The menus
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Good Checksum. This indicates that the SIP contains valid error checked data. Note that this
does not confirm that the data is meaningful; simply that it has arrived without error. Two
hexadecimal numbers are displayed. The first is the checksum that ‘The Wife’ DMX Tester
calculates from the packet contents. The second number is the checksum retrieved from data
slot 24 of the SIP.
Bad Checksum. This indicates that there has been a data reception error in this SIP packet.
Packet checksum
The Packet checksum is used to confirm that the preceding DMX packet contains good data.
The level of confidence that this check provides depends upon the frequency at which SIP’s
are transmitted. Maximum confidence in the data integrity is achieved by transmitting a SIP
after each zero start code (lighting data) packet. In this situation, the integrity of all lighting
data can be checked. This is a realistic proposition as the SIP is only 24 bytes in length. This
scenario would only reduce lighting data bandwidth (or refresh rate) by 5%.
The decision on the frequency of the SIP transmission is a decision that is left to the
manufacturer of the transmitting equipment subject to the provision that if SIP’s are
transmitted, they must be sent at least once in 15 seconds.
Packet checksums are available in two types; 8 bit and 16 bit. The 16 bit version provides a
greater degree of confidence. However, it requires slightly more processing power to
calculate at both the transmitter and receiver.
‘The Wife’ DMX Tester is able to decode and test both types of checksum.
The Packet Checksum display can show six distinct checksum states:
No Checksum. This shows that the Control Bits field of the SIP indicates that no checksum
has been sent by the transmitter. It is not an error condition; but simply confirms the
manufacturer of the transmitting equipment choose not to implement this feature.
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