API frame format

The firmware supports two API operating modes: without escaped characters and with escaped characters. Use the AP command to enable either mode. To configure a device to one of these modes, set the following AP parameter values:

The API data frame structure differs depending on what mode you choose.

API operation (AP parameter = 1)

The following table shows the data frame structure when you enable AP = 1:

Frame fields Byte Description
Start delimiter 1 0x7E
Length 2 - 3 Most Significant Byte, Least Significant Byte
Frame data 4 - n API-specific structure
Checksum n + 1 1 byte

The firmware silently discards any data it receives prior to the start delimiter. If the device does not receive the frame correctly or if the checksum fails, the device replies with a device status frame indicating the nature of the failure.

API operation-with escaped characters (AP parameter = 2)

This mode is only available on the UART, not on the SPI serial port. The following table shows the data frame structure when you enable AP = 2:

Frame fields Byte Description
Start delimiter 1 0x7E
Length 2 - 3 Most Significant Byte, Least Significant Byte Characters escaped if needed
Frame data 4 - n API-specific structure
Checksum n + 1 1 byte

Escape characters

When you are sending or receiving a UART data frame, specific data values must be escaped (flagged) so they do not interfere with the data frame sequencing. To escape an interfering data byte, insert 0x7D and follow it with the byte to be escaped XOR’d with 0x20.

Data bytes that need to be escaped:

Byte Description
0x7E Frame Delimiter
0x7D Escape
0x11 XON
0x13 XOFF

Example: Raw serial data before escaping interfering bytes:

0x7E 0x00 0x02 0x23 0x11 0xCB

0x11 needs to be escaped which results in the following frame:

0x7E 0x00 0x02 0x23 0x7D 0x31 0xCB

Note In the previous example, the length of the raw data (excluding the checksum) is 0x0002 and the checksum of the non-escaped data (excluding frame delimiter and length) is calculated as:
0xFF - (0x23 + 0x11) = (0xFF - 0x34) = 0xCB.

Length

The length field specifies the total number of bytes included in the frame's data field. Its two-byte value excludes the start delimiter, the length, and the checksum.

Frame data

This field contains the information that a device receives or transmits. The structure of frame data depends on the purpose of the API frame:

 

Start

delimiter

 

Length

Frame data

 

Checksum

API identifier

Identifier-specific Data
1 2 3 4 5 6 7 8 9 ... n n+1
0x7E MSB LSB

cmdID

cmdData

Single

byte

The cmdID frame (API-identifier) indicates which API messages contains the cmdData frame (Identifier-specific data). The device sends multi-byte values big endian format.

The XBee®-PRO 900HP RF Module supports the following API frames:

API frame names and IDs sent to the module

API frame names

API ID

AT Command

0x08

AT Command - Queue Parameter Value

0x09

TX Request

0x10

Explicit TX Request

0x11

Remote Command Request 0x17

API frame names and IDs received from the device

API frame names

API ID

AT Command Response

0x88

Modem Status

0x8A

Transmit Status

0x8B

Route information packet

0x8D

Aggregate Addressing Update frame 0x8E

RX Indicator (AO=0)

0x90

Explicit Rx Indicator (AO=1)

0x91

Data Sample Rx Indicator frame 0x92

Node Identification Indicator (AO=0)

0x95

Remote Command Response

0x97

Note Requests are less than 0x80, and responses are always 0x80 or higher.