Rockwell Automation 1770-HT8 SMART TRANSMITTER - Manual (Page 118)

Appendix
C
B
C-1
Cable Length and Power Supply Requirements
This appendix discusses cabling requirements between the Terminal
Blocks and the Communications Controller for control signals and 24VDC
power, explains how to determine when a separate 24VDC power supply is
needed for a Terminal Block and describes how to connect it. It also
provides further details about supplying 4-20mA loop power to 2 wire
HART field devices from a Terminal Block.
The digital communications cable carries both the control signals and
power for the Terminal Blocks. To carry the control signal clearly, the
length of cable between the Communications Controller and all Terminal
Blocks added together must not exceed 1000 feet. How the modules are
connected (star, linear or a combination) does not change this requirement.
The 1000 foot maximum cable length applies only to the control signals.
When power (24 VDC) is being supplied over the cable to the Terminal
Blocks, the maximum cable length is further restricted because of the
power drawn by the Terminal Blocks. The exact length allowed depends
upon the number of Terminal Blocks being powered, the gauge of wire
being used and the wiring configuration.
The simplest cabling scheme between the Terminal Blocks and
Communications Controller (Figure C.1.) is one where the Smart
Transmitter Interface products can be placed close enough together for the
cable to carry both control signals and power. See Chapter 2, Figure 2.3 to
Figure 2.5 for examples.
Figure C.1
Cable Carrying Control Signals and Power
90061
HT1 HT8
For all plugs and cables:
line 1 Ć shield
line 2 Ć power
lines 3 to 17 Ć control
CONTROL LINES
HT16
Cabling Requirements
Between the Communications
Controller and the Terminal
Blocks