KROHNE BM 702 - HART Communication Methods

Page: 8 BM 70 X Project Engineering Guidelines 01/2001
Load
≤
(U
S
- 13 V) / 20 mA (U
S
= external supply voltage)
Effect of supply voltage
≤
0.02 % (rel. 20 mA for U = 13...30 V)
Ex-i safety limits
Signal circuit in type of protection: Intrinsic Safety EEx ia IIC/IIB or EEx ib IIC/IIB
for connection to a certified intrinsically safe circuit with the following peak values:
U
i
= 30 V; I
i
= 100 mA; P
i
= 1 W
Effective inner capacitance C
i
= 11 nF; effective inner inductance L
i
≈
0
4.4 HART
®
communication
In accordance with the Rosemount Standard, the HART
®
communications protocol can be used with a
BM 70 A/P, BM 702.
Electrical connection: see Section 7.6.
There are two ways of using the HART
®
communication:
a) As a
point-to point
connection between the BM 70 A/P, BM 702 and the HART
®
Master
equipment.
A BM 70 A/P, BM 702 can be used either with an Ex-e-current output HART
®
or with an Ex-i (IS)
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, BM 702 or
other HART
®
equipment), in parallel on a 2-wire
bus
:
A BM70 A/P, BM 702 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, RS 485 version
(no communication capability)
Function Level, distance, volume
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or reflection,
and error identification
Current 4-20 mA; without or with error message 2 mA or 22 mA
Accuracy/linearity 0.3 % (rel. 20 mA; at T = 25°C/77°F, 100
Ω
load and rated operating voltage)
Temperature drift
≤
200 ppm/K (typically 70 ppm/K)
Load
≤
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 switching output (digital output):
Function Limit switch, alarm contact or error message
Low state Current < 2 mA
High state Current = 22 mA (when load
≤
250
Ω
)
No-load voltage
≤
18 V