Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 19)

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AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 19
For systems conforming to AANSI/ISA non-incendive hazardous area specifications, the T8801
replaces the fuses with resistors to limit the available power. In this case, the field zener diodes will
only dissipate 100mW on a full short circuit to zero volts.
T8423 35-120V dc Digital Inputs
The T8423 is similar to the T8403, but is rated for any supply voltage from the upper range of the
T8403 up to 140 volts dc maximum. It has an internal impedance to earth of 60Kohm, so when two
modules are paired, the impedance will drop to 30Kohm. Therefore, volt-free contact inputs should use
zener diodes, which should be specified at approximately one third of the supply voltage. The
thresholds should then be positioned around open and closed voltages of two-thirds and one-third of
the supply voltage, as for 24Vdc inputs.
It can be used with the T8821 FTA, which has neon power supply indicators designed for 120V dc.
This requires I/O cables TC-201, 203, 501 or 503.
The T8423 can also be terminated at fused terminals (providing the same functions as T8821), using
I/O cables TC-202, 204, 502 or 504.
Field Supply
60K (60K)
Module (Module)