---
title: "General Concept of Handshake"
product: "LumiScan Cam Driver"
component: "lumiscan-cam-driver"
version: "2.2.1"
display_version: "2.2.1.10"
latest_version: true
language: "en"
url: "https://docs.hdvisionsystems.com/en/lumiscan-cam-driver/2.2.1/lumiscan-camdriver/general-concept-of-handshake.html"
---

# General Concept of Handshake – LumiScan Cam Driver

The communication process in the LumiScan CamDriver application is based on the general concept of handshake. The naming convention for handshake parameters usually consists of the **functionality** and the names **-ready** or **-request**.

The handshake works for every operation except `trigger`. Most operations can also be started with a single write to their `execute` node — see [Execute a Method](https://docs.hdvisionsystems.com/en/lumiscan-cam-driver/2.2.1/lumiscan-camdriver/execute-a-method.html) for a comparison of the two ways.

Each operation has a `<parent address>/<functionality name>/ready` node (read-only, set by the application: `true` when the functionality may be started) and a `<parent address>/<functionality name>/request` node (writable, set by you to start it). The general handshake - its addresses, sequence and rules - is described in [Handshake](https://docs.hdvisionsystems.com/en/lumiscan-keryx/1.0.0/handshake.html). This page describes how the Cam Driver follows it.

## Functionality Execution {#functionality-execution}

> [!NOTE]
> To avoid repeating the entire address, in this section the **invariant part** containing the parent address and the **variable functionality name** (capture, connect-camera, disconnect-camera) have been replaced by **XXX**.\
> Thus, the address: **`hdvisionsystems/lumiscan-cam-driver/<camera name>/control/connect-camera/ready`** will be written as **XXX/ready**.

Accordingly, the address: **`hdvisionsystems/lumiscan-cam-driver/<camera name>/control/connect-camera/request`** will be written as **XXX/request**.

> [!NOTE]
> Make sure there is only one process is accessing XXX/request at a time.\
> The result of execution is **undefined when**:

a. ProcessA set XXX/request to True.

b. ProcessB set XXX/request to False without considering ProcessA.

c. Our application might therefore miss XXX/request from ProcessA.

In order to execute a functionality, you need to run the following steps (see also [Functionality Execution Steps](https://docs.hdvisionsystems.com/en/lumiscan-cam-driver/2.2.1/lumiscan-camdriver/general-concept-of-handshake.html#functionality-execution-steps)):

1. Wait until XXX/ready is set to **True**.
2. Set XXX/request to **True**.
3. Wait until XXX/ready changes to **False**: the application has seen the request.
4. Set XXX/request to **False**.
5. Wait for the functionality to finish, and read its outcome: see the table below.

**Functionality Execution Steps**

```mermaid
%% The ready/request handshake. Rendered by Mermaid in the browser; edit and preview at https://mermaid.live
sequenceDiagram
  autonumber
  participant C as Client (PLC or program)
  participant M as Module
  M-->>C: ready = true: the function can be started
  C->>M: request = true
  M-->>C: ready = false: the request was seen
  C->>M: request = false
  Note over M: runs the function
  M-->>C: ready = true: the function has finished
  C->>M: read control/status/code and output/
```

## When a Functionality Runs {#when-a-functionality-runs}

Only one request is served at a time, in this order: `reset`, `connect-camera`, `disconnect-camera`, `save`, `capture`, `load`, `list-images`, `remove`.

**Table 1. When each functionality runs**

| Functionality | Runs | At the end |
| --- | --- | --- |
| `capture` | At once, as soon as the application sees the request: it does not wait for XXX/request to fall. `status/code` is `2` (running) while it runs, then `3` (waitingForClient) while the application waits for XXX/request to fall. | Success: `status/code` `0`, `capture/ready` **True**, and `output/package-identifier` has grown by one.<br> **Failure: `status/code` stays `3`**, not the capture’s error code, and `capture/ready` stays **False** until a `reset` succeeds. |
| `connect-camera`, `disconnect-camera`, `reset`, `save`, `load`, `list-images`, `remove` | After XXX/request has fallen. `status/code` is `3` until then, and `2` while it runs. | `status/code` `0`, or an error (below `0`). |

A failed `capture`, `reset`, `save` or `load` is latched: its status stays until a `reset` succeeds. (A `save` before anything was captured, `-10005`, is not latched.) Any other error stays on `status/code` for about 100 ms only, then `0` replaces it. So check whether the camera is connected with `hdvisionsystems/lumiscan-cam-driver/<camera name>/device-info/is-connected`, not with the status.

To read the outcome of a capture directly, use `control/capture/execute` instead. The status then keeps the capture’s own code. See [Execute a Method](https://docs.hdvisionsystems.com/en/lumiscan-cam-driver/2.2.1/lumiscan-camdriver/execute-a-method.html).