Scmbus / Fast Scmbus; Physical Interfaces; Scmbus And Fast Scmbus Features; Byte Format - Scaime eNod4-F ETHERNET Software User Manual

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
http://www.scaime.com.

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
..... 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:
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
8 +
G(x) = x
14/113
User manual SCAIME:
NU-Soft-eNod4F-ETH-E-0716_235709-C.doc
..... 39 ) and ASCII hexadecimal characters (3A
H H
7 4 3
x + x + x + 1
H