Apex Digital 5708009-C manual Header

Apex 1 to 5.5 kW Generator

Header

The first byte in each packet contains two pieces of information: five bits contain the packet address, and three bits contain the data byte count. If the message packet originates with the host computer (master), the address specifies the packet’s destination (to an Apex generator, for example). If the packet is going to the host, the address specifies the packet’s origin (from the Apex generator). The address section of the Header field is five bits long (bits 3-7), which allows a total of 32 distinct addresses. Address 0 (zero) is reserved for the network broadcast address; when this address is used in a host-originated packet, all units execute the packet (but do not respond back to the host).

The remaining three bits (bits 0, 1, and 2) tell the receiving unit how long the Data field is so that the unit can determine when the entire message has been received.

Note: The value in these bits should refer only to the number of actual data bytes. Do not include the checksum byte when calculating the value for these bits (see “Checksum” on page 4-42).

Command

This field contains a one-byte value: 00h to FFh (0 to 255). If the message packet originates with the host computer, this value specifies the purpose of the message packet. If the message originates with the Apex generator, the value specifies the command to which it is responding. See “Host Port Commands for RS-232 with AE Bus” on page 4-45 for a complete list of commands.

Optional Length Byte

This field supplements the Header field and exists only when the length bits in the Header field contain a value of 7. Under those circumstances, the Optional field contains a one-byte value (between 0 and 255) indicating the number of data bytes.

Data (Data Bytes)

The Data field can contain from 0 to 255 bytes of binary data, which are interpreted in various ways, depending on the value that appears in the Command field. The Data field typically contains data or a Command Status Response (CSR) (see “Creating an Ideal Communications Transaction” on page 4-42), depending on what was requested. Since some commands do not require data, sometimes the Data field is not present.

If the value specified in the length bits of the Header field is 0 to 6, the Apex generator expects 0 to 6 bytes of data. However, if the value in the Header field is 7, the Apex generator looks for an additional eight-bit byte after the Command field (the Optional field) and uses this value for the data byte count.

5708009-C 4-41