FMT230, FMT250
THERMAL MASS FLOWMETER | CI/FMT230/250-EN REV. B
43
Modbus® communication
Note
The Modbus® protocol is an unsecured protocol, as such the
intended application should be assessed to ensure that these
protocols are suitable before implementation.
Modbus is an open standard owned and administrated by an
independent group of device manufacturers styled the Modbus
Organization (
www.modbus.org
).
Using the Modbus protocol allows devices made by different
manufacturers to exchange information via the same
communication bus, without the need for any special interface
devices to be used.
Modbus protocol
Terminals
V1 / V2
Configuration
Via the Modbus interface or via the local operating
interface in connection with Asset Vision Basic
(DAT200) and a corresponding Device Type Manager
(DTM)
Transmission
Modbus RTU - RS485 serial connection
Baud rate
2400, 4800, 9600, 19200, 38400, 56000, 57600,
115200 baud
Factory setting: 9600 baud
Parity
None, even, odd
Factory setting: odd
Stop bit
One, two
Factory setting: One
IEEE format
Little endian, big endian
Factory setting: Little endian
Typical response time
< 100 ms
Response delay time
0 to 200 milliseconds
Factory setting: 10 milliseconds
1
Modbus master
2
Terminating resistor
3
Modbus slave 1
4
Modbus slave n to 32
Figure 31: Communication with the Modbus protocol
Cable specification
The maximum permissible length is dependent on the baud rate,
the cable (diameter, capacity and surge impedance), the number
of loads in the device chain, and the network configuration
(2-core or 4-core).
• At a baud rate of 9600 and with a conductor cross-section of
at least 0.14 mm
2
(AWG 26), the maximum length is 1000 m
(3280 ft).
• When using a 4-core cable as a 2-wire wiring system, the
maximum length must be halved.
• The spur lines must be short, a maximum of 20 m (66 ft).
• When using a distributor with ‘n’ connections, each branch
must have a maximum length of 40 m (131 ft) divided by ‘n.’
The maximum cable length depends on the type of cable used.
The following standard values apply:
• Up to 6 m (20 ft):
cable with standard shielding or twisted-pair cable.
• Up to 300 m (984 ft):
double twisted-pair cable with overall foil shielding and
integrated earth cable.
• Up to 1200 m (3937 ft):
double twisted-pair cable with individual foil shielding and
integrated earth cables. Example: Belden 9729 or equivalent
cable.
A category 5 cable can be used for Modbus RS485 up to a
maximum length of 600 m (1968 ft). For the symmetrical pairs in
RS485 systems, a surge impedance of more than 100
Ω is
preferred, especially at a baud rate of 19200 and above.
