How to Use a Modbus/TCP Module as Remote I/O with a Honeywell Controller – 8400640

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

The Acromag 967EN has eight analog 4-20mA inputs. It is a Modbus/TCP slave; thus 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.

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

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 967EN and read directly as scaled values. If the scaling in the 967EN is correct, no further scaling will be needed in the HC-900. Signed integers values are ranged 1 – 20,000 counts across the 4-20mA input span; so scaling to user-friendly engineering units has to be done on the HC-900i.

The Acromag 967EN setup is done via web browser, therefore software is not needed. Typical input and scaling setup changes made in the web browser download and execute quickly; on the order of seconds. Input configuration changes do not require an ON-OFF power cycle.

Acromag’s sample time is 8mS; thus the Acromag data tends to be noisier than the same signal wired into the HC-900. Consequently, 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. However, it 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.


The implementer has:

  • Familiarity configuring the HC-900 using Honeywell’s Hybrid Control Designer (HCD) software
    • HC-900’s development software
    • function block configuration
    • loading configurations, etc.
  • Familiarity with ethernet network connections (devices on the same subnet)
  • Familiarity with engineering unit scaling of industrial signals


From its introduction in 2002, the HC-900 has supported Modbus/TCP. Honeywell HC-900’s added Serial Modbus Master functionality in June 2003. This note covers Modbus/TCP connections, only.

Furthermore, we strongly advise using the latest hardware firmware revision for communications. Firmware updates are available at no cost on Honeywell’s public website.


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