Hatteland Display 26 inch - HD 26T21 MMC (Widescreen) - Manual (Page 11)

11
Serial/Ethernet Communication (SCOM) Interface
IND100084-14
LEN
Data Length (LEN)
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
Inverse Header Checksum (IHCHK)
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. This does not apply to Broadcast messages in RS-485 mode, in which
case there will be no reply.
DATA
Data Field (DATA)
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
Inverse Data Checksum (IDCHK)
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. This requirement does not reply to
broadcast messages in RS-485 (for units that support it) mode, in which case there will be no reply. Basically this
byte is located at the very end of a received stream.
NOTE: A complete HEX, ASCII, BIN and Character table overview are available in the APPENDIX chapter.