Westermo GDW-11 - Manual (Page 356)

356 Technical appendixes 6615-2220
Technical appendixes
Data / Commands multiplexing protocol
Introduction:
The multiplexing protocol operates between a DCE (Data Communication Equipment:
the product) and a DTE (Data Terminal Equipment). It allows a double session over a
serial link interface: one for AT commands and one for DATA communications.
AT+WMUX=1 activates the Multiplexing Mode. With this mode, AT commands and
DATA are encapsulated into packets. The header of these packets allows to recognize
whether it is a DATA packet or an AT command packet. AT+WMUX=0 deactivates the
Multiplexing Mode and gets the product back to the default mode.
This appendix presents how the multiplexing mode handles the DATA and the AT com-
mands flow. It also describes the format of DATA packets and AT command packets.
AT command packets:
An AT command is encapsulated into a packet with a header which allows to separate it
from DATA packets. This packet is formed by a header (3 bytes), the AT command itself
and a checksum (1 byte):
B7 B6 B5 B4 B3 B2 B1 B0
Start pattern → 0xAA
AT command length LSB
AT command pattern → 0x1D AT command length MSB
AT command
Checksum
The 3 bytes of the header are:
… The first byte (0xAA) is used to identify the packet.
… The second byte represents the 8 LSB (Low Significant Bits) bits of the length of
the AT command.
… The third byte is made of 2 parts:
• The 3 LSB bits are the 3 MSB (Most Significant Bits) bits of the length of the
AT command.
• The 5 MSB bits (0x1D which equals to 0xE8 with the 3 bits offset) are used
to identify an AT command.
The maximum length of an AT command could be 2047 bytes which is greater than all
the existing AT commands.
The checksum is the addition (modulo 256) of all the transmitted bytes (header bytes
and AT command bytes).