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Operation and Control using the Enhanced Parallel Interface
Enhanced Parallel Interface
The Enhanced Parallel interface is similar to the RS485 Serial interface, as all instructions received
by the switch are command packets containing a command opcode, a length code, and parameters.
Since the command packet structure is the same as for the RS485 Serial
Note:
Interface, the sections under RS485 that deal with Command Packet Format,
Response Packet Format, Parameter Data Types, and Byte Order also apply to the
Enhanced Parallel Interface
Enhanced Parallel Interface Voltage Levels
All parallel interface signals are TTL compatible. The active state of each signal is outlined in
Table 11.
Table 11: Enhanced Parallel Interface States
Signal Name
Active State
D0 - D7
/STROBE
BUSY
ERROR
/SOP
R/W
/RESET
Enhanced Parallel Timing
The SKB switch uses bidirectional parallel lines for the data bus (D0 to D7) and dedicated input/out-
put lines for flow control. Data flow control is implemented on a byte-by-byte basis, using the BUSY
output line of the SKB switch as the primary mechanism of controlling the data transfer rate. By using
control signals, the actual signal timing can vary (within defined tolerances) according to the avail-
ability of the SKB switch, which is indicated with a low-level BUSY signal.
The primary control signals of the master are the /SOP and /STROBE lines. Transitions of the /SOP
line are used to delimit the beginning and end of packets. The /STROBE line is used to signal the
SKB switch to either read bytes from the data lines or write bytes to the data lines.
Timing dependencies are minimized, allowing use of control signals to regulate communication.
However, the following timing requirements must be observed:
•
A hold time of 5 µs must be applied when toggling SKB input lines (for example, R/W, D0-D7, /
SOP, /STROBE, /RESET).
•
The maximum period between sequential bytes (indicated by transitions of the /STROBE line) is
500 ms.
Inactive State
high
low
high
high
low
high = read
low = write
low
Operation and Control Instructions
low
high
low
low
high
N/A
high
33
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