20 The
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Checksum analysis
The final two display fields provide analysis of checksum information.
A checksum is a method designed to ensure data integrity. This is a new feature of
DMX512-A. The checksum simply adds together all of the data in a packet and sends this
total along with the data to the receiver. The receiver can then recalculate the checksum by
adding together all of the received data. If this number matches the checksum sent by the
transmitter, the data is known to be good.
If the numbers do not match, it is assumed that data corruption has occurred and that the
frame of DMX should be discarded.
There are two distinct types of checksum:
SIP Checksum. The SIP checksum is used to confirm that the data within a single SIP is
correct.
Packet Checksum. The Packet checksum is used to confirm that the preceding DMX packet
contains good data.
SIP Checksum
The SIP checksum is used to confirm that the data within a single SIP is correct. The
checksum is calculated by adding together the first 23 data slots of the SIP plus the start
code. The SIP contains the transmitter’s version of this calculation at the end of the packet.
The actual slot that contains this information is defined by the Byte Counter field. This is
done in order to allow the size of the SIP to be increased in future versions.
‘The Wife’ DMX Tester is designed to analyze the standard 24 byte SIP.
The SIP Analysis display indicates three modes:
Too Complex. This indicates that the SIP is longer than the standard 24-byte packet. In most
instances this can be assumed to mean that the transmitter is sending corrupt data.
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