How to Use Acromag’s 967EN Modbus/TCP Module as Remote I/O with Honeywell HC-900 Application Note

Honeywell HC-900 with Acromag 967EN Modbus/TCP Module as Remote I/O

How to Use Acromag's 967EN Modbus/TCP Module as Remote I/O with Honeywell HC-900


The Acromag 967EN has 8 analog 4-20mA inputs. It is a Modbus/TCP slave, ideal for use as remote I/O with a Honeywell HC-900 Hybrid Controller (Process Automation Controller) that runs as a Modbus/TCP master over a standard Ethernet network connection.

The Acromag 967EN provides data in either floating point or signed integer formats.

Floating point values can be scaled to engineering units in the Acromag and read directly as scaled values. If the scaling in the Acromag is correct, no scaling is needed in the HC-900. Signed integers values are ranged 1 to 20,000 counts across the 4-20mA input span; so scaling to user-friendly engineering units has to be done on the HC-900i.

Acromag setup is done via web browser, no software is needed. Typical input and scaling setup changes made in the web browser are downloaded and executed quickly, on the order of seconds. An ON-OFF power cycle is not required for input configuration changes.

Acromag’s sample time is 8mS, so the Acromag data tends to be noisier than the same signal wired into the HC-900. The solution is to let the Acromag average multiple samples. Start with 20 as a number to average with.

The 200 ohm 4-20mA input resistance will not unduly load a loop powered circuit, but will inhibit HART communications (HART requires 230 ohms, minimum).

Acromag’s i2o remote messaging function writes values only to other Acromag products. Hence, i2o is not applicable for use with the HC-900.


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