Motorola SE955 Integration Manual page 197

10 - 10 SE955 Integration Guide
CMD_ACK
Description: Positive acknowledgment of received packet
Packet Format
Length Opcode
0x04 0xD0
Field Descriptions
Field Name
Length
Opcode
Message Source
Status
Data
Checksum
This message is sent to the SSI packet transmitter when the received packet passes the checksum check and
no negative acknowledgment conditions apply (see CMD_NAK). If the data is in response to a command (e.g.,
PARAM_REQUEST, REQUEST_REVISION, etc.), no ACK is sent.
NOTE ACK/NAK handshaking can be disabled, but this is not recommended.
It is not necessary to respond to a valid ACK or NAK message.
Host Requirements
The decoder must send a CMD_ACK or response data within the programmable Serial Response Time-out to
acknowledge receipt of all messages, unless noted otherwise in the message description section. If the host
sends data and does not receive a response within the programmable serial response time-out, it resends the
message (with the retransmit status bit set) before declaring a failure. The host should limit the number of
retries.
Decoder Requirements
The decoder must send a CMD_ACK or response data within the programmable Serial Response Time-out to
acknowledge receipt of all messages, unless noted otherwise in the message description section. If the
decoder does not receive an ACK within this time period, it sends the previous message again. The decoder
retries twice more (with the retransmit status bit set) before declaring a transmit error.
Message Source
Format
Length of message
(not including checksum).
0xD0
0 = Decoder
4 = Host
Bit 0: Retransmit
Bit 1-7: unused
2's complement sum of message
contents excluding checksum.
Status Data Checksum
Size
1 Byte Length Field
1 Byte Identifies this Opcode type.
1 Byte Identifies where the message is coming
from.
1 Byte Identifies the transmission status.
All unused bits must be set to 0.
None
2 Bytes Checksum of message.
Description