KROHNE BM 70 M - Manual (Page 12)

12 BM 70 M
B) Connection of several BM 70 M in the RS 485 bus system via stubs to the bus cable:
C) Connection of several BM 70 M in the RS 485 bus system, with bus cable looped through to
further field instruments:
• If other devices are operated together with the BM 70 M Level-Radar on one
bus, all devices must use the same Communications protocol (e.g. ModBus
devices of different makes or BM 70 M and MTM 70 with KROHNE
Protocol).
•
When "HART
®
" is set as the protocol type, use only the protocol in keeping
with the "HART ® Specification". The "Hardware Implementation RS 485"
does not conform to the HART
®
Standard (see Sections 4.2 and 4.3).
• Since signal reflections at the line ends can interfere with Communication,
both the line beginning (at the computer system) and the line end (at the last
device) should be terminated with the typical surge impedance of the cable
(120 Ohm). To do this inside the BM 70 M: see Section 7.7.
4.7 Breakdown signal
Breakdown information can be called up via the following interfaces:
• Local display: flashing display, plain text information
•
Current output: error signal 2 mA (Exi = 3.6 mA) or 22 mA
• Switching output: contact opens or closes
•
Digital interfaces: interrogation of error flags
Computer
system
...........
RS485 connection must
be terminated with 120 ohms
(if necessary 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
...........
with termination
resistance
120 ohms
without
resistance
termination
without
resistance
termination
without
resistance
termination
Bus line (2 shielded wires) Computer
system
RS485 connection must
be terminated with 120 ohms
(if necessary insert resistor
between A and B)