Instruction manual

15
INM4840-5 Nov 2007
15
4.12 Cabling recommendations
4.12.1 HART signals cable
HART signal cables are used to connect the BPHM64 backplane with
a HART connection unit or an IS backplane. This connection is made
with a 20-way ribbon cable.
20-way HART ribbon cables
Standard lengths of this 20-way cable are available from MTL using
the part number HM64RIB20-x.x, where the x.x denotes lengths
between 0.5 and 6.0 metres.
4.12.2 Backplane interface bus
The interface bus provides a connection between a single MTL4841
communications module and up to 16 MTL4842 HART interface
modules/backplanes. This connection is made via a ten-way ribbon
cable.
10-way HART ribbon cables
Standard lengths of this 10-way cable are available from MTL using
the part number HM64RIB10-x.x, where the x.x denotes lengths
between 0.5 and 4.0 metres.
Note: The ribbon cables connecting the HCUs or
MTL4000 backplanes to the BPHM64 and interface Bus
cables are not suitable for running outside of the cabinet
or under control room floors. Generally this will not be necessary, as
to maintain integrity, each group of cabinets would interface their
HART signals via independent MTL4841 communication modules.
Each MTL4841 module is linked by standard communications cables
suitable for RS485 data at 9.6, 19.2 or 38.4 kbaud.
4.12.3 RS485 connection
The RS485 physical layer
Most PCs running instrument maintainance software masters use
RS232C for serial communication; however, this interface type does
not allow the communications network to extend beyond 10 to 20
metres in length.
MTL have chosen to use an RS485 serial interface, which includes
tri-state operation, and allows network lengths of up to 1000
metres and operates with data rates between 1200 baud and
39.4 kbaud when used with the MTL4841. RS485 also allows the
simple parallel connection of a number of units.
Note: When an RS232 master is used, a data converter
must be inserted.
The RS485 serial interface standard
For the purposes of this document RS485 can be regarded as a 2-
wire, half-duplex, differential, multi-drop (32 nodes), communications
standard. The RS485 standard defines the characteristics of the
drivers and receivers that can be connected to the bus. It does not
define the cabling or connectors used, nor does it specify a particular
data rate or signal format.
Terminations
RS485 interfaces should ideally be provided with a ‘matched’
termination to prevent reflections and ringing of the signal on the bus
cabling. The termination is usually a simple resistive terminator, with
an impedance that matches the characteristic of the cable - this will
normally be in the range of 100 - 200Ω.
Biasing
When no communication is taking place, the bus is in an undefined,
floating state and, consequently, noise on the bus may be decoded
as real characters. Well-written software 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.
MTL4841 communication modules from MTL have no built-in facility
for terminating or biasing the network; as this is often provided by
the RS232/RS485 converter.
4.12.4 RS485 2-wire interconnection
The MTL4840 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 MTL4841 communications module connects to the maintenance
workstation via a 2-wire link, accessible from the RS485 Port
connections on the backplane. Up to 31 MTL4841 modules (slaves)
can be connected to a single Master maintenance workstation, as
shown in Figure 4.22.
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 1 on the backplane - see Figure 4.23.
Connect the cable screen to terminal C.
If multiple backplanes are to be connected, use Port 2 to loop
to Port 1 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.
4.12.5 Data converters
Many RS232/485 data converters are available but the one that has
received the most extensive testing and which is recommended for
use with the MTL4841 is the Westermo MA45 RS232/485.
Follow the manufacturers instructions for setting up the RS232
connection to the controlling PC. The RS485 connection should be
configured to suit the MTL4841 settings, which are discussed in
Section 5 of this manual.
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•
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Connector type 20-way DIN41651 bump polarised
Cable 0.05 inch pitch standard ribbon
Maximum length 15m/50ft
Connector type 10-way DIN41651 bump polarised
Cable 0.05 inch pitch standard ribbon
Maximum length 4m/13ft (between modules 1 and 16)
Figure 4.22 - 2-wire bus
Slave 1
Slave 31
Master
T T
Command
from Master
Response
from Slave
Figure 4.23 - RS485 port connectors
ABC ABC
RS485
PORT 1
RS485
PORT 2