Select Page

Modbus Design Considerations

by | Jul 27, 2017 | Communications-Group, Modbus Communications-Group

Modbus Devices can access the OpenADR signal values either through Modbus TCP over Ethernet (All Series) or RTU RS485 9600 8N1 (Series 113/135 only)

The default station number is 5. If it is necessary to change then the GRIDlink can be reconfigured through GRIDview when on line or using Toolkit locally. Contact support.

Modbus Integration - Quick Start

    • The simplest way to integrate OpenADR signals into any control system is to poll the 3 Digital Outputs: event_pending 0:0111, event_moderate 0:0112 & event_high 0:0113. Shown in the violet shaded area.
    • event_special 0:0114 is not commonly used but could be included for future use.
    • Pending offers the system time to prepare like pre-cooling the AC before the Event or just notifying personnel.
    • Depending on the signals used by the Utility (Moderate or High) the logic for the reduction on the load should be based on binary states of those registers in accordance with the participation agreement.

Opt Out

Control systems can write to the opt_out 0:0103 register. ON will exclude the system from the Event or end the Event if in progress. All registers will return to non-Event states. The VTN will be notified. It is important to return the opt_out 0:0103 register to OFF after the Event as it will remain ON indefinitely.

Pending Notice

In addition to the event_pending 0:0111 binary which ON=Event Notification and remains ON throughout the Event, 2 additional Analog Inputs give additional information about the Pending phase.

  1. event_status 3:0117 1 = far 2 = near & 3 = Active. Near and far are non-precise terms and vary greatly among Utility VTNs. They are more of a guide that anything else. Active is when the Event goes from the Pending to actual Moderate, High or Special.
  2. next_op_mode 3:0118 foretells what the Event will be when it becomes Active; 1 = Moderate, 2 = High & 3 = Special. This could be utilized in the overall control strategy particularly if the Utility uses more than 1 signals in their programs.

Epoch time variables

These variables provide epoch times in seconds for use in the control system

    • current_time 3:5021 - the GRIDlink onboard clock Current time
    • event_start_time 3:5023 - Event Start time in epoch seconds
    • event_end_time 3:5023 - Event End time in epoch seconds

VTN Connection and Communications Status

There are 2 registers which indicate VTN connection status in the orange shaded area.

  •  vtn_connect 0:0003 - ON = good connection and notifications are within parameters
  •  comm_status 0:0115 - where -1=Server Refused Connection, -2=Error in Payload, -3=Certs Fail Authentication.

Binary Relay Control Register 113/135 Series only

USER_TRANSF_IO – Allow the user to isolate the relays from the Event Signal by setting to FALSE. This way an test Event signal can be sent from the VTN without actually shedding the load. It also allows the installer to simply exercise the relays from GRIDview to test the load shed without having to schedule an Event.

Loss of Internet Connection During an Event

GRIDlink OpenADR software has 2 control strategies for loss of Internet connection.

  1. The default is ride through which the Event continues and all related registers hold their signal. The only caveat is that if the Utility sends an Cancel notice in the middle of the Event the GRIDlink will be unaware.
  2. If requested, tech support will set the OpenADR logic to return all registers to their non-Event state on loss of connection through GRIDview. The event_start_time and event_end_time will return to zero even though the Event is technically still active.