KROHNE BM 70 A_P EN - Manual (Page 11)

Installation and operating instructions BM 70 A/P Page: 11 11/00
4.4 HART
®
communication
The HART

Communications protocol can, in accordance with the Rosemount Standard, be used with
a BM 70 A/P.
Electrical connection: see Section 7.8.
There are two ways of using the HART
®
Communication:
a) As a point-to-point connection between the BM 70 A/P and the HART
®
master equipment.
A BM 70 A/P can be used either with an Ex-e current output HART
®
or with an Ex-i current output
HART
®
.
HART
Master
4...20 mA
b) As a multipoint connection (multidrop) with up to 15 devices (BM 70, BM 70 A/P or other
HART
®
equipment), in parallel on a 2-wire bus:
A BM70 A/P is normally used with Ex-i current output, but an Ex-e current output can also be used
(constant 4 mA in both cases.)
HART
Master
4mA 4mA 4mA 4mA
other
HART
device HART
device
other
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 Ω load impedance and nominal operating voltage)
Temperature drift < 200 ppm/K (typically 70 ppm/K)
Load impedance < 250
Ω
Effect of load impedance < 0.1 % (rel. 20 mA for R
B
= 0...250
Ω
)
Effect of power < 0.1 % (rel. 20 mA for U
B
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
Ω
)
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 A/P
Level-Radar. When the computer system sends out a request, the BM 70 A/P transmits: level,
distance; volume (conversion table), reflection, setting 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.