Understanding MDB Flags in Nayax Core

Configure MDB flags in Nayax Core to control how your payment devices handle card transactions on vending machines, from authorization method to VMC error handling and charge behavior.

General

Device Configuration › Nayax Core

Understanding MDB Flags in Nayax Core

Control how your payment devices interact with vending machines. The right MDB flag determines transaction flow, customer experience, and authorization method, making it one of the most impactful configuration decisions for unattended retail operators.

👤 Operators
📦 Nayax Core
JUMP LINKS
TRANSACTION MODESCONFIGURE MDB FLAGSADVANCED FLAGSBEST PRACTICES 


Prerequisites

Before You Start

  • You have Administration access in Nayax Core.
  • You know which machines you need to configure.
  • You have checked whether your machines are older models that may require specific flag settings.
  • You are prepared to run a test transaction after any flag change to verify correct behavior.
ℹ️
What is MDB? MDB (Multi-Drop Bus) is the communication protocol between a Nayax payment device and the vending machine controller (VMC). MDB flags tell the device how to handle authorization, pricing, and VMC error states during each transaction.
Part 1

Understanding the Two Main Transaction Modes

Every MDB flag option is built on one of two transaction modes. Understanding the difference helps you pick the right setting the first time.

Feature Preauthorization — Flags Disabled Preselection — Flag 02
Authorization Type Real Default Credit Virtual Default Credit
Card Taps Required Single tap Double tap
Temporary Hold Yes — holds maximum amount, then adjusts to actual price after vend No — charges exact product price only
Best For Older machines; simplified customer flow Operators who want exact charges with no customer complaints about holds

Use Preauthorization (Flags Disabled) when:

  • Your machines are older models that require it
  • Customers prefer or need a single-tap experience
  • Simplicity matters more than avoiding temporary holds

Use Preselection (Flag 02) when:

  • You want to avoid temporary holds on customer accounts
  • Customer retention is a priority
  • Your machines support preselection (most modern machines do)
⚠️
Important: Many older vending machines will not process card transactions unless Flags Disabled is selected. Always confirm machine compatibility before changing this setting.
💡
Operator tip: Flag 02 (Preselection) is the most popular choice among operators. Customers appreciate seeing only the exact charge — no confusing temporary holds that generate support inquiries.
Part 2

How to Configure MDB Flags

  1.  
    Navigate to Operations > Machines.
  2.  
    Enter your operator details in the search fields to filter and locate the relevant machines.
  3.  
    Click on the machine name you want to configure.
  4.  
    Select the MDB tab in the machine settings panel.
  5.  
    Select the appropriate value from the MDB Flags dropdown menu. Refer to Part 3 for the full flag reference table.
  6.  
    Click Save to store the configuration.
    ✅
    What you should see: A confirmation that the machine settings have been saved. The configuration is stored in Nayax Core but has not yet been applied to the physical device.
  7.  
    Select all MDB attributes that need to be updated.
  8.  
    Go to Actions > Send to Queue to push the update to the physical device.
    ⚠️
    Required: The MDB flag change does not take effect until you complete Send to Queue. Saving alone does not update the machine.
Part 3 — Reference

Advanced MDB Flag Options

Beyond the two main modes, additional flags address edge cases involving VMC error states, vend failures, and communication issues. Use these only after standard flags have not resolved the problem.

Flag Name and Use Case
OFF
Preauthorization — Flags Disabled
Single tap. Real Default Credit. Standard mode for older machines and straightforward transactions.
02
Preselection
Double-tap. Virtual Default Credit. Preferred by operators who want exact charges and no temporary holds on customer accounts.
04
Preauthorization + Ignore VMC Messages
Processes transactions even when the VMC reports errors such as "ongoing transaction" or "fail to vend." Use when a machine frequently sends false error states.
08
Preauthorization + Display Price Charge
Charges the customer the exact price shown on the machine display rather than the maximum authorization amount. Use when display pricing is accurate and reliable.
256
Ignore VMC Action State
Forces a new transaction to start regardless of the current VMC action state. Use when the VMC repeatedly gets stuck in an erroneous state that blocks new transactions.
258
Preselection + Ignore VMC Messages
Combines preselection (double tap, exact charge) with error tolerance. Processes transactions even when the VMC reports an ongoing transaction or vend failure.
514
Charge Regardless of Vend Success
Charges the customer whether or not the product vends successfully. Use only when vend success cannot be verified — this flag can charge customers who receive nothing, significantly increasing dispute risk.
770
Maximum Override
Combines charging regardless of vend success with ignoring all VMC messages. The most aggressive setting for unreliable or poorly communicating machines. Use only as a last resort — bypasses all safety checks and can generate customer disputes.
⚠️
Flags 514 and 770: These settings override vend-success verification. Only use them when machine vending reliability genuinely cannot be confirmed, and ensure the operator understands the risk of charging customers who may not receive a product.
Part 4 — Decision Guide

How to Choose the Right Flag

Flag selection comes down to two factors: machine compatibility and operator preference. Resolve compatibility first, then use operator priorities to choose between the flags your machines support.

1. Check Machine Compatibility First

  • Modern machines: Typically support any flag — full flexibility.
  • Older machines: Often require Flags Disabled (Preauthorization) or card transactions will fail completely.
  • Unknown compatibility: Contact the machine manufacturer for official MDB flag requirements before making changes.
💡
Pro tip: Machine manufacturers always know their specific MDB requirements. When in doubt, ask the operator to contact the manufacturer directly before you change any flag that is currently working.

2. Apply Operator Preferences

  • Reduce customer complaints about temporary holds - Use Flag 02 (Preselection). Customers see only the exact charge.
  • Prioritize a simple, single-tap experience - Use Flags Disabled (Preauthorization). Fewer taps, straightforward flow.
  • Machine sends frequent VMC error states - Try Flag 04 or Flag 258 to process transactions despite false errors.
  • VMC gets stuck and blocks new transactions - Try Flag 256 to override the VMC action state.
  • Vend success is genuinely unverifiable - Consider Flag 514 or 770 only as a last resort, with full operator awareness of the risks involved.
Guidance

Best Practices

  • Start with standard flags. Always try Flags Disabled or Flag 02 before escalating to advanced options.
  • Document your choices. Keep a record of which flag was selected for each operator and machine, and why.
  • Test after every change. Run a test transaction after each flag update to confirm correct behavior before returning the machine to service.
  • Avoid aggressive flags unless necessary. Flags 514 and 770 carry real dispute risk. Only use them when no other option resolves the issue.
  • Confirm machine compatibility first. Always check older machine requirements before changing a flag that is currently working.
  • Always Send to Queue. Saved settings in Nayax Core are not applied to the device until you complete the Send to Queue step.
Expected Outcome

Result

✅

MDB flag configured and applied to the device

After completing all steps, including Send to Queue, the payment device receives the updated MDB flag setting. The machine processes card transactions according to the new mode, and customer-facing behavior reflects your configuration. Run a test transaction to confirm everything is working as expected before returning the machine to full operation.

FAQ

FAQ

Did you find this article helpful?

Related articles

Product Maps, MDB Codes & PA Codes | How They Work Together