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The first byte is always an opcode, which can be any number between 0 and 127. The most signifi-
cant bit (MSB) is never set in a command packet opcode.
The second byte is always the length, in bytes, of the following parameter data. The value of the
length byte corresponds to the number of bytes in the parameter data section of the command
packet, regardless of the actual data type represented by the binary parameter data.
The minimum length of a command packet is two bytes. The opcode and length (even if zero) must
be transmitted to the SKB module.
Command packets appear as follows:
OP
LEN
where OP = opcode and LEN = length.
The communication architecture of the SKB switch does not implement a command-response
scheme. Each command packet sent is not necessarily answered with a response packet. Only
those opcodes that represent a query generate a response packet.
Response Packet Format
Response packets refer to packets transmitted by the SKB switch for the purposes of returning query
data. Response packets have an identical format as a standard command packet, with the MSB of
the opcode byte set.
If, for example, an opcode of 0x01h was sent to the SKB switch and this opcode represented a
query, the response packet would have an opcode of 0x81h.
The length byte of a response packet represents the total length of the parameter response data,
regardless of the actual data types embedded in the parameter area.
Parameter Data Types
Parameter data of the command packet is limited to 253 bytes, which can be allocated at the discre-
tion of the application according to standard data types shown in Table 6.
Table 6: Parameter Data Types
Data Type
Size in Bytes (Bits)
signed char
1 (8)
unsigned char
1 (8)
signed int
2 (16)
unsigned int
2 (16)
signed long
4 (32)
unsigned long
4 (32)
float
4 (32)
Byte Order
When utilizing data types larger than one byte, the byte order follows "little-endian" convention: the
low-order byte (which holds the least significant bit, or LSB) is referenced as byte 0. Subsequent
bytes progress toward the high-order byte (which holds the MSB). This sequence is illustrated as fol-
lows.
DATA
Operation and Control Instructions
25
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