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- Analog Input Module
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Product Overview
The Honeywell 2MLF-AC4H MasterLogic-200 Analog Input Module is a four-channel current-input module with HART communication support for compatible Honeywell MasterLogic PLC systems. It acquires analog signals from field instruments and converts them into digital values that the PLC program can process.
The module is particularly useful when a control system needs both conventional current measurement and access to digital information from HART-enabled instruments. For example, a chemical dosing skid may use 4–20 mA transmitters to measure flow, pressure, and tank level. With the 2MLF-AC4H, the PLC can acquire these measurements while communicating with supported smart instruments through the same analog wiring.
The module accepts 0–20 mA and 4–20 mA input signals. HART communication is available when a channel is configured for the 4–20 mA range; it is not supported in the 0–20 mA range. This distinction matters when integrating smart transmitters because selecting the correct input range affects both analog measurement and access to digital instrument data.
Each of the four channels can be configured for its required input range, and the converted values can be represented in different digital formats. The module also supports high-resolution conversion, with a specified resolution of 1/64,000 and accuracy of ±0.1% at an ambient temperature of 25°C.
For maintenance and commissioning teams, the 2MLF-AC4H combines process-variable acquisition with HART instrument access in one PLC module. It is suited to applications where operators need current measurements for control while maintenance personnel need additional instrument information for diagnostics and configuration.
Technical Specifications
| Parameter | Specification |
|---|---|
| Manufacturer | Honeywell |
| Model / Part Number | 2MLF-AC4H |
| Product Family | MasterLogic-200 |
| Product Type | HART Analog Input Module |
| Number of Analog Input Channels | 4 |
| Input Signal Types | Current input |
| Analog Input Ranges | 0–20 mA, 4–20 mA |
| Input Resistance | 250 Ω |
| HART Support | Yes, on channels configured for 4–20 mA |
| HART Communication Mode | Monodrop |
| HART Master Role | Primary master only |
| Maximum Resolution | 1/64,000 |
| Resolution at 4–20 mA | 250 nA |
| Resolution at 0–20 mA | 312.5 nA |
| Accuracy at 25°C | ±0.1% or less |
| Accuracy at 0–55°C | ±0.25% or less |
| Conversion Time | Maximum 100 ms per 4 channels |
| Maximum Analog Input | ±30 mA |
| Digital Output Data Formats | Signed, precise, and percentile values |
| Input-to-PLC Power Isolation | Photocoupler isolation |
| Channel-to-Channel Isolation | Not provided |
| Terminal Connection | 18-point terminal |
| Occupied I/O Points | 64 fixed points or 16 non-fixed points |
| Internal Current Consumption | 340 mA at 5 V DC |
| Operating Ambient Temperature | 0–55°C |
| Storage Temperature | −25 to +70°C |
| Dimensions | 113 × 27 × 98 mm |
| Weight | 0.145 kg |
Product Features
- Four current-input channels: Acquires up to four analog current signals through one PLC module.
- HART instrument communication: Supports digital communication with compatible HART field devices when the input range is configured for 4–20 mA.
- Two selectable current ranges: Supports 0–20 mA and 4–20 mA measurement, with channel-specific range settings.
- High-resolution conversion: Provides a specified resolution of 1/64,000 for detailed process-variable representation.
- Specified conversion accuracy: Achieves ±0.1% accuracy at 25°C and ±0.25% over the stated 0–55°C ambient range.
- Selectable digital data formats: Makes measurement values available in signed, precise, or percentile formats according to the configured application.
- PLC power isolation: Uses photocoupler isolation between the analog input terminals and PLC power, while the input channels themselves are not isolated from one another.
- Software-based configuration: Supports input-range and operating-parameter settings through the PLC’s I/O parameter configuration.
- Integrated diagnostic support: Allows input values and supported module operating information to be examined through the PLC engineering environment.
Working Principle
The 2MLF-AC4H measures current signals from field instruments and converts them into digital values for use by the PLC program. Each input channel receives a current signal, measures it within the configured range, and makes the resulting value available in the selected data format.
