Modbus Diagnostic Restart Causes Slave Lockup

In this tutorial, you'll learn about Modbus Diagnostic Restart Causes Slave Lockup. We cover key concepts, practical examples, and best practices.

The Problem

Modbus FC 08 sub-function 01 (Restart) causes the slave to become unresponsive.

Quick Fix

Wrong

// Restart slave with invalid option
mb.diagnostic(1, 0x0001, 0xFF00);  // Invalid restart option```

Slave restarts but with unexpected behavior or lockup.

### Right

```cpp
#include <ModbusRtu.h>

Modbus master(1, Serial, 2);

void setup() {
  Serial.begin(9600);
  master.begin(9600, SERIAL_8N1);
}

void attemptRestart(int slaveId) {
  Serial.print("Restarting slave ");
  Serial.println(slaveId);

  // Sub-function 01: Restart Communications Option
  // Data 0x0000: Restart with all counters cleared
  // Data 0xFF00: Restart with all counters preserved

  uint16_t diag[8];

  // First, save diagnostics
  master.diagnostic(slaveId, 0x0000, 0x0000, diag);

  // Send restart (clear all counters)
  int result = master.diagnostic(slaveId, 0x0001, 0x0000, diag);

  if (result == 0) {
    Serial.println("Restart command sent");

    // Wait for slave to reboot (typically 1-5 seconds)
    Serial.println("Waiting for slave to restart...");
    delay(3000);

    // Re-establish communication
    for (int attempt = 0; attempt < 5; attempt++) {
      uint16_t test;
      int r = master.readHoldingRegisters(slaveId, 0, 1, &test);
      if (r == 0) {
        Serial.println("Slave is back online");
        return;
      }
      Serial.print("Waiting... attempt ");
      Serial.println(attempt + 1);
      delay(1000);
    }
    Serial.println("Slave did not restart properly!");
  }
}

void loop() {
  attemptRestart(1);
  delay(30000);  // Wait 30 seconds between restarts
}```

Restarting slave 1 Restart command sent Waiting for slave to restart... Slave is back online

## Prevention

Modbus FC 08 sub-function 01 restarts the slave's communication. Data 0x0000 = restart with counters cleared. Data 0xFF00 = restart with counters preserved. After restart, the slave is offline for 1-5 seconds while the UART resets. Always wait 3+ seconds before retrying. Do NOT use on slaves that are actively controlling equipment — unexpected restart could cause unsafe states.

DodaTech engineers apply these same patterns across Doda Browser, DodaZIP, and Durga Antivirus Pro for production IoT reliability.

## FAQ

<details style="margin-bottom:12px;border:1px solid #e2e8f0;border-radius:10px;overflow:hidden"><summary style="cursor:pointer;padding:14px 18px;font-weight:600;font-size:1.05rem;background:#f8fafc;border-bottom:1px solid #e2e8f0;color:#1e293b">### What is the restart delay?</summary><div style="padding:14px 18px;color:#475569;line-height:1.7;background:#fff"><p>Most slaves restart in 1-3 seconds. Some industrial devices may take 10+ seconds. Start with 3s wait, increase if needed.</p>
<h3 id="can-restart-damage-equipment">Can restart damage equipment?</h3><p>Yes. A Modbus restart only affects the communication interface — the slave's controlled outputs may enter a safe (or unsafe) default state. Use with extreme caution on PLCs and motor drives.</p>
<h3 id="does-fc-08-restart-work-on-all-slaves">Does FC 08 restart work on all slaves?</h3><p>No. FC 08 is optional. Many slaves do not implement diagnostic functions. Check the device manual before attempting.</p>
</div></details>