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Serial/Ethernet Communication (SCOM) Interface
IND100084-14
Unit Response and Addresses
When individual unit addressing is supported by an installed configuration of units in a RS-485 (for units that support
it) system, a separate ACK or NAK message for each unit will be transmitted providing each unit’s individual address
in response to any broadcast addressed Command.
NAK messages will not be generated when an error in a Broadcast message is detected. When individual unit
addressing is not supported, the unit will only respond to the broadcast address and will include the broadcast
address in the ACK and NAK messages. NAK messages will not be generated when an error in a Broadcast
message is detected.
When a unit receives an incomplete message and the next byte is not received until after a time equal to the
Intermessage Gap, the next bytes received shall be processed to check for the start of a new command (0x07, ASCII
Bell).
If the header checksum is valid, but the first byte of the command message is not 0x07, as specified, the unit may wait
until after the next inter-message gap to resume checking. A NAK message shall not be generated.
If the header checksum is valid, but the value of the CMD field does not equal one of the defined commands, the unit
shall reply by generating a NAK message as though a VER command had been received.
If the header checksum is valid, but the value of the LEN field is greater than the maximum allowed, the unit shall
ignore the message. A NAK message shall not be generated.
If the data checksum is valid, but the value in the DATA field associated with a command is invalid (out of range,
undefined, etc.), the unit shall generate a NAK message indicating the current data value in the DATA field.
Additional Commands
In time, additional commands and corresponding data fields may be defined. These additions will not conflict with the
operation of the interface as defined herein in this document.
