Hatteland Display 20 inch - HM 20T07 CMD - Manual (Page 57)

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Serial Communication (SCOM) Interface
Communication Interface
IND100084-1
Broadcast Command Response
In response to RS-485 (for units that support it) broadcast command messages, after the last byte of the command
message is received, all units will reply within the time period defined for Te, below. Further more, any gap between
these individual responses will be less than the Intermessage Gap, defined below.
Te = (Tr + Lr) * N, where
Lr = length of the ACK/NAK message response
Tr = response time
N = the total number of units. *
*) As the units reply in order to their address, the units must be given subsequent addresses, starting at zero, for N to
equal the total number of units. If not, N = the highest unit address + 1.
The maximum Lr for each command are shown in the table below:
Command BRT MAN VER TYP POT MCC
Lr 9 13 11 28 9 17
Example: For the BRT command, and 8 units, this corresponds to
Te = (2.5 * 10 + 9 * 10) * 8 / 9600 = 95.8 ms
MCC Response
For broadcast messages in RS485 (for units that support it) mode, the data field of an ACK is limited to 9 bytes, and
LEN of command + LEN of ACK is limited to 14. This leads to the response time: 102ms + N * 21ms.
Inter-message Gap
Following an individually addressed command, the next command shall not be issued until at least Tg = 5 character
periods after the ACK or NAK message received. At 9600, that is 5 * 10 / 9600 = 5.2ms.
Following the issue of a broadcast command message, the next command shall not be issued until at least Tc = Te +
Tg, where Te is as defined for Broadcast Command response and Tg is defined above.
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.