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2.4 Maximizing Message
Integrity
MaxIMIzIng Message InTegRITY
For additional reassurance and/or maximum message integrity, you can insert one or two special
characters:
aCKnoWleDgMenTs: If you want
assurance from the projector (or group
of projectors) that a set message has
been processed, request this simple
acknowledgment by inserting a "$"
just after the start code "(". When the
projector executes the required action
(such as a source switch, for example), the projector will send a "$" (only) back. This is a
quick way to confirm success with set messages, and is particularly useful with long-distance
communication links or where the projectors and/or images are not visible from the controller.
Acknowledgements can also be a type of flow control. Note that requesting an acknowledgement
serves no purpose when included in a request message, since the acknowledgement will be
redundant to the actual reply from the projector. However, if requested, the "$" acknowledgement
from the projector will follow the reply.
CheCKsUMs: For maximum message integrity, add a checksum character "&" just after the "("
as shown below. You must then also include the correct checksum total (0-255) just before the
")" end code. Make sure to add a space before the calculated checksum to separate it from the
last data parameter:
The checksum is the low byte of the sum of the ASCII values of all characters between the "("
and the beginning of the checksum. Calculate the checksum for the above "set contrast to 64"
command as follows:
CheCKsUM exaMPle
The projector collects all of the message bytes as defined in the first byte of the message, then
creates its own checksum value for comparison with the checksum included in the controller's
message. If the values match, the message is considered to have been correctly received—
otherwise the message is discarded.
noTes: 1) 'h' indicates a hex number. 2) If a "request" message has a checksum so will the
reply. 3) If using both "acknowledge" and "checksum", either character can occur first.
B-7
Message from Controller
(
$
Code
(&con64 240)
= & + c + o + n + 6 + 4 + 'space'
= 26h+63h +6Fh +6Eh +36h +$34h +$20h
= 01F0h
= F0h when only the low byte is used
= 240
Appendix B ► Serial Communications
Next Message from Controller
Message
Data
)
(
$
Code
Processed
$
Simple Acknowledge from Projector
Data
)
$
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