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The value of the number is thus
Sign
(Exponent-127)
{ (-1)
* 2
Note the 'assumed 1' before the mantissa. The exception to this is the special value 0.0, which is represented as 4
zeroes.
eg. a floating-point number of -12345.678 is represented as – [hex] C6, 40, E6, B6
This is transmitted in nibble format as – [hex] 0C, 06, 04, 00, 0E, 06, 0B, 06.
End of Data Identifier
As the protocol has no fixed length or length identifiers the last nibble of data sent to the device has its MS nibble
set. This indicates to the device that all data has been received & the next two bytes will be checksum data.
ACK & NAK
Mantrabus-II supports ACK & NAK, sending ACK (06h) at the end of a successful operation, and NAK (15h) for an
unknown command or failed operation. These are always preceded by the station number (see examples below).
N.B. Mantrabus-II will not transmit a NAK for invalid checksum data, but instead remains silent. (This is different
from the behaviour of the older Mantrabus-I).
Writing to Variables
Station number and command number are followed by 8 bytes of nibble data (the last having its MS bit set),
followed by the two checksum nibbles.
eg. To write the value 100.0 (Floating point 100.0 = 42h, C8h, 00h, 00h) to variable CGAI (command number
40) at station 20, send the following
FEh,
14h,
|
|
[EXOR CS]
Frame
Station
number
number
the response is then –
14h, 06h
Reading of Variables
To read an individual variable, the command number is sent with the MS bit set (i.e. no data following).
eg. To read CGAI (command number 40) from station number 20, send the following –
FEh,
14h,
|
|
Frame
Station
char
number
Assuming the value was -12345.678 (coded as C640E6B6h, representing -1 * 2
response will be –
14h,
0Ch, 06h, 04h, 00h, 0Eh, 06h, 0Bh, 06h,
|
[
Station
number
53
Micron Meters
* 1.Mantissa },
28h,
04h, 02h, 0Ch, 08h, 00h, 00h, 00h, 80h,
|
Cmd
i.e. 'station number', 'ACK'.
A8h,
0Bh, 0Ch
[ | ]
[EXOR CS]
Cmd with
MS bit set
of last byte
DATA
DCell & DSC User Manual
[
MS bit
of last Byte set
13
01h, 0Fh
]
[EXOR CS]
0Bh, 0Eh
DATA
|
]
char
* 12345.678 / 8192), the
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