Rainbow Electronics DS31256 - Manual (Page 147)

DS31256
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11. LOCAL BUS
11.1 General Description
The local bus can operate in two modes, either as a PCI bridge (master mode) or as a configuration bus
(slave mode). This selection is made in hardware by connecting the LMS pin high or low.
Figure 11-1
shows an example of the local bus operating in the PCI bridge mode. In this example, the host can access
the control ports on the T1/E1 devices through the local bus.
Figure 11-2 also shows an example of the
PCI bridge mode, but the local bus arbitration is enabled, which allows a local CPU to control when the
host can have access to the local bus. To access the local bus, the host must first request the bus and then
wait until it is granted.
Figure 11-3 features an example of the configuration mode. In this mode, the
CPU on the local bus configures and monitors the DS31256. The host on the PCI/custom bus cannot
access the DS31256 and the PCI/custom bus is only used to transfer HDLC packet data to and from the
host.
Table 11-A lists all the local bus pins and their applications in both operating modes. The local bus
operates only in a nonmultiplexed fashion; it is not capable of operating as a multiplexed bus. For both
operating modes, the local bus can be set up for either Intel or Motorola type buses. This selection is
made in hardware by connecting the LIM pin high or low.
Table 11-A. Local Bus Signals
SIGNAL FUNCTION
PCI BRIDGE
MODE (LMS = 0)
CONFIGURATION
MODE (LMS = 1)
LD[0:15] Data Bus
Input on Read/Output on
Write
Input on Write/Output on
Read
LA[0:19] Address Bus Output Input
LWR (LR/W) Bus Write (Read/Write Select) Output Input
LRD (LDS) Bus Read (Data Strobe) Output Input
LBHE Byte High Enable Output Three-stated
LIM Intel/Motorola Select Input Input
LINT Interrupt Input Output
LMS Mode Select Input Input
LCLK
Bus Clock (Use of PCLK is
recommended in bridge mode.)
Output Three-stated
LRDY Bus Ready Input Ignored
LCS Chip Select Ignored Input
LHOLD(LBR) Hold Request (Bus Request) Output Three-stated
LHLDA(LBG) Hold Acknowledge (Bus Grant) Input Ignored
LBGACK Bus Acknowledge Output Three-stated
Note: Signals shown in parenthesis ( ) are active when Motorola Mode (LIM = 1) is selected.