Modbus Diagnostic Clear Counters Not Resetting

In this tutorial, you'll learn about Modbus Diagnostic Clear Counters Not Resetting. We cover key concepts, practical examples, and best practices.

The Problem

Modbus FC 08 sub-function 01 (Clear Counters) does not reset diagnostic counters.

Quick Fix

Wrong

// Sending clear counters command
mb.diagnostic(1, 0x0001, 0x0000);  // Sub-function 01 = Clear Counters```

Diagnostic response echoes the request but counters are not cleared.

### Right

```cpp
#include <ModbusRtu.h>

Modbus master(1, Serial, 2);

void printDiagnostics(uint16_t *diag) {
  Serial.println("=== Modbus Diagnostics ===");
  Serial.print("Bus Message Count:   "); Serial.println(diag[0]);
  Serial.print("Bus CRC Error Count: "); Serial.println(diag[1]);
  Serial.print("Bus Exception Count: "); Serial.println(diag[2]);
  Serial.print("Slave Message Count: "); Serial.println(diag[3]);
  Serial.print("Slave No Response:   "); Serial.println(diag[4]);
  Serial.print("Slave NAK Count:     "); Serial.println(diag[5]);
  Serial.print("Slave Busy Count:    "); Serial.println(diag[6]);
  Serial.print("Bus Char Overrun:    "); Serial.println(diag[7]);
}

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

void loop() {
  uint16_t diag[8];

  // Read diagnostic counters (sub-function 00)
  int result = master.diagnostic(1, 0x0000, 0x0000, diag);
  if (result == 0) {
    Serial.println("Before clear:");
    printDiagnostics(diag);
  }

  // Clear counters (sub-function 01)
  result = master.diagnostic(1, 0x0001, 0x0000, diag);
  Serial.println(result == 0 ? "Clear counters sent" : "Clear failed");

  delay(100);

  // Read again to verify clear
  result = master.diagnostic(1, 0x0000, 0x0000, diag);
  if (result == 0) {
    Serial.println("After clear:");
    printDiagnostics(diag);
  }

  delay(5000);
}```

Before clear: Bus Message Count: 150 Bus CRC Error Count: 3 ... Clear counters sent After clear: Bus Message Count: 0 Bus CRC Error Count: 0 ...

## Prevention

Modbus FC 08 (Diagnostics) sub-function 00 returns diagnostic counters. Sub-function 01 clears them. Not all slaves support diagnostic clear. The diagnostic counters track: Bus Message Count, CRC Error Count, Exception Count, Slave Message Count, Slave No Response Count, Slave NAK Count, Slave Busy Count, Character Overrun Count. Clear after reading to establish a baseline for monitoring.

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">### Why won't counters clear?</summary><div style="padding:14px 18px;color:#475569;line-height:1.7;background:#fff"><p>Some slaves implement counters as read-only. Check the device manual for diagnostic support. Some industrial devices require password authentication to clear counters.</p>
<h3 id="what-diagnostic-sub-functions-exist">What diagnostic sub-functions exist?</h3><p>00=Return Query Data, 01=Restart Comm Option, 02=Return Diagnostic Register, 03=Change ASCII Delimiter, 04=Force Listen Only, 10=Clear Counters, 11=Return Bus Message Count, 12=Return Bus CRC Error Count, 13=Return Bus Exception Count, 14=Return Slave Message Count, 15=Return Slave No Response Count, 16=Return Slave NAK Count, 17=Return Slave Busy Count, 18=Return Bus Character Overrun Count.</p>
<h3 id="can-i-read-counters-on-any-device">Can I read counters on any device?</h3><p>No. FC 08 is optional. Many simple Modbus slaves (sensors, actuators) do not implement diagnostics.</p>
</div></details>