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Device communication
10.5 BS bus
The BS bus is used to extend Bender measuring devices (e.g. ISOMETER®) It is an RS-485 in-
terface with a specially developed protocol for Bender devices. On the BS bus, the trans-
mission of alarm messages takes priority over the transmission of all other messages. For
further information, refer to the BS bus manual (document number: D00278) at
www.bender.de/en/service-support/downloads
When using interface converters, a galvanic separation is required.
CAUTION
The compatibility of the BS bus and the BMS bus is restricted!
10.5.1 Master-slave principle
The BS bus works according to the master-slave principle. This means that the measuring
device operates as the MASTER, while all sensor devices operate as SLAVES. The master is
responsible for the communication necessary for the measuring function. It also provides
the required bus bias voltage for the operation of the BS bus.
10.5.2 Addresses and address ranges on the BS bus
Address 1 is assigned to the master. All sensor devices receive unique addresses starting
with address 2, assigned in consecutive order without gaps. In the event of a device failu-
re, a maximum gap of 5 addresses is permissible.
10.5.3 RS-485 specifications/cables
The RS-485 specification restricts the cable length to 1200 m and requires a daisy chain
connection. The number of devices on the BS bus is only limited by the BS bus master.
Use twisted pair, shielded cables for bus cabling. For example, cable type J-Y(St)Y
n x 2 x 0.8 is suitable. The shield must have a single-ended connection to earth. The BS
bus must be terminated at both ends with terminating resistors (120 Ω, 0.25 W). The ter-
minating resistors are connected in parallel to the terminals A and B. Some devices featu-
re integrated terminating resistors and can be activated or deactivated via the "R" button.
43
.
Device communication
10.5.4 Cable routing
The optimum cable routing for the BS bus is a double-terminated bus topology. The
length of the branch line is limited to 1 m. These branch lines do not have to be
terminated.
Bus topology examples:
2
2
2
3
3
2
2
2
Termination
Terminating resistor activated via switch on device (ON) or external ter-
1
Master
minating resistor between terminals A and B
2
Slave
Terminating resistor deactivated via switch on device (OFF)
Terminating resistor activated via switch on device (ON) or external ter-
3
Slave
minating resistor between terminals A and B
Only the first and last device may be terminated. Therefore, check all
devices.
CAUTION
10.6 Modbus RTU
Modbus RTU is implemented on the RS-485 interface. The data transmission is binary/se-
rial. Error-free and continuous data transmission must be guaranteed.
Measured values, messages and parameters are stored in virtual register addresses. Data
can be read out with a read command on the register address. With a write command,
data can be written into a register address. The register addresses of the individual meas-
ured values und parameters can be found in the "iso685-D Annex A" manual with the title
"ISOMETER® iso685 device family - Modbus settings" at
port/downloads.
2
2
2
2
2
2
www.bender.de/en/service-sup-
isoRW685W-D_D00178_04_M_XXEN/02.2019
3
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