NFAI135-S50

The Yokogawa NFAI135-S50 is an 8-channel isolated analog input module for receiving 4 to 20 mA signals from field transmitters. It provides individual channel isolation, overcurrent protection, and HART communication support. Its 10 ms data refresh cycle and 100 ms input step response time define its signal acquisition characteristics for process control applications.

Input Signal Processing and Channel Isolation

The NFAI135 accepts 4 to 20 mA analog signals through eight isolated input channels. Channel isolation separates the electrical input paths, while the specified 500 V AC withstand voltage applies both between input/output and the system and between individual channels, each for one minute.

The module provides overcurrent protection and supports an allowable input current of 25 mA. Its input resistance depends on the module power state: 250 ohm when powered on and at least 500 kohm when powered off. This distinction is relevant when checking transmitter loop loading and circuit conditions during power-down.

HART Communication and Transmitter Supply

HART communication is supported, allowing compatible field devices to exchange digital information over the analog current loop. The module also provides a transmitter power supply with a specified output range of 20.2 to 29.3 V. The actual loop configuration should account for transmitter requirements, wiring resistance, and the available supply voltage.

Accuracy and Response Characteristics

The specified accuracy is +/-0.1% of full scale, with a maximum temperature drift of +/-0.01% per deg C. The 10 ms data refresh cycle describes how frequently input data is updated, while the 100 ms input step response time describes the response to a change in the input signal. These are separate characteristics and should not be treated as interchangeable timing specifications.

Electrical Characteristics and Power Consumption

The module has a maximum current consumption of 360 mA at 5 V DC and 450 mA at 24 V DC. These values should be considered separately when checking the power budget of the I/O assembly. The specified weight is 0.3 kg.

Technical Specifications

Parameter	Specification
Model	NFAI135-S50
Base Model	NFAI135
Brand	Yokogawa
Product Type	Analog Input Module
Number of Input Channels	8
Channel Configuration	Isolated channels
Input Signal	4 to 20 mA
Allowable Input Current	25 mA
Overcurrent Protection	Provided
Input Resistance (Power ON)	250 ohm
Input Resistance (Power OFF)	500 kohm or more
Accuracy	+/-0.1% of full scale
Data Refresh Cycle	10 ms
Input Step Response Time	100 ms
Temperature Drift	Max. +/-0.01%/deg C
Withstanding Voltage	500 V AC for 1 minute
Withstanding Voltage - Input/Output to System	500 V AC for 1 minute
Withstanding Voltage - Channel to Channel	500 V AC for 1 minute
Transmitter Power Supply	20.2 to 29.3 V
Maximum Current Consumption (5 V DC)	360 mA
Maximum Current Consumption (24 V DC)	450 mA
External Connection	Pressure clamp terminal, MIL connector cable
Weight	0.3 kg

Model and Suffix Identification

The NFAI135 model designation identifies an 8-channel, 4 to 20 mA analog input module with isolated channels. Yokogawa documentation defines the suffix codes as follows:

Code	Description
-S	Standard type
5	With no explosion protection
0	Basic type

For the specified NFAI135-S50, the ordering configuration is standard type, without explosion protection, and basic type. The -S designation should not be confused with the -H version, which is the HART communication configuration.

Frequently Asked Questions

Q: Does the NFAI135-S50 support HART communication?

A: The -S suffix identifies the standard version. Yokogawa specifies -H for the HART communication version, so HART capability should not be assumed for the NFAI135-S50.

Q: What is the difference between NFAI135 and NFAI141?

A: Both are 4 to 20 mA analog input modules, but the NFAI135 has eight isolated input channels, whereas the NFAI141 has 16 non-isolated input channels. Channel isolation and channel count are important selection criteria.

Q: What should be checked when replacing an NFAI135-S50?

A: Verify the complete model and suffix, input channel configuration, terminal or connector arrangement, and explosion-protection requirements. Also check the installed terminal block and cable configuration before ordering a replacement.

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