SI-INTERBUS User Guide
35
Issue Number: 1
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Table 8-5 CT Single Word format
8.1.3
16-bit parameter access
16-bit data can be accessed using only 4 telegrams. Telegram 4 becomes the low data
byte request (or value, for a write sequence) and telegrams 5 and 6 are not required.
If an attempt is made to access a 32-bit parameter, the parameter value will be
returned, provided that the parameter value does not exceed the signed 16-bit limits
(-32768 to 32767). If the value is not within the signed 16-bit value, the ERR bit will be
set.
When writing data to a 32-bit parameter using 16-bit mode, the 16-bit data will be
treated as a signed 16-bit data value. This limits the range that can be written to a 32-bit
parameter.
8.1.4
Reading parameters using CT Single Word
To read 32-bit parameters using CT Single Word non-cyclic channel, the following
“telegrams” must be transmitted to construct the final message.
•
Telegram 1 Define menu number.
•
Telegram 2 Define parameter number.
•
Telegram 3 Request high data byte.
•
Telegram 4 Request mid-high data byte (32-bit) or low data byte (16-bit).
•
Telegram 5 Request mid-low data byte (32-bit).
•
Telegram 6 Request low data byte (32-bit).
shows the normal sequence for reading a
parameter using CT Single Word non-cyclic.
Bit
Function
Value
Description
0 to 7
Data
0 to 255
Depending on the stamp number of the telegram,
this byte contains the menu, parameter or data byte.
8 to 11
Stamp
Number
0 to 6
Indicates the stamp number of the telegram.
This shows which part of the message is currently in
progress. Setting the stamp number to 0 resets the
internal non-cyclic state machine.
12
32-BIT
0 = 16-bit
1 = 32-bit
Specifies whether a 16-bit or 32-bit data value is to
be accessed. If 32-BIT is set to 1, telegrams 5 and 6
will be used to transfer the additional data bytes.
If 32-BIT is set to 0, then the data bytes will be
transferred in telegrams 3 and 4.
13
Reserved
0
Reserved for future use. Always set to 0.
14
ERR
0 = Data OK
1 = Error
Indicates the success or failure of the message.
Failure could occur if the parameter does not exist,
or is read-only or write-only parameter. This bit will
also be set if the parameter value is out of range in
16-bit mode.
15
READ
0 = Write
1 = Read
Defines whether the data word is part of a READ or
WRITE cycle.
If a telegram produces an error at any point in the sequence the reply to the master will
have the error bit ERR set to a 1 and the data bits will have no significance. Setting the
stamp number to 0 will reset the internal non-cyclic state machine.
NOTE
NOTE
