Apex Digital 5708009-C - Manual (Page 87)

5708009-C 4-41
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.