Apex Digital 5708009-C manual HOST/APEX Communications Transaction Example

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.

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