Westermo GDW-11 - Manual (Page 357)

Data packets:
Like for AT commands, DATA are encapsulated into packets. These packets are composed
of a header (3 bytes), the data bytes and the checksum (1 byte):
B7 B6 B5 B4 B3 B2 B1 B0
Start pattern → 0xDD
Data packet length LSB
Data packet type Data packet length MSB
Data Bytes
Checksum
The 3 bytes of the header are:
… The first byte (0xDD) used to identify the packet.
… The second byte represents the 8 LSB bits of the length of the data field.
… The last byte is made of 2 parts:
• The 3 LSB bits represent the 3 MSB bits of the length of the data field.
• The 5 MSB bits represent the packet type.
Data packets can have different values according to the type of packet:
0: DATA packet: the packet contains the data to transmit on the radio link or
received from the radio link.
1: STATUS packet: the packet contains the status of SA, SB, X bits* and the break
condition coding as follow:
SA SB X BRK RI Spare Spare Spare
• The length of data for the status packet is always equal to 1.
• Whenever a status changes (except break), all the status bits are included.
• These bits are off by default (and therefore the bits DTR and RTS), so it is
necessary to send a status packet to the target at the beginning of the
multiplexing to start the transmission.
2: READY packet: the packet indicates that the target is ready to receive data:
• No data are transmitted in this packet (so the length is null).
3: BUSY packet: the packet indicates that the target is busy and can not receive data:
• Like the ready packet, no data are transmitted.
Other values: currently, these values are not used (reserved for future enhancement).
NOTE:
The checksum is calculated like the AT command packet checksum (addition of all the
transmitted bytes including the header bytes).
Technical appendixes 357 6615-2220