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== Frame Structure ==
== Frame Structure ==
An ALDL frame consists of a message ID, length, an optional number of data bytes (up to 64 bytes), and a 2's compliment checksum. The table below illustrates the structure of an ALDL frame.
An ALDL frame consists of a message ID, length, an optional number of data bytes (up to 64 bytes), and a 2's compliment checksum. Any multi-byte fields are always big-endian unless otherwise noted. The table below illustrates the structure of an ALDL frame.
{| class="wikitable"
{| class="wikitable"
=== Mode 1 ===
=== Mode 1 ===
Mode 1 is used to retrieve live data.
Each module maintains a set of tables containing live data as well as stored parameters. When a device issues a Mode 1 request to a module, it provides the number of the table being requested. The module responds by transmitting the requested table number, followed by the contents of the table. The exact contents and structure of the table depend on both the table in question, and the module. Tables are numbered sequentially, however most modules have less than 10 tables. Requests for table numbers higher than the total number of tables on a given module will not receive a response{{Dubious}}. The table below illustrates the full structure of a Mode 1 request.
{|class="wikitable"
! Length (bytes) !! Field !! Value !! Notes
|-
| align="center" | 1 || ID || align="center" | <code>F1</code> ||
|-
| align="center" | 1 || Length || align="center" | <code>57</code> ||
|-
| align="center" | 1 || Mode || align="center" | <code>01</code> ||
|-
| align="center" | 1 || Table || align="center" | ''n'' || Table number
|-
| align="center" | 1 || Checksum || align="center" | ||
|}
=== Mode 2 ===
=== Mode 2 ===