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User manual SCAIME:
NU-Soft-eNod4F-E-0716_235704-C.doc
4
SCMBUS / FAST SCMBUS
4.1
Physical interfaces
SCMBus and fast SCMBus communication protocols can be used either through
eNod4
USB port and AUX port.
USB port behaves as a full duplex interface whereas the DB9 and AUX ports support half-duplex RS485
communication. Supported baud rates are 9600, 19200, 38400, 57600, and 115200.
For a complete description of the recommendations about
eNod4
RS485 connexion, please refer to the user manual
“characteristics and functioning” of the
eNod4
.
Note :
using
eNod4
through USB requires installing first the necessary USB drivers available on the website
4.2
SCMBus and fast SCMBus features
SCMBus and its variant fast SCMBus can be imbricate into ModBus RTU protocol if the setting
‘communication
protocol’
is set to SCMBus or fast SCMBus. That means that
eNod4
continues answering Modbus RTU frames but it
also allows the device to send frames coded according to SCMBus/fast SCMBus format.
Each protocol has its advantages:
•
in SCMBus measurements are transmitted as ASCII with the decimal point and the unit integrated to the
frame
•
fast SCMBus is dedicated to fast measurement transmission as the frames are the most compact as
possible
•
both protocols allow to communicate without any master request (continuous transmission or sampling
triggered by a logical input)
4.3
Byte format
Data transmitted to
eNod4
thanks to SCMBus or fast SCMBus communication protocol must respect following format:
•
1 start bit
•
8 data bits
•
no parity
•
2 stop bits
in SCMBus protocol, data is encoded as ASCII numeral characters (30
H
..... 39
H
) and ASCII hexadecimal characters (3A
H
..... 3F
H
).
in fast SCMBus protocol, data is encoded as signed hexadecimal (see frame structure paragraph) below.
SCMBus CRC-8 byte is generated by the following polynomial:
G(x) = x
8 +
x
7
+ x
4
+ x
3
+ 1
The CRC-8 polynomial result can be determined by programming the algorithm corresponding to the following
diagram:
Note:
The frame error detection can be ignored. Value
0xFF
of the CRC-8 always is admitted by
eNod4
and a received
frame which is ended by such CRC-8 is considered as a frame without any error.
•
Fast SCMBus checksum byte is obtained by summing all the frame previous bytes then setting b7 bit to 1.
4.4
Frames structure
4.4.1
Transmission organization
•
frame :
eNod4
address first
•
byte : lsb first
