KROHNE BM 702 - RS 485 Digital Interface

BM 70 X Project Engineering Guidelines 01/2001 Page: 9
4.6 Digital interface RS 485
RS 485 is a bus allowing two-way communication (half-duplex operation) with the BM 70 A/P Level-
Radar. When the computer system sends out a request, the BM 70 A/P transmits: level, distance,
volume (conversion table), reflection, parameters and status.
A PC can configure any BM 70 A/P via the bus (RS 485/RS232 converter required) by means of the
PC-CAT program, provided no Master is active. The maximum cable length - excluding amplifier - is
2000 m (6500 ft).
Transmission rate: 1200 to 38400 bauds
Address: 0 to 255
Protocols: KROHNE Protocol, Modbus RTU, HART
®
Add’l information: see "BM70A/P Communication RS 485" instructions
Additional current output: The current output is conductively coupled to the RS 485 interface!
Technical data: see Section 4.5
Note: Switching contact and digital input not available. However, the current
output can be configured as a switching output.
Various bus configurations
A) Connection of a BM 70 A/P as a single RS 485 user:
RS485 connection must
be terminated with 120 ohms
(if necessar
y
insert resistor
between A and B
)
Computer
s
y
stem
Bus line (2 shielded wires)
with termination
resistance
120 ohms
B) Connection of several BM 70 A/P in the RS 485 bus system via stubs to the bus cable:
Computer
s
y
stem
...........
RS485 connection must
be terminated with 120 ohms
(if necessar
y
insert resistor
between A and B)
Bus line (2 shielded wires)
with termination
resistance
120 ohms
without
resistance
termination
without
resistance
termination
without
resistance
termination
C) Connection of several BM 70 A/P in the RS 485 bus system, with bus cable looped through to
further field instruments:
...........
with termination
resistance
120 ohms
without
resistance
termination
without
resistance
termination
without
resistance
termination
Bus line (2 shielded wires)
Computer
s
y
stem
RS485 connection must
be terminated with 120 ohms
(if necessar
y
insert resistor
between A and B)
Notes:
•
If other devices are operated together with the BM 70 A/P Level-Radar on one bus, all devices
must use the same communications protocol (e.g. ModBus devices of different manufacturers, or
BM 70 and TTM70 with KROHNE protocol).