Configuring Modbus Output Commands in Rapid SCADA 6

Overview

Rapid SCADA 6 is an open-source industrial automation platform written in C# on .NET, distributed through the official repository at RapidScada/scada-v6 on GitHub. It is designed for building distributed automation systems that run on servers, embedded computers, and cloud instances. Writing outputs from the operator interface down to a Modbus device requires three coordinated configuration objects:

1. An Output Channel registered in the configuration database.
2. A Scheme Component bound to that channel number with its Action property pointing to a command.
3. A Command declared inside the Modbus device template that translates the channel write into the appropriate Modbus function code and register.

This article walks through each step with the XML structures, configuration table parameters, and verification checks required to push a value from a Rapid SCADA web scheme to a Modbus slave.

Prerequisites

• Rapid SCADA 6 Administrator utility (ScadaAdmin) with read/write access to the configuration database.
• Rapid SCADA 6 Server (ScadaServer) and Agent (ScadaAgent) installed and running as Windows services or Linux daemons.
• Modbus TCP or Modbus RTU driver configured in ScadaComm; license-less trial mode is sufficient for development work.
• Knowledge of the target Modbus device's data block, register address (zero-based or one-based depending on driver conventions), and required function codes.
• Web scheme editing access through ScadaWeb or the Scheme Editor shipped with the Administrator package.

Rapid SCADA 6 Architecture for Write Operations

Understanding how a command propagates from the operator's browser to the field device prevents mis-configuration. The flow is:

Configuration Database Objects

The configuration database is the single source of truth. Three tables are directly involved in write operations:

For commands, additional lookup logic ties CnlNum to a CmdNum through the template's command table; the channel's CmdType must match the template's command type or the write will be rejected by the server.

Step 1 — Create the Output Channel

Open ScadaAdmin, connect to the configuration database, and navigate to Channels → New Channel. Configure the following parameters:

Save and publish the configuration. The channel is now known to ScadaServer but is not yet wired to a physical write.

Step 2 — Configure the Modbus Template Command

Templates are XML files inside the project's template directory. A Modbus command element is required for every output channel. The structure follows the schema defined in the Rapid SCADA 6 ScadaComm driver for Modbus.

<DeviceTemplate>
  <Template>
    <Name>ModbusTcp</Name>
    <DevType>Modbus</DevType>
    <Commands>
      <Cmd cmdNum="1" name="SetCoil" cmdCode="05" dataBlock="0" address="0"/>
      <Cmd cmdNum="2" name="SetRegister" cmdCode="06" dataBlock="1" address="0" readCmdCode="03"/>
      <Cmd cmdNum="3" name="SetRegistersArray" cmdCode="16" dataBlock="1" address="0" cmdType="Array">
        <CmdEl cnlNum="100" name="Setpoint1" dataType="Double" dataLen="2"/>
        <CmdEl cnlNum="101" name="Setpoint2" dataType="Double" dataLen="2"/>
      </Cmd>
    </Commands>
  </Template>
</DeviceTemplate>

Key attributes:

Reload the configuration in ScadaAdmin so that ScadaServer recognizes the new template commands. The driver compiles the template and exposes cmdNum identifiers to the server's command dispatcher.

Step 3 — Bind the Scheme Component to the Channel and Action

Open the Scheme Editor, select the button, dynamic text, or input control that will trigger the write, and configure the following properties:

A common scheme-side binding looks like this in the scheme's XML:

<Component type="Button" cnlNum="100" action="SendCommand" cmdNum="2" cmdVal="1">
  <Caption>Start Pump 1</Caption>
</Component>

On click, the browser posts a request to ScadaWeb, which hands it to ScadaServer. The server resolves the channel number to the linked device, looks up the command by cmdNum, packages the value according to the Modbus function code, and hands the request to the ScadaComm Modbus driver for transmission.

Modbus Function Codes Used for Writes

Always confirm the target device supports the chosen function code; some inexpensive RTU slaves accept only 06 and 16.

Verification Procedure

1. In ScadaAdmin, publish the configuration and confirm no validation errors are reported.
2. Restart ScadaServer and ScadaComm services if the template changed; channel-only edits typically do not require a restart.
3. Open the web scheme, sign in as an operator with command privileges, and trigger the bound component.
4. Watch the server log (ScadaServer.log) for a Command processed entry containing the CnlNum and CmdNum.
5. Use a Modbus packet sniffer (Wireshark with the mbtcp dissector, or a vendor diagnostic tool) to confirm the function code, address, and value on the wire.
6. Check the channel's OutVal in ScadaAdmin; it should reflect the acknowledged written value.

Troubleshooting Matrix

Security and Role Configuration

Writes must be restricted to authorized roles. In ScadaAdmin under Users → Roles, ensure the operator role has the SendCommand permission and that the role can act on the device or channel in question. Always enable audit logging on write operations; Rapid SCADA 6 records every command with the originating user, timestamp, channel, and value.

Performance and Timing Considerations

• Modbus TCP round-trip on a LAN is typically 5–20 ms; over WAN expect 50–200 ms.
• Avoid issuing more than one write per second to the same register to prevent Modbus exception 06 (busy) on slower slaves.
• For multi-register array commands, prefer function code 16 over multiple 06 transactions to reduce bus traffic.
• Large arrays (greater than 100 registers) may need to be split depending on the slave's PDU limit; classic Modbus caps a single 16 transaction at 123 registers.

Field-Proven Cautions

• Never bind a write action to an input channel; the server silently discards writes to read-only channels.
• If the Modbus driver restarts while a command is in flight, the request is lost — design HMI feedback (status indicator channel) for critical actuators.
• When upgrading between Rapid SCADA minor versions, re-validate templates: schema attributes occasionally gain or rename fields.
• Test with the Modbus Simulator shipped with Rapid SCADA examples before pointing at a live PLC.

FAQ