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5708009-C 4-43
Apex 1 to 5.5 kW Generator
• A command and data that change a parameter setting
• An executable command
Once the Apex generator receives the message packet, the Apex generator verifies that
the message is intended for it and not for another unit on the network. At this time, the
Apex generator also analyzes the checksum to verify that the message was received
correctly.
If the address does not match, the Apex generator does not respond to the host; the
Apex generator resets and resumes waiting for a message addressed to it. If the
address matches but the exclusive-or (XOR) sum of the bytes in the packet (including
the checksum) is not zero, the Apex generator sends a negative acknowledgment
(NAK), hex code 15h, to the host. If the address matches and the message is intact, the
Apex generator sends an acknowledgment (ACK), hex code 06h, to the host.
If the Apex generator receives a request for data or status information, it gathers and
sends the requested information. Otherwise, it evaluates the incoming command and
sends a message-packet that contains a 1-byte data value (CSR code) to the host (see
“Command Status Response (CSR) Codes” that follow). CSR code 0 is sent when the
command has been accepted.
If the host receives a NAK from the Apex generator, the host either retransmits the
packet or does whatever else it has been programmed to do in this situation. If the host
receives an ACK, it waits for the requested data or status information or for the CSR
code telling it whether or not the new parameter was accepted. If the host receives no
response within a reasonable period, it takes whatever action it has been programmed
to take.
Meanwhile, the Apex generator has prepared a message packet with the requested
information or appropriate CSR code, which it then transmits to the host. The host
determines by means of the checksum if the message is complete. If the host detects
an error in the transmission (by using the Checksum), it can request the packet be sent
again by transmitting a NAK.
If the Apex generator receives an ACK, it returns to the normal waiting state. If the
Apex generator receives a NAK, it retransmits the message packet. The Apex
generator continues to retransmit in response to NAK transmissions until the host
stops the cycle. If the Apex generator receives no response, it assumes an ACK and
returns to the waiting state.
HOST/APEX COMMUNICATIONS TRANSACTION EXAMPLE
Figure 4-7 is a simplified graphic showing the steps in an example communications
transaction between a host computer and an Apex generator.