KROHNE MFC 010 C Converter - Manual (Page 12)

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MFC010 Interface Manual
3.4 Connection to the Modbus Bus
The MFC010 is designed to be connected as a Slave device onto the 2-wire bus implementation of the
Modbus physical layer definition. In this configuration the receiver and transmitter lines for each device
are connected together, Transmitter A to Receiver A and T ransmitter B to Receiver B, and operated in
Half Duplex mode, where the master transmits a request and only after receiving it does the nominated
slave device transmit a reply. When not responding to a direct request from the Master device, the Slave
devices remain passive, monitoring the bus and aw aiting a suitable request from the Master device. In
addition to the A and B signal lines the bus MUST include a “Common” signal line to act as a ground
reference point for the A and B signals.
 The m aster bus m ust be t erminated at i ts phy sical end poi nts by suitable termination networks
connected between the A (D0) and B (D1) signal lines. When not using bus-biasing resistors, see next
paragraph, each termination network may consist of a single 150 O hm, 0.5W resistor. H owever, when
bus-biasing resistors are required, a more suitable termination network would consist of a 1nF capacitor in
series with a 120 Ohm, 0.25W resistor. NB It is common that the Host system “Master” is physically at
one end of the bus, so one of the termination resistors is fitted at its terminals, but it should be r ealised
that this is not always the case and care should be taken to ensure that the termination network is at the
physical end of the bus. In a point-to-point configuration, when only one Slave device is fitted to the bus,
then the terminating networks can simply be situated at the connecting terminals of the master and slave
devices.
 Some slave devices require that Bus-Biasing resistors are fitted to ensure that the bus is in a defined
state w hen none of t he t ransmitting devices are active. The MFC010 does NOT require Bus-Biasing
resistors to be f itted but is compatible with their presence on t he bus if one or more of the other slave
devices on the bus require them to be fitted, as long as they comply with the Line Polarization
requirements of the Modbus specification.
 In a m ultidrop bus c onfiguration t he s lave dev ices ar e c onnected t o t he m ain bus c able by branch
connections at intervals along the length of the main bus. T he branch connections, Derivations as they
are termed in the Modbus specification, must be l ess than 20m in length from the main bus cable to the
slave device. Some slave devices permit direct connection to the main bus, known as “Daisy Chaining”,
in some cases by providing extra terminals and c able ac cess poi nts. H owever, as i ndicated i n t he
previous s ections, ac cess t o t he t erminal c ompartment of t he M FC010 i s limited; therefore i t i s not
practical to directly connect the MFC010 to the main bus. Instead, the installation should utilise a short
branch c onnection. I f a greater length of cable is required between the MFC010 device and the main
Modbus bus, the user should install a suitable RS485 repeater between the MFC010 and the bus (refer to
the diagram below).