DRAFT - 11 December 2014
DRAFT - 11 December 2014
20
DRAFT - 11 December 2014
INM MTL4850_54 Rev4
4.11 Cabling recommendations
4.11.1 HART signals cable
HART signal cables are used to connect the CPH-SC backplane with a HART connection unit or
an IS backplane. This connection is made with a 20-way ribbon cable.
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.
Connector type
20-way DIN 41651 bump polarised
Cable
0.05 inch pitch standard ribbon
Maximum length
15m / 50ft
4.11.2 Backplane interface bus
The interface bus provides a connection between two CPH-SC16 or CPH-SC16(R) backplanes
to provide an additional 16 modules to the MTL4850/54. This connection is made via a 34-way
ribbon cable between connector P2 on the backplane carrying the multiplexer and connector P1
on the backplane carrying only the additional modules as shown in Figure 4.19.
Standard lengths of this 34-way cable are available from MTL using the part number
HM64RIB34-x.x, where the x.x denotes length, as shown below.
Connector type
34-way DIN 41651 bump polarised
Cable
0.05 inch pitch standard ribbon
Length
0.2, 0.5, 1.0 or 2.0 metres
4.12 RS485 connection
The RS485 physical layer
Most PCs running instrument maintenance software masters use RS232 or USB 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 and
38k4 baud when used with the MTL4850 or MTL4854. 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 - 200
Ω
.
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
Figure 4.19
Redundant power
supply wiring
for backplanes
MTL4850/54
