 6-0: number of bytes to follow (
n
)
Data
n
The
n
bytes that were received in the time window leading up to
this packet frame.
End Sequence
2
0xFFFF (invalid mSec_Count)
Checksum
2
Fletcher checksum calculated between Run Time and End
Sequence, inclusive.
2
With the introduction of the high-speed unit, the 7 bit count value is no longer sufficient to represent all of the
bytes that could be received in a 2ms frame when baud rate is greater than 635k. When more than 127 bytes are
received in a frame, a second block for the frame will be generated with the same mSec value. The first block will
have 127 bytes, and the second block will contain the remainder of the bytes received in the frame.
Note that a software utility, including source code, is provided with the SSR-1U for parsing
time tagged archives into a variety of useful formats. See section 6.3 The STTP Utility for
details.