Bender ISOMETER iso1685DP-425 Manual page 68

9.1.2 BMS protocol
This protocol is an essential part of the Bender measuring device interface (BMS bus protocol).
Data transmission generally makes use of ASCII characters.
Interface data are:
• Baud rate: 9600 baud
• Transmission: 1 start bit, 7 data bits, 1 parity bit, 1 stop bit (1, 7, E, 1)
• Parity: even
• Checksum: Sum of all transmitted bytes = 0 (without CR and LF)
The BMS bus protocol works according to the master-slave principle. Only one master may
exist in each network. All bus devices are identified by a unique BMS address. The master cyc-
lically scans all other slaves on the bus, waits for their response and then carries out the corre-
sponding commands.
A device receives the master function by assigning bus address 1 to it.
9.1.3 BMS master
A master can query all measured values, alarm and operating messages from a slave. If bus
address 1 is assigned to a device, this device automatically operates as master, i.e. all addresses
between 1 and 150 are cyclically scanned for alarm and operating messages via the BMS bus.
If the master detects incorrect answers from a slave, the fault message "Fault RS-485" will be
output via the BMS bus.
The following fault causes may exist:
• Addresses are assigned twice
• A second master exists on the BMS bus
• Interference signals occur on the bus lines
• A defective device is connected to the bus
• Terminating resistors are not activated or connected
9.1.4 Commissioning of an RS-485 network with BMS protocol
• Interconnect terminals A and B of all bus devices in one line.
• Switch the terminating resistors on at the start and the end of the RS-485 network. If a
device at the end of the bus is not terminated, connect a 120 Ω resistor to terminals A
and B.
• Switch the supply voltage on.
• Assign the master function and address 1 to a bus-capable device.
• Assign addresses (2...90) to all other bus devices in consecutive order.
68 isoxx1685Dx-x25_D00272_07_M_XXEN/09.2022
Device communication