> ## Documentation Index
> Fetch the complete documentation index at: https://nayax-44d6e37b-docs-marshall-faq-redistribution.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.
# Link and Events Configuration
This page will explore the essential components and configurations needed to establish and manage the connection between your application and Nayax's payment devices using the Marshall SDK. This includes:
* Handling events through the `vmc_link_events_t` interface.
* Understanding the structure of `vpos_config_t` class.
Refer to the following sections for more information.
## Link Events Interface
The `vmc_link_events_t` Interface defines the events related to the communication link between the vending machine controller (VMC) and Nayax devices. Implementing this interface enables your application to respond to critical events that occur during the device's operation.
The following code block presents how to add the interface:
```java Java theme={null}
public interface vmc_link_events_t
void onReady(vmc_link.vpos_config_t config);
void onCommError();
```
```csharp C# theme={null}
public interface vmc_link_events_t
void onReady(vpos_config_t config);
void onCommError();
```
### Interface Methods
The table below describes the available methods of the interface.
| Method | Description |
| :-------------- | :----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `onReady()` | This method is called when the link with the VPOS (Vend Point of Sale) is successfully established and the device is ready for vending operations. It provides the VPOS's configuration details. |
| `onCommError()` | This method is invoked when a communication error occurs between the VMC and the VPOS, allowing your application to handle such errors appropriately. |
## VPOS Configuration Class
The `vpos_config_t` class encapsulates the configuration settings for the VPOS, which are communicated between the VMC and the device. The following code block lists all available configurations:
```java Java theme={null}
public static class vpos_config_t
public byte prot_ver_major;
public byte prot_ver_minor;
public byte language;
public byte[] country_code;
public byte[] currency_code;
public byte decimal_place;
public byte[] vpos_serial;
public int machine_type;
public byte[] merchant_id;
public byte[] acquirer_terminal_id;
public byte[] machine_location;
```
```csharp C# theme={null}
public static class vpos_config_t
public byte prot_ver_major;
public byte prot_ver_minor;
public byte language;
public byte[] country_code;
public byte[] currency_code;
public byte decimal_place;
public byte[] vpos_serial;
public int machine_type;
public byte[] merchant_id;
public byte[] acquirer_terminal_id;
public byte[] machine_location;
```
```c C theme={null}
vpos_config_t
uint8_t language;
uint8_t[] country_code;
uint8_t[] currency_code;
uint8_t decimal_place;
uint8_t[] vpos_serial;
uint8_t[] merchant_id;
uint8_t[] acquirer_terminal_id;
uint8_t[] machine_location;
```
### Class Parameters
The table below provides a detailed description of the parameters.
| Parameter | Type (inferred) | Description |
| :--------------------- | :-------------- | :--------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `prot_ver_major` | int | The major version of the Marshall protocol. |
| `prot_ver_minor` | int | The minor version of the Marshall protocol. |
| `language` | String | The language setting of the VPOS. |
| `country_code` | String | The country code in ISO format. |
| `currency_code` | String | The currency code in ISO format. |
| `decimal_place` | int | The price is represented as 10 exponents. For example, if the decimal place is 2 and the requested price is 100, then the actual cost is 1.00 (with 2 decimal places). |
| `vpos_serial` | String | The serial number of the VPOS. |
| `machine_type` | String | The type of machine connected to the VPOS. |
| `merchant_id` | String | The merchant ID. |
| `acquirer_terminal_id` | String | The acquirer terminal ID. |
| `machine_location` | String | The location of the machine. |
## Callback function
To register the callback events for link operations, use the following method:
```java Java theme={null}
public vmc_link set_events(vmc_link_events_t events);
```
```csharp C# theme={null}
public vmc_link set_events(vmc_link_events_t events);
```
```c C theme={null}
void vmc_link_register_callback(vmc_vend_event_handler_cb_t cb)
```
By understanding and implementing these configurations and methods, you can effectively manage the communication link and event handling for Nayax payment devices, ensuring reliable and efficient vending operations.
## Communication Loss
The Marshall protocol answers every command with an ACK. When your peripheral sends a command to the device it gets an ACK back, and the same happens in the other direction. The ACK is the receiver telling the sender that the command arrived.
When no ACK comes back, the command is transmitted twice more. If none of the three attempts is acknowledged, the SDK stops sending keep-alive commands and the device sends a reset command to start the pairing process again.
What happens to a transaction that was in progress when the ACKs stopped depends on the stage it had reached. See [Create Payments](/docs/integrate-pos-device/marshall/marshall-create-payments).