Abaco Systems / SBS / Bit 3 400-202 Dual Port RAM Module

The Abaco Systems / SBS / Bit 3 400-202 is a dual-port RAM module that enables high-speed inter-system memory sharing and data exchange through Bit 3 Adaptor cards. The module creates a shared memory space accessible by random access reads and writes from either connected system, consuming only the bandwidth of the accessing bus to maximize transfer speeds while minimizing performance impact on the non-accessing system’s bus. Simultaneous access from both systems is supported, with the Bit 3 Adaptor arbitrating all accesses.

– Technical Specifications

• Form Factor: 3″ × 5″ printed circuit card
• Installation Method: Component-side-down mounting on Adaptor card; standoff-secured
• Memory Window Size: 64K to 1M byte (variable, programmable)
• Data Access Width: 32-bit on most systems; 16-bit on PC/AT, MULTIBUS, and Q22-bus systems
• Addressing Modes:
– Direct Mode: One-to-one address mapping to each system’s bus
– Page Mode: Small address range “window” for full dual-port RAM access
• Address