# Connection & handshake _Open, find, and handshake_ The handshake confirms the device on the serial port is a Medius box and speaks a protocol version you understand: one request, one reply. Open the port and start talking. - No baud negotiation. - No login. - No `115200` startup step or baud-switch command. ## Handshake _One round-trip to confirm the box_ 1. Open the serial port at `4,000,000` baud ([Transport](/native/transport.md)). The box speaks [framed binary](/native/frame.md) from the first byte. 2. Catch the unsolicited hello the box sends on its own ([below](/native/connection.md#hello)), or send a [`QUERY(VERSION)`](/native/commands/requests.md#version) yourself. Both produce the same [`RESP(VERSION)`](/native/commands/requests.md#version) frame. 3. Read `proto_ver` from that reply and check it equals `3`. | Reply | Meaning | | --- | --- | | `proto_ver == 3` | Speaks the protocol these pages describe. | | `proto_ver != 3` | Speaks a protocol these pages don't cover; don't assume the commands behave as described. | | No reply | Not a Medius box, or the port or baud is wrong. | #### THE REPLY: RESP(VERSION) The first byte echoes the `what` selector you asked for (the byte that chose which thing to query), then the protocol and firmware version, the box MAC, and the box name follow. Full detail on the [Requests](/native/commands/requests.md#version) page. | Offset | Field | Type | Notes | | --- | --- | --- | --- | | 0 | `what` | `u8` | the selector byte, echoed back; `0x00` = `VERSION` | | 1 | `proto_ver` | `u8` | protocol version, expected `3` | | 2 | `fw_major` | `u8` | firmware major | | 3 | `fw_minor` | `u8` | firmware minor | | 4 | `fw_patch` | `u8` | firmware patch | | 5 | `mac` | `u8[6]` | the box MAC, a stable per-box id | | 11.. | `name` | `ascii` | the box's human-readable name (may be empty), delimited by the frame `LEN` | ## The ready hello _Unsolicited RESP(VERSION) on link-up_ The box sends one [`RESP(VERSION)`](/native/commands/requests.md#version) on its own as soon as its serial link is up. Treat it as "box is here and ready" and skip your own [`QUERY(VERSION)`](/native/commands/requests.md#version). | Trigger | When it fires | | --- | --- | | Power-on | Once, as the box boots. | | First contact | On the first valid frame after a program opens the port, so a program that connects after the power-on hello still gets one. | The hello carries [`SEQ=0`](/native/frame.md#seq) since no request prompted it, and its payload is identical to a queried reply. > **Note** > > The [medius library](/library/connection.md) does all of this inside [`open`](/library/connection.md#open) and [`find`](/library/connection.md#open): it sends [`QUERY(VERSION)`](/native/commands/requests.md#version), retries a few times, and checks `proto_ver == 3` before handing you a working connection.