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  3. Lesson 4
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Beginner12–18 min

Lesson 4. MT5 Architecture for EAs — What Beginners Must Know

Understand how MT5 "holds" a robot: terminal/account/symbol, netting vs hedging, global AutoTrading vs chart-level attachment, magic numbers, inputs, logs/permissions, and multi-chart behavior.

You chose a robot style. Now learn how MT5 actually runs an EA, so you avoid common pitfalls.

4.1. Big Picture: Terminal → Account → Chart → EA

Understand the hierarchy before attaching any robot.

Terminal (MT5 App)
Account (demo/live; netting/hedging)
Chart A (EURUSD, M15)EA instance (inputs, magic)
Chart B (XAUUSD, H1)EA instance (inputs, magic)
  • •Terminal: the MT5 app runs your accounts, charts, and EAs.
  • •Account: login to a broker server (demo/live), with its own mode (netting/hedging) and permissions.
  • •Symbol + Timeframe: each chart is a 'view' of one symbol on one timeframe.
  • •EA on chart: each EA instance runs on a specific chart and timeframe; it listens to ticks and timer events there.
One EA file can be attached to many charts; each chart instance is independent.

4.2. Netting vs Hedging (Quick Refresher Applied to EAs)

Netting

One net position per symbol; opposite trades reduce/increase the single position. EA should manage partial closes by sending opposite orders.

Hedging

Separate long and short positions; EA must track multiple tickets and manage SL/TP per position.

Using a hedging-oriented EA on a netting account can lead to unexpected behavior (e.g., closes instead of hedges). Always check account mode.

4.3. AutoTrading, Chart Attachment, and Scope

Global AutoTrading button (toolbar)

Master switch — OFF stops all EAs from trading; ON allows EAs to trade (if chart-level permissions also allow it).

Chart-level

When attaching an EA (Navigator → Expert Advisors → drag to chart), allow "Algo trading" in the EA settings dialog; the EA runs only on that chart's symbol/timeframe.

Remove/disable per chart

Right-click chart → Expert List → remove; or uncheck "Algo trading" for that EA.

If nothing happens, check both switches: global and EA-level permissions.

4.4. Magic Number and Inputs (Distinct Instances)

Magic Number
What: An identifier the EA uses to track its own orders (so it doesn't touch your manual trades or another EA's trades).
Why: Running multiple EA instances requires unique magic numbers per instance/symbol.
Rule of thumb: Unique per EA per symbol/timeframe (e.g., 1001 for EURUSD M15, 1002 for XAUUSD H1).
Inputs (Parameters)
Where: When attaching an EA, the Inputs tab shows configurable parameters (risk %, lot mode, filters, time windows, magic number, etc.).
Save/load presets: Save to .set file, Load to reuse configurations.
Micro example: Two charts of the same EA with magic numbers 2001 and 2002 → each manages only its own positions.

4.5. Logs, Journals, Experts, and Permissions (Debug Basics)

Journal

Terminal/Toolbox → Journal

Terminal-level messages (connection, orders, errors).

Experts

Terminal/Toolbox → Experts

EA-specific messages (print logs, order results, errors like "Trade disabled" or "Invalid stops").

Common Issues
"AutoTrading disabled"→ Turn on global/EA switch.
"Trade is not allowed"→ Permissions in EA settings, markets closed, symbol trade disabled.
"Invalid stops"→ SL/TP too close to price (check symbol's stop level).
"No prices / Market closed"→ Trying to trade outside hours or no ticks.
Use logs first when something "doesn't work."

4.6. Symbols, Timeframes, and Multi-Chart Behavior

  • •Each EA instance is bound to its chart's symbol/timeframe; it won't trade other symbols unless coded for it.
  • •Multi-symbol EAs exist, but standard EAs watch only their chart's symbol.
  • •More charts/EAs = more CPU. Close unused charts.
  • •Restart behavior: when MT5 restarts, attached EAs re-load with previous inputs if AutoTrading is on and permissions allow it.
Running the same EA with the same magic on multiple charts can cause conflicts.

4.7. Practice: Attach an EA Twice with Unique Magic Numbers

Practice — 10–15 min

4.8. Quick Diagnostics Checklist

Quick Diagnostics Checklist
  • 1.Global AutoTrading ON?
  • 2.EA settings "Allow Algo trading" ON?
  • 3.Correct account mode for the EA logic (netting/hedging)?
  • 4.Unique magic numbers per instance?
  • 5.Logs show errors? (Journal/Experts)
  • 6.SL/TP not violating minimum stop levels?
  • 7.Market open? Symbol trading allowed?

Lesson 4 Quiz

Test your understanding with 3 questions. Pass with 2/3 correct.

What's Next?

You understand how MT5 hosts an EA — hierarchy, permissions, magic numbers, and logs. Next, we'll do the hands-on: install a real EA, configure it, and launch it on a demo chart.

Next: Install & Basic Configure an EA
Educational content only. Not financial advice. Trading involves risk of capital loss. Backtests and past performance do not guarantee future results.