KROHNE BM 70 M - Manual (Page 11)

BM 70 M 11
RS485 connection must
be terminated with 120 ohms
(if necessary insert resistor
between A and B)
Computer
system
Bus line (2 shielded wires)
with termination
resistance
120 ohms
4.5 Current output (non-communicable)
Function Level, distance, volume (conversion table) or reflection, and error
identification
Current 4-20 mA; without or with error message (2 mA and 22 mA)
Accuracy/linearity 0.3 % (rel. 20 mA; at T = 20°C, 100 Ohm load impedance and
nominal operating voltage)
Temperature drift < 200 ppm/K (typically 70 ppm/K)
Load impedance < 250 Ohm
Effect of load impedance < 0.1 % (rel. 20 mA for RB = 0...250 Ohm)
Effect of power < 0.1 % (rel. 20 mA for UB within limits of tolerance)
Hazardous-duty version Ex e
When operated as a switching output (digital output):
Function limit switch, alarm contact or error message
Low status current < 2 mA
High status current = 22 mA (when load < 250 Ohm)
No-load voltage < 18 V
4.6 Digital interface RS 485
RS 485 is a bus that allows two-way Communication (half-duplex operation) with the BM 70 M
Level-Radar. When the computer system sends out a request, the BM 70 M transmits: level,
distance; volume (conversion table), reflection, setting parameters and status.
A PC can configure any BM 70 M 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.
Transmission rate 1200 to 38400 bauds
Address 0 to 255
Protocols KROHNE Protocol, HART
®
, Modbus RTU (from firmware
V.3.02/4.02)
Additional information: see "BM 70 A/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 are not available.
However, the current output can be configured as a switching
output.
Various bus configurations
A) Connection of a BM 70 M as a single RS 485 user: