Siemens SIMATIC NET SPC 4-2 LF, Руководство пользователя

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PROFIBUS Controller SPC 4®-2 LF
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C79000-G8976-C157-3
11.3 Dual-Port RAM Controller
11.3.1 Function
The internal maximal 3-Kbyte RAM of the SPC 4-2 is a single-port-RAM. Control by
an integrated dual-port RAM controller (DPC) , however, allows almost
simultaneous access by both ports (bus interface and microsequencer interface
MS). Since access via the external interface takes significantly longer than the MS
access, they can be interleaved so that the externally connected processor does
not recognize any delay.
An external bus request is generated by the BIU and passed on to the DP-RAM
controller. A request is formed differently depending on the mode:
In the configuration mode (1,1) for Motorola microcontrollers, the rising edge of the
e-clock pulse is evaluated as a request if the CS signal is present at the same time.
In the Motorola asynchronous configuration mode (1,0) the falling edge of AS is
evaluated as a request if chip-select is also activated.
In the Intel 80C32 configuration mode (0,1), the falling edge of the read-write signal
to be synchronized is evaluated as a request when the internal CS decoder
generates a CS signal at the same time.
In the Intel X86 configuration mode (0,0), the falling edge of the read-write signal
to be synchronized is differentiated and in this case the CS signal is evaluated.
In the 80C165 configuration mode (0,0), the falling edge of ALE triggers access
and the chip-select signal is evaluated.
Notice:
The local microprocessor interface is decoupled from the actual DPRAM by a
buffer and by the DPRAM controller. When the DPRAM is written to, the
information is only physically present when at least four active 0-1 edges of the
SPC4-2 clock pulse have occurred since the active write edge of the local interface
(regardless of the local microprocessor interface). This means that back-to-back
accesses to the DPRAM cannot be faster than is required for two successive write
edges with at least four clock periods between them. The same applies following
write when data is to be read from the same cell immediately.