IMP Pumps NMTC module - Manual (Page 32)

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Figure 7: Example of Modbus network
Typically, only one master device is connected to the serial bus, and one or several slaves are
also connected to the bus. Slaves do not communicate with each other and will never transmit
data without receiving a proper request from the master device.
Up to 32 single load devices can be connected to one RS-485 Modbus
system without using a repeater. As this module is a 1/8 load device, up to 256 modules can be
connected to the bus. Repeaters can be used to extend the maximum
transmission distance and increase device count if needed.
7.3 Connection to Modbus
A screened, twisted-pair cable should be used. The cable screen must be connected to the COM
terminal and connected to safety ground at one point.
7.4 Speed, parity and address
By default, each device is set to 19200-E-1 (even parity), address 245.
Properly set registers in section “7.7 NMTC Configuration register block” to configure each device
before connecting it to existing network. Optionally power each device one by one, configuring the
settings before adding another one.
There are several ways to restore forgotten Modbus connection settings:
1) Configure the module over Web interface. See section 6 “Ethernet”.
2) Reset the module to factory defaults. See Mode 9, section “4.3 Module mode selection”.
3) If only address is unknown, Modbus “broadcast” (0x00) address can be used to write new
address. Use carefully as the value will be written to all connected devices.
7.5 Termination
NMTC module contains neither termination nor bias circuitry. RS-485 wiring should be externally
terminated if needed.
For short wiring and/or low baud rate, interface can operate without termination. However it is
recommended that termination (~150 ohm resistor) is added on both ends of bus wiring. There are
wiring length limits regarding to speed and termination:
Maximum speed [baud] Maximum cable length [m]
38400 1200, terminated cable
9600 1200
19200 500
38400 250
Master Slave 1 Slave 2 Slave 3
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