INSTALLATION MANUAL User Manual

Chapter 5
Serial Communications
5–14
Response From the DL50
To every command the DL50 successfully receives, the DL50 will provide a
response. The response packet has the following format

ACK or NAK
Field 1
1 Byte
Status Byte
Field 2
1 Byte
Field 3
3 Bytes
Checksum
1. If the DL50 Display successfully received the message packet, and is
ready to process it, the DL50 sends an acknowledge response (ACK
6 decimal). In this case, the contents of the status byte are irrelevant.
2. If the DL50 successfully received the message packet but is not able to
display it, a negative acknowledge response (NAK 21 decimal) is sent.
This indicates one of the following:
• A Display FAULT has been detected, or
• Display is in the BUSY state.
A NAK will be accompanied by a status byte with the bits set as defined
here:
Bit Number Meaning
0 0 = No Fault; 1 = Fault Detected
1 0 = Normal; 1 = Display is Busy
2 Unused
3 Unused
4 Unused
5 Unused
6 Unused
7 Always 1
3. If no response is sent from the DL50, it indicates one of the following:
• The message was not received
• CHECKSUM error was detected
• DL50 address is set to 255
• Message address is 255.
Both ACK and NAK responses are also accompanied by a three byte
Checksum. The first byte (dummy byte) is normally 0 and is only used when
either or both checksum bytes contain a value equivalent to a Carriage
Return. Refer to Appendix F for a description of how the checksum bytes
are calculated.
Note: If a DL50 has an address of 255, or if a DL50 receives a message with
an address of 255, it does not send a response. This is to prevent multiple
devices from trying to transmit simultaneously on the half-duplex RS-485
network.
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