400-206 Bit 3 Dual Port RAM Card

The GE Fanuc / SBS / Bit 3 Model 400-20x Series Dual Port RAM Cards are optional memory cards for computer systems that are interconnected by Bit 3 Adaptors. Optional Dual Port RAM provides a memory buffer; saves the cost of additional memory cards; and for some systems, such as VMEbus systems, requires no additional card slots.

The Dual Port RAM card installs on an Adaptor card and is accessible to either system. As shared memory, either system can address Dual Port RAM via random access reads and writes. Access to the memory card uses only the bandwidth of the accessing bus. Consequently, transfer speed is increased and data can be exchanged with minimal impact on the performance of the other system's bus. Both systems can access Dual Port RAM simultaneously with the Adaptor arbitrating accesses.

Bit 3 Adaptors provide access to Dual Port RAM by a chassis at either end of the Adaptor cable. Dual Port RAM is treated as global memory and access to it is accomplished identically to bus memory access. Direct Mode, a method of communication used by all Bit 3 Adaptors, allows Dual Port RAM addresses to be mapped one-for-one to address space on each system's bus. The target addresses need not be the same for both buses.

Most Bit 3 Adaptors also support Page Mode Dual Port RAM access. Page Mode addressing allows a relatively small range of addresses on a bus to be used as a "window" for access to all Dual Port RAM. Window size is variable, from 64K to 1M byte. When a system accesses an address within the window, the Adaptor decodes only the lower 16 - 20 address bits. The upper address bits for the Dual Port RAM destination address are provided by a programmable Adaptor register. The number of upper address bits used can be changed by an I/O write instruction.