Rockwell Automation 1770-HT8 SMART TRANSMITTER - Manual (Page 119)

Cable Length and Power Supply
Requirements
Appendix C
C-2
If the Terminal Blocks are to receive power via the cable, the power cable
length requirements explained below must be adhered to. If a Terminal
Block is too far from the Communications Controller to meet these cabling
requirements, an external 24 VDC power supply must be used. In any
given Smart Transmitter Interface installation, there may be some Terminal
Blocks receiving power from the Communications Controller via the cable,
and others receiving power from an external power supply.
Figure C.2 shows an example installation with a 1770-HT16 Terminal
Block close enough to the Communications Controller to receive power via
the cable and a 1770-HT8 Terminal Block requiring an external power
supply. If you are not using line 2 of the digital communications cable for
power to a particular Terminal Block, do not connect that wire to its
connector.
Figure C.2
Power Provided to a Terminal Block by an External Power Supply
90062
HT1
HT8
line 2 (power) not attached
to connector at HT8 end.
CONTROL LINES
HT16
+ -
+24 VDC
2
3
For all connectors and cables:
line 1 - shield
lines 3 to 17 - control
line 2 (power) attached to
connector at HT16 end
Important: No matter which method you choose, the total length of the
digital communications cable between the Communications Controller and
all the Terminal Blocks must not exceed 1000 feet (300 meters).
Maximum Power Cable Lengths
Figure C.3 gives the formula for determining maximum power cable
lengths and illustrates its application in a linear connection configuration of
four 1770-HT8 units. If fewer units are used, the variables that do not
apply (D, C etc.) are set to 0.