5: ACK

Used to acknowledge a previously sent message. Empty ACKs are useless and should not be sent. Either the Response Message ID field, or the ACK Type field should be populated. Multiple fields can be populated if desired. The Header status field can be used to indicate OK, or an error condition. In the case that a response is being sent immediately, an ACK is not required. But the response should contain the Response Message ID header field. If a sender does not receive an ACK or response, it may resend the message. ACKs should not themselves be ACKed.

Response conventions

See Responses and acknowledgments for correlation, Status, typed replies and when an ACK is sent. In particular, omitting Message ID does not guarantee that no error ACK or typed response will be sent. This page defines the MT5 packet format.

Header

Field	Name	Description	Type
3	Response to	ID of the message that is being responded to (Message ID of the request or subscription)	uint32

This is an extract of header fields that are relevant to this message type, you can find them all documented in the Headers section.

Payload

Field	Name	Description	Type	Unit
1	ACK Type	Type of previous message being ACKed	uint16

Examples

ACKing a heartbeat message type 13, ID 567, with an OK status

Protocol

3

3

Length

240

24 bytes

Type

50

5 (ACK)

Header Count

20

2

Header Types

34

3, 4

Response to

455200

567

Status

10

0 (OK)

Payload Count

10

1

Payload Types

1

1

ACK Type

2130

13

Checksum

201198

50889

📡 Real example from on 2025-10-09This example is captured from an RH2 device as part of normal communications over I2C with the ESP32 co-processor, in response to a heartbeat message that was sent by the ESP.

Message ID

Protocol

3

3

Length

170

17 bytes

Type

50

5 (ACK)

Header Count

10

1

Header Types

3

3

Response to

4234000

234

Payload Count

00

0

Checksum

22259

15326

Message ID and type

Protocol

3

3

Length

210

21 bytes

Type

50

5 (ACK)

Header Count

10

1

Header Types

3

3

Response to

4234000

234

Payload Count

10

1

Payload Types

1

1

ACK Type

2320

32

Checksum

86231

59222

Message type only

Protocol

3

3

Length

150

15 bytes

Type

50

5 (ACK)

Header Count

00

0

Payload Count

10

1

Payload Types

1

1

ACK Type

2110

11

Checksum

184159

40888

Sequence

Simple ACK

The simple case is that a sender sends a message, and the receiver responds with an ACK message.

Response

An immediate typed response can replace a separate ACK. Its Response to header correlates it with the request when a nonzero Message ID was supplied.

In such cases the response will not have an ACK message type, instead it will have the message type of the response (often the same as the request).

Retries

If a sender does not receive an ACK or response, it may resend the message.

Code

For convenience, the following constants can be used to reference this message type.