13
2 6 6 H S H MODBUS TR A N S M IT TE R |
P R E SSU R E/ T EM P ER AT U R E M U LT I VA R I A B L E | O I/ 2 6 6 H SH M T- EN R E V. A
Cable specification
The maximum permissible length depends 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 mm2 (AWG 26), the maximum
length is 1000 m (3280 ft).
— If a 4-core cable is used in a 2-wire system, the
maximum length must be halved.
— The spur lines must be short (maximum of 20 m
[66ft]).
— When using a distributor with “n” connections, the
maximum length of each branch is calculated as follows:
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 19,200 and
above.
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.
Up to 32 devices can be connected on one Modbus line.
The Modbus network can be expanded using repeaters.
Configuration
(HART-RS485)
Via an RS485 interface in
connection with Asset Vision Basic
(DAT200) a HART
communication DTM and a
corresponding Device Type
Manager (DTM)
1200, 2400, 4800, 9600, 19200,
38400,
57600, 115200 bps.
None, even, odd
Factory setting: none
0 ... 200 milliseconds
Factory setting: 50 milliseconds
1 ... 247
Factory setting: 247
One base
< 100 milliseconds
Modbus RTU – 2-wire half-duplex
RS485 serial connection
Operating
(Modbus
communication)
Baud Rate
Parity
Typical response
time
Response delay time
Device address
Register address
offset
Modbus
