AMM42T | Yokogawa | 2-wire Transmitter Input Multiplexer Module

Description

The Yokogawa AMM42T, also cataloged as the AMM42T 2-wire Transmitter Input Multiplexer Module, operates as a dedicated hardware component for localized current loop regulation within CENTUM Distributed Control System networks. This module interfaces directly with downstream 2-wire field transmitters, distributing a 23.5 to 24.5 V DC loop supply while simultaneously multiplexing the incoming -4 to 20 mA DC analog current telemetry across non-isolated measurement channels on a 1-second data update cycle.

Hardware Specifications

4-20 mA HART Loop Protocol and Channel-to-Channel Isolation

The AMM42T features a non-isolated architecture requiring strict grounding discipline, as it provides zero isolation between the control system and the field, or between individual measurement paths. Optimized for direct 4-20 mA HART loop protocol field interfaces, each multiplexed node maintains a base 250 Ohm input resistance to transform current inputs into proportional internal voltages, which can be modified to 70 Ohm when paired with external intrinsic safety isolation barriers. The integrated transmitter power supply output circuit delivers up to 60 mA per loop to power field hardware, driving the shared analog-to-digital conversion matrix to refresh host register values concurrently every 1 second.

Frequently Asked Questions

Q: What are the installation conditions and circuit risks associated with the non-isolated design of the AMM42T?

A: Because the module provides no system-to-field or channel-to-channel isolation, all connected current loops share a common electrical ground reference. Any ground potential differences or high-voltage transient faults on one field wire will directly affect adjacent channels and can propagate into the host DCS backplane, necessitating centralized external isolation or single-point grounding schemes.

Q: How must the internal input resistance parameter be altered when routing field signals through an intrinsic safety barrier?

A: When connecting the AMM42T current inputs to safety barriers, the default 250 Ohm input resistance must be changed to the 70 Ohm specification via hardware termination parameters. This resistance drop offsets the added impedance of the barrier, keeping the total loop loop voltage and current parameters within correct operational and safety tolerances.

Field Installation Guidelines

• System Power Budget Validation: Verify that the primary 5.0 V DC power supply in the host I/O nest can handle the module's maximum logic current consumption of up to 4 A under full load.

• Ground Potential Equalization: Bond all field transmitter chassis grounds to a single, centralized reference earth bar before making terminal connections to prevent ground loop currents from destabilizing the non-isolated measurement matrix.

• Output Current Limit Constraints: Limit external loop loads to ensure the total current draw on the internal transmitter power supply stays below the 60 mA maximum output ceiling per channel.

• External Overcurrent Mitigation: Install external fast-acting fuses on the field loop wiring to protect the internal conversion resistors against field shorts that exceed the 40 mA allowable input current threshold.