Delta DOP HMI — Communication Error 3 with the PLC

Delta · intermediate · 20 min

A Delta DOP HMI popping 'Communication Error 3' means it can't read device addresses from the PLC — almost always mismatched station number, baud rate or protocol, or reversed RS485 wiring. Learn the exact settings to match and how to prove the link.

ℹ️ On a Delta DOP panel, Communication Error 3 means the HMI can't read device addresses from the PLC. It's a settings/wiring mismatch: station number, baud rate, protocol, or the RS485 D+/D- polarity. Match them and the error clears.

What 'Communication Error 3' means

On a Delta DOP HMI (programmed in DOPSoft), Communication Error 3 specifically indicates the HMI cannot read device addresses from the PLC. That narrows it down fast: the panel is talking, but the PLC isn't answering in a way the panel accepts — typically a mismatched station number, baud rate, or communication protocol.

Match the communication parameters exactly

For reliable data transfer the serial parameters must be identical on both ends — baud rate, data bits, parity, stop bits, and station number. As a concrete reference, the default RS485 settings for a Delta ES2 PLC are:

  • Baud rate: 9600
  • Data bits: 7
  • Stop bits: 1
  • Parity: Even
  • Station number: 1

If your HMI is set to 9600/8/N/1 talking to an ES2 left at 9600/7/E/1, it will never read — match every field.

Check the addressing and protocol

  • Station / slave address: the Modbus slave address of the PLC set in the HMI must match the PLC's actual address. A mismatch here is a classic Error 3.
  • Mapped addresses: verify the memory addresses the HMI is reading are valid and mapped in the PLC, and that read/write permissions allow it.
  • Protocol: the driver selected in DOPSoft must be the correct one for that PLC.

Check the physical link

  • RS485 polarity: D+ and D- are reversed on some cables — swap them if the settings are correct but there's still no comms.
  • PLC in RUN: confirm the PLC is actually running.
  • Right project: make sure the HMI has the correct project downloaded, with device addresses that match the PLC.

Step-by-step

  1. Match serial params on HMI and PLC — baud, data bits, parity, stop bits, station (e.g. ES2 default 9600/7/Even/1, station 1).
  2. Confirm the station/slave address in the HMI equals the PLC's address.
  3. Verify the DOPSoft driver/protocol matches the PLC model.
  4. Check RS485 wiring — try swapping D+/D- if settings look right.
  5. Confirm the PLC is in RUN and the correct project is on the HMI, then re-test.

Never do this

Don't suppress the Communication Error popup with a macro just to stop it appearing while leaving the link broken — that hides a dead HMI from the operator. Fix the actual comms (settings, address, wiring) so the screen shows live PLC data.

Key points

  • Delta DOP 'Communication Error 3' means the HMI can't read device addresses from the PLC — usually a mismatched station number, baud rate or protocol.
  • Serial parameters must be identical on both ends: baud, data bits, parity, stop bits and station number.
  • Reference: a Delta ES2 PLC's default RS485 settings are 9600 baud, 7 data bits, 1 stop bit, Even parity, station 1.
  • Confirm the Modbus slave/station address in the HMI matches the PLC, the DOPSoft driver is correct, and the read addresses are mapped.
  • Check RS485 polarity (D+/D- are reversed on some cables), that the PLC is in RUN, and the correct project is downloaded — don't just mute the error with a macro.

Codes and symptoms

Communication Error 3 — HMI can't read PLC addresses
The HMI cannot read device addresses from the PLC — typically a mismatched station number, baud rate, or protocol. Match all serial params and the slave address.
RS485 D+/D- reversed — Wiring polarity
On some cables the RS485 D+ and D- are reversed. If the comm settings are correct but there's still no data, swap the two lines and re-test.