A typical signal path is:
Field Transmitter → 4–20 mA Current Loop → 2MLF-AC4H → Digital Measurement Value → PLC Control Program
Consider a pressure transmitter installed on a process pipeline. As pressure changes, the transmitter adjusts its output current between 4 mA and 20 mA. The 2MLF-AC4H converts that current into a digital measurement, which the PLC can use to display pressure, regulate a valve, or generate an alarm when the configured limit is exceeded.
The module supports three digital representation formats. Signed values use a range of −32,000 to 32,000. Precise values represent the 4–20 mA range as 4,000 to 20,000 and the 0–20 mA range as 0 to 20,000. Percentile values use a range of 0 to 10,000. The PLC application must interpret the selected format correctly when converting raw values into engineering units.
HART communication adds a second layer of information to the 4–20 mA loop. The analog current carries the primary process measurement, while digital HART messages allow supported instrument information to be exchanged over the same wiring. The module operates in monodrop mode and supports the primary-master role; HART communication is unavailable when a channel is configured for 0–20 mA.
The module’s maximum conversion time is 100 ms for four channels. This makes it suitable for many process-measurement tasks, but the actual control-loop response must also account for PLC scan time, transmitter response, filtering, and the application requirements.
Role in the MasterLogic-200 PLC Architecture
The 2MLF-AC4H occupies the analog signal-acquisition layer between field instrumentation and the PLC control program. It converts current signals into digital process data and supports HART communication with compatible instruments.
Field Transmitters → 2MLF-AC4H Analog Input Module → MasterLogic-200 CPU → Control Logic → Output Modules and Field Devices
| System Component | Function |
|---|---|
| Honeywell 2MLF-AC4H | Acquires four current-input signals and supports HART communication |
| MasterLogic-200 CPU | Executes the PLC program and processes acquired measurements |
| Analog Field Transmitters | Convert physical variables into current signals |
| 4–20 mA Current Loops | Carry process measurements to the input module |
| HART-Enabled Instruments | Exchange supported digital instrument information over the current loop |
| PLC Engineering Software | Configures input ranges, data formats, and module parameters |
| Operator Interface | Displays process measurements and instrument status when configured |
| Analog Output Modules | Supply current or voltage commands to compatible actuators |
| Digital I/O Modules | Acquire discrete status signals and control digital devices |
| Control Cabinet and Terminal Wiring | Houses the PLC hardware and connects field circuits |
The input module does not independently execute control logic or operate final control elements. Its measurements are processed by the CPU, and any subsequent control action depends on the application program and the connected output hardware.
Industrial Applications
| Application | Typical Use |
|---|---|
| Chemical Processing | Acquiring pressure, flow, and level measurements from current-output transmitters |
| Water and Wastewater Treatment | Monitoring process instrumentation and communicating with supported smart instruments |
| Pharmaceutical Production | Collecting process measurements for programmed control and monitoring |
| Oil and Gas Facilities | Acquiring analog measurements from compatible field transmitters |
| Food and Beverage Processing | Monitoring flow, pressure, and other process variables |
| Boiler and Utility Systems | Receiving current signals from pressure and temperature transmitters with current outputs |
| Pumping Stations | Monitoring discharge pressure, tank level, and flow |
| Tank Farm Automation | Acquiring level and pressure measurements from field instruments |
| Process Skids | Combining several analog measurements within a compact PLC configuration |
| Instrument Maintenance Systems | Accessing supported HART information while retaining analog process measurement |
Installation and Maintenance
- Verify the module reference: Confirm that the installed hardware is 2MLF-AC4H and that the application requires four current-input channels with HART support.
- Check the PLC compatibility: Verify that the CPU, base, and installed module arrangement support the 2MLF-AC4H within the MasterLogic system configuration.
- Identify each field transmitter: Record the instrument connected to each channel, including its output range and whether HART communication is required.
- Configure the input ranges: Set each channel to 0–20 mA or 4–20 mA as appropriate. Use 4–20 mA when HART communication is required.
- Inspect the current-loop wiring: Check polarity, terminal assignments, loop power arrangements, and wiring integrity before connecting the field circuits.
- Check grounding and isolation: Review the installation’s grounding and shielding arrangements, remembering that the module does not provide isolation between individual input channels.
- Configure the digital data format: Select signed, precise, or percentile representation according to the PLC program and required engineering-unit scaling.
- Verify the HART configuration: Confirm that the connected instrument supports HART, the channel is set to 4–20 mA, and the communication arrangement follows the module’s monodrop and primary-master limitations.
- Check the power budget: Account for the module’s 340 mA consumption from the 5 V DC backplane supply when assessing the installed PLC hardware load.
- Validate the analog readings: Compare the PLC values with known transmitter outputs or a suitable current source to verify scaling and channel assignment.
- Investigate abnormal measurements: If a reading is unstable or incorrect, check the transmitter, loop current, wiring, configured range, data format, and module diagnostics before replacing hardware.
- Record commissioning results: Document the channel assignments, input ranges, HART settings, data formats, calibration checks, and any maintenance actions.
Compatible System Components
| Component | Function |
|---|---|
| Honeywell MasterLogic-200 CPU | Executes control logic and processes analog measurements |
| Compatible MasterLogic Base | Provides the module mounting and electrical interface |
| 4–20 mA Pressure Transmitter | Supplies a current signal representing process pressure |
| 4–20 mA Flow Transmitter | Supplies a current signal representing process flow |
| 4–20 mA Level Transmitter | Supplies a current signal representing liquid level |
| HART-Enabled Field Instrument | Provides analog measurement and supported digital instrument data |
| 24 V DC Loop Supply, Where Required | Powers compatible two-wire transmitter loops |
| Shielded Instrument Cable | Connects field transmitters to the analog input terminals |
| PLC Engineering Software | Configures the module and monitors input values |
| Operator Interface | Displays measurements and supported instrument information |
Recommended Related Models
| Model | Product Type | Main Difference |
|---|---|---|
| Honeywell 2MLF-AC4H | HART Analog Input Module | Four current-input channels with HART support |
| Honeywell 2MLF-AC8A | Analog Input Module | Eight current-input channels without the AC4H HART feature set |
| Honeywell 2MLF-AD8A | Voltage/Current Analog Input Module | Eight channels for supported voltage and current signals |
| Honeywell 2MLF-AD16A | Voltage/Current Analog Input Module | Sixteen channels for supported voltage and current signals |
| Honeywell 2MLF-AD4S | Voltage/Current Analog Input Module | Four voltage/current input channels |
| Honeywell 2MLF-AV8A | Voltage Analog Input Module | Eight voltage-input channels |
| Honeywell 2MLF-DC4H | HART Analog Output Module | Four current-output channels with HART support |
| Honeywell 2MLF-DC4A | Analog Output Module | Four current-output channels without the AC4H input function |
Key Advantages
- Acquires four current signals in one MasterLogic-200 module.
- Supports HART communication over compatible 4–20 mA loops.
- Provides a specified resolution of 1/64,000.
- Offers accuracy of ±0.1% at 25°C under the specified conditions.
- Supports selectable digital formats for PLC processing.
- Provides photocoupler isolation between input terminals and PLC power.
Technical FAQs
Can HART communication be used when a channel is configured for 0–20 mA?
No. HART communication is supported when the channel is configured for 4–20 mA. A channel using the 0–20 mA range can acquire the analog current signal, but the module does not support HART communication in that range.
Are the four input channels electrically isolated from one another?
No. The module provides photocoupler isolation between the analog input terminals and PLC power, but it does not provide isolation between individual input channels. The field-loop design should account for this when connecting transmitters with different grounding or potential conditions.
What does the 100 ms conversion specification mean for a control application?
The module’s maximum conversion time is 100 ms for four channels. When designing a control loop, engineers should also account for the transmitter response, PLC scan time, configured filtering, and the required process response.
Why might the PLC display a different value from the transmitter’s expected measurement?
Check the selected input range, raw-data format, engineering-unit scaling, and current-loop wiring. For HART applications, also verify that the channel is configured for 4–20 mA and that the instrument communication arrangement is supported.