4 Wiring
Varec, Inc. 21
RTD Wiring
To use a 4-wire RTD with the 2920 FTT, connect one of the 2 parallel leads to RTD A and leave
the other lead disconnected, as shown in the following figure.
EIA-485 MODBUS The 2920 FTT uses a 2-wire EIA-485 hardware
interface to communicate with the MODBUS master.
EIA-485 is a high speed differential communications
network which allows up to 32 devices to operate
on one network. The 2920 FTT and MODBUS master
communicate over a maximum distance of 4000 feet
(1230 meters).
The EIA-485 communications interface is compatible
with the RTU 8130 (along with any MODBUS–
compatible device) and other MODBUS masters.
This connection can typically be accomplished with
two (2) shielded, twisted pairs of 18 AWG wires.
Mark/Space Two shielded, twisted pairs of 18 AWG wire are
recommended. This option provides compatibility
with the Varec Tank Scanning Unit (TSU), RTU 8130
or Tank Gate interface devices. The 2920 FTT can
then be multi-dropped on the same Mark/Space
data highway as the Varec 1800/1900 MWT/4000
ATT /2900/2910 FTT or 4100/4200 HIU/MFT devices.
Up to 50 devices can be connected. Mark/Space
Communications requires 48 VDC.
L&J TankWay L&J TankWay is a 4-wire system which includes
power and ground connections. Fifty or more
devices can be connected on the field
communication bus.
The network is typically installed using two (2)
shielded, twisted pairs of wires.
Communications Protocol Description
3 Wire RTD
A
B
C
A
B
C
Disconnected
4 Wire RTD
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