DRAFT - 11 December 2014
DRAFT - 11 December 2014
INM MTL4850_54 Rev4
21
DRAFT - 11 December 2014
should discard most of these characters, but the system may be further protected by biasing
the bus to a known state and thereby preventing the reception of ‘false’ characters.
MTL4850 multiplexer modules from MTL have no built-in facility for terminating or biasing the
network; as this is often provided by the RS232/RS485 converter.
4.12.1 RS485 2-wire interconnection
The MTL4850 system uses a 2-wire connection as shown below:
This 2-wire bus uses the same pair of wires to transmit queries from the master and responses
from the slave.
The MTL4850/54 multiplexer module connects to the maintenance workstation via a 2-wire link,
accessible from the RS485 Port connections on the backplane. Up to 63 MTL4850/54 modules
(slaves) can be connected to a single Master maintenance workstation, as shown in Figure 4.20.
Connect the bus to the backplane as follows using a screened cable, for example Belden 8132 or 9841.
• Connect the two cores of the RS485 cable to screw terminals A & B of Port A on the
backplane - see Figure 4.21.
• Connect the cable screen to terminal C.
• If multiple backplanes are to be connected, use Port B to loop to Port A of the next
backplane. To avoid spending time in troubleshooting, it is suggested that the same
colour convention is used for all connections to backplane terminals A and B, e.g. red
to A, black to B.
NOTE
No damage will occur if the signalling lines are connected with the wrong polarity.
The system will not operate in this case but all that has to be done is to reverse the two
connections to make it operate correctly.
4.12.2 Data converters
Most PCs used to run instrument maintenance software masters do not have a built-in RS485
interface so the following conversion options should be considered.
• Ethernet is in widespread use in modern installation and an Ethernet to RS485 gateway
enables the workstation to be located at greater distances and use the installed wiring
to pass data effectively across a site when necessary, without the need for additional
wiring and repeaters to support the RS485 protocol. MTL can provide a suitable
gateway on request – model number ET-485.
• USB to RS485 converters are convenient for local connection to a laptop computer
when carrying out maintenance; however, this interface type does not allow the
communications network to extend beyond 10 to 20 metres in length.
• RS232 to RS485 data converters are also available for use with earlier installations
where RS232 is provided on the PC’s serial COM port.
In all cases, follow the manufacturer’s instructions for the installation and operation of the
appropriate converter.
Figure 4.20
2-wire
bus system
Figure 4.21
RS485 port
connectors
