Hatteland Display HT B22 (Fanless) - Manual (Page 57)

57
API / VCOM Commands
Appendix
IND100077-132
Message Format
The basic message format shall be as follows and sent to the Internal Virtual COM (VCOM) port:
Byte # 0 1 2 3 4 5 6 7..etc End Byte
ATTN ADDR CMD CMD CMD LEN IHCHK DATA IDCHK
The minimum message size is 7 bytes (0x07). The maximum message size is 82 bytes (0x52), consistent with the
EN61162-1 standard. Every byte sent are viewed in this document as HEX values and are based on standard
characters in the ASCII table (0 to 255) to send or receive messages in a human readable input/output. No further
decoding or decrypting functionality is needed or required. Every command sent are always ended with a 0x00 (null
byte terminator).
Message Format Sending / Receiving- Breakdown
ATTN, Byte 0 Description - Attention
0x07
Sent Command, also known as ASCII BELL
0x06 Acknowledge (Command successfull), also known as ASCII ACK
0x15 Negative Acknowledge (Command not successfull), also known as ASCII NAK
ADDR, Byte 1 Description - Address
0xFF
Broadcast - Addressed to local unit
CMD, Byte 2,3,4 - Command
Byte 2
Byte 3 Byte 4 ASCII Description I/O
0x53 0x57 0x49 "SWI" Retrieve Internal Software version Read Only
0x51 0x44 0x55 "QDU" QM57 Chipset Duplex Mode Write Only
LEN, Byte 5 Description - Data Length
0x?? This single byte defines the length of DATA in the message in bytes. The maximum value for this field is 74 bytes (0x4A in
HEX). The minimum value is 0 bytes (0x00 in HEX).
IHCHK, Byte 6 Description - Inverse Header Checksum
0x??
This single byte is a simple 8-bit checksum of the header data, message bytes 0 to 5 on which a bit-wise inversion has been
performed. The checksum shall be initialised to 0. The 8-bit sum (without carry) of bytes 0, 1, 2, 3, 4, 5 and 6 shall be 0xFF
(255 in value). If the unit receives a message with an incorrect checksum, the unit will reply with the attention code set to NAK
and no data field.
DATA, Byte 7..etc Description - Data Field
0x??
The single byte is the DATA field which shall only be transmitted if LEN is greater than 0. This field depends on the CMD
transmitted.
IDCHK, End Byte Description - Inverse Data Checksum
0x??
This single byte shall only be transmitted if LEN is greater than 0. This is a simple 8-bit checksum of the data field,
message bytes 7 to 7+(LEN-1) on which a bit-wise inversion has been performed. The checksum shall be initialised to 0.
The 8-bit sum (without carry) of bytes 7 through 7+LEN inclusive shall be 0xFF. The receiver will reply to any message
that the checksum has failed with the attention code set to NAK. Basically this byte is located at the very end of a received
stream.