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dCS 954 User Manual Manual for Standard Software Version 1.5x
dCS Ltd June 2000
Manual part no: DOC136954 iss 2B1
Page 57
file 135954ma2b1.pdf available from website
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dCS
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Transmit Message -
The system employs the following protocol - all transactions are initiated by the
PC. The PC is the transmitter and units on the daisy chain are receivers.
Byte 1 : ID of unit to process command
Byte 2 : Command (single byte)
Byte 3 : Length of parameter string
Byte 4 : List of parameters
.
.
Last byte : Checksum of parameter list
The minimum length of a transmit message is 4 bytes, maximum 64 bytes,
limited by a buffer within the receiving unit. There are two cases for parameter
length mismatch.
(a) Receiver expects n parameters, transmitter sends more.
This condition may arise when a later version remote tries to communicate with
an earlier version of receiver. The receiver acts on first n parameters in list,
ignores rest. The full parameter list is check summed.
(b) Receiver expects n parameters, transmitter sends less.
This condition may arise when a earlier version remote tries to communicate
with an later version of receiver. The receiver acts on all parameters in list, and
uses sensible defaults for the rest (ideally, no action except where this is silly).
Do not truncate a command sequence expecting the receiver to do something
sensible. The full parameter list is check summed.
The checksum is the sum of the bytes in the parameter list (bytes 4 to (last-1)
byte) modulo 256. The receiving unit checks the checksum and will only act
upon a command if the command is complete and the checksum is correct.
Acknowledge Message -
The addressed receiving unit (ADC, DAC, DDC, Master Clock, etc)
acknowledges within 50 msecs of the last transmitted byte in the transmit
message. For some special cases (dCS use only, see “Special Commands and
Protocols” below) commands do not acknowledge. If the checksum is incorrect
the receiving unit will ignore the command, clear its buffer and will not
acknowledge. Only valid command bytes will generate an acknowledge, other
command bytes will be ignored, clearing the receiver buffer.
The acknowledge response starts with:
Byte 1 : 101010xx – indicates successful transmission to physical
address, with xx indicating the time the unit may take to respond to the
command
and then, with a command dependant part:
Byte 2 : ID of unit that processed command
Byte 3 : Command (single byte)
Byte 4 : Length of response string
Byte 5 : List of response bytes
.
.
Last byte : Checksum of response list