13
Serial/Ethernet/USB Communication (SCOM) Interface
IND100084-18
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 will be initialised to 0. The 8-bit sum (without carry) of bytes 0, 1, 2, 3, 4, 5 and 6
will 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
Data Field (DATA)
The bytes is the DATA field which will only be transmitted if LEN is greater than 0. This field depends on the CMD
transmitted.
IDCHK
Inverse Data Checksum (IDCHK)
This single byte will 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 will be initialised to
0. The 8-bit sum (without carry) of bytes 7 through 7+LEN inclusive will 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.
NOTE: A complete HEX, ASCII, BIN and Character table overview are available in the APPENDIX chapter.
