Command is an 8-byte array of the following format:

Byte 0: GPIO_GET_FR_CNT_VAL (0x18)

This byte contains command ID (command type). It should be set to 0x18.

Byte 1: ECHO

This byte is used to associate a response received from GPIO-24 adapter with a specific command. Response will contain exactly the same value in its echo byte.

Byte 2: FR_CNT_NUMBER

The ID number of the frequency counter to retrieve frequency from:

• 0 – frequency counter on A.3 pin;

• 1 – frequency counter on A.4 pin.

Bytes 3..7: Reserved

These bytes are reserved for future usage and must be set to 0.

Response is an 8-byte array of the following format:

Byte 0: GPIO_GET_FR_CNT_VAL (0x18)

Response ID (response type). This byte will contain 0x18.

Byte 1: ECHO

Echo byte copied from the corresponding command. It is used to associate the response with the specific command.

Byte 2: ST

Status of the command execution:

• GPIO_ST_SUCCESS (0x00) – Frequency has been succesfully retrieved;

• GPIO_ST_INVALID_FR_CNT_NUMBER (0x0A) – Invalid ID number of the frequency counter has been specified (only 0 and 1 are valid).

Byte 3: FR_CNT_NUMBER

The ID number of the frequency counter, from which current frequency has been retrieved:

• 0 – frequency counter on A.3 pin;

• 1 – frequency counter on A.4 pin.

Byte 4..6: FREQUENCY

The current frequency in Hz.

Byte 4: FREQUENCY[2] – least significant byte (low byte) of frequency.

Byte 5: FREQUENCY[1] – middle byte of frequency.

Byte 6: FREQUENCY[0] – most significant byte (high byte) of frequency.

Byte 7: Reserved

This byte is reserved for future usage.