Guralp Systems CMG-DM24 - Manual (Page 111)

Operator's guide
Block sequence number : An unsigned integer, which increments by 1
after each block, wrapping round to 0 from 255.
Block size : The size of the block in bytes, excluding the transmission
header and checksum tail, but including the GCF block header.
After the transmission header, the GCF block is sent (with a length
equal to the number of bytes specified in Block Size), followed by a
two-byte checksum. This value is the sum of all bytes in the block
header and body, modulo 65536, and is presented in big-endian byte
order.
To optimise the use of available transmitter bandwidth the transmitted
data block is truncated to the actual data length. This reduction is only
applied to the difference records - the first and last absolute values are
still transmitted as 32 bit values.
ACK and NACK signals
After transmission of the block is complete, the transmitter waits a
nominal 150 ms for either a NACK or ACK signal before starting to
transmit the next block.
If it receives an ACK, transmission of the next block commences
immediately. If NACK is received, the block is re-transmitted.
The receiver should read the block, calculate the data checksum, and
compare that with the value sent in the transmission header. If the two
values match, the receiver should send an ACK signal, otherwise a
NACK.
ACK and NACK signals can be in two formats: an older 2-byte format
and a newer 6-byte format. The 2-byte signal is simply the first 2 bytes
of the 6 byte signal. When both the transmitter and receiver use the 6
byte version, the link is said to be operating BRP (Block Recovery
Protocol), which allows recovery of lost data (from up to 255 blocks
ago) as well as error correction on a per-block basis.
The format of an ACK or NACK message is
Byte 1 2 3 4 5 6
ACK 0x01
Stream ID
LSB ^S (0x13)
or 0 Stream
ID Stream
ID Stream ID
MSB
NACK 0x02
Stream ID
LSB Block
number Stream
ID Stream
ID Stream ID
MSB
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