Hatteland Display 20 inch - HM 20T07 MIL-H - Manual (Page 52)

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Serial Communication (SCOM) Interface
Communication Interface
IND100084-1
Data Length (LEN)
This byte defines the length of DATA in the message in bytes. The maximum value for this field shall be 74 bytes. The
minimum value is 0 bytes.
Inverse Header Checksum (IHCHK)
This 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.
To any message that fails the checksum, the receiver shall reply with the attention code set to NAK and no data field.
This requirement does not apply to Broadcast messages in RS-485 (for units that support it) mode, in which case
there will be no reply.
Data Field (DATA)
The DATA field shall only be transmitted if LEN is greater than 0. This field is dependent upon the CMD transmitted.
Inverse Data Checksum (IDCHK)
These bytes 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 initialized
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.
Message Commands
The following sections define the message formats for each of the specific commands.
Brightness Command “BRT”
This command shall be sent to the unit to command the backlight brightness control setting.The brightness value shall
be sent as 1 byte in the DATA field. A setting of 0x00 will indicate off. A setting of 0xFF will indicate maximum
brightness. Intermediate values will control brightness over the range from minimum to maximum luminance.
If the data checksum is valid and the brightness was set, the unit will reply to this command with an ACK attention
code. The DATA field in the reply shall indicate the resulting brightness control setting. If an invalid checksum was
received and the message was not Broadcast and RS-485 (for units that support it), the unit will reply with an NAK
attention code. The DATA field in the reply will indicate the current brightness control setting.
Examples:
Command 60% Brightness:
0x07 0xFF 0x42 0x52 0x54 0x01 0x10 0x99 0x66
Acknowledge 60% Brightness:
0x06 0xFF 0x42 0x52 0x54 0x01 0x11 0x99 0x66
Negative Acknowledge 40% Brightness:
0x15 0xFF 0x42 0x52 0x54 0x01 0x02 0x66 0x99