What is DE communication protocol of Filed Transmitter?

It is a kind of proprietary communication protocol used to communicate with Honeywell transmitters, DE means digitally enhanced protocol

The DE protocol modulates the 4-20mA signal where a logic 1 is 20mA and a logic 0 is 4mA when communicating in DE mode. It is a very robust means of communicating data digitally, but slow.

One selects (configures) either DE mode or 4-20mA mode for communicating with the receiver. One or more process variables (primary, secondary, etc.) can be communicated digitally in DE mode at fairly slow rate, about 2-3 PV values per second. The alternative 4-20mA mode uses DE only for configuration or troubleshooting, where the instrument’s process variable (PV) is communicated with a conventional 4-20mA signal but a DE modem or handheld communicator can be used on occasion to interrogate DE smart device.

The display of the software/modem or handheld warns the user to “put the loop in manual (mode)” when first connecting to the device because DE mode communication will oscillate the 4-20mA signal used for the process variable and cause continuing upsets if the loop is left in automatic mode.

The only receiver devices that use DE are Honeywell’s earlier DCS systems.

Field instruments using DE are the 1987 - 2015 era Honeywell ST3000 series pressure, temperature and magnetic flow transmitters and Honeywell’s current Smartline transmitters which have an option card for DE communications. Azbil (the former Yamatake) continues to the DE protocol for its pressure and flow meters.

A breakout box could be used with the ST3000 Multivariable DP flow meter that would read multiple variables from the via DE and break them out as 2-10Vdc signals.