9. API Operation
As an alternative to Transparent Operation, API (Application Programming Interface) Operations are available. API
operation requires that communication with the module be done through a structured interface (data is communicated in
frames in a defined order). The API specifies how commands, command responses and module status messages are sent
and received from the module using a serial port Data Frame.
Please note that Digi may add new API frames to future versions of firmware, so please build into your software interface
the ability to filter out additional API frames with unknown Frame Types.
API Frame Specifications
Two API modes are supported and both can be enabled using the AP (API Enable) command. Use the following AP
parameter values to configure the module to operate in a particular mode:
•AP = 1: API Operation
•AP = 2: API Operation (with escaped characters)
API Operation (AP parameter = 1)
When this API mode is enabled (AP = 1), the serial port data frame structure is defined as follows:
Serial Port Data Frame Structure:
Start Delimiter
(Byte 1)
Length
(Bytes 2-3)
Frame Data
(Bytes 4-n)
Checksum
(Byte n + 1)
0x7E
MSB
LSB
API-specific Structure
1 Byte
MSB = Most Significant Byte, LSB = Least Significant Byte
Any data received prior to the start delimiter is silently discarded. If the frame is not received correctly or if
the checksum fails, the module will reply with a module status frame indicating the nature of the failure.
API Operation - with Escape Characters (AP parameter = 2)
This mode is only available on the UART, not on the SPI serial port. When this API mode is enabled (AP =
2), the UART data frame structure is defined as follows:
UART Data Frame Structure ‐ with escape control characters:
Start Delimiter
(Byte 1)
Length
(Bytes 2-3)
Frame Data
(Bytes 4-n)
Checksum
(Byte n + 1)
0x7E
MSB
LSB
API-specific Structure
1 Byte
Characters Escaped If Needed
MSB = Most Significant Byte, LSB = Least Significant Byte
Escape characters. When 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.
© 2010 Digi International, Inc.
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DCD [ DIGITAL CORE DESIGN ]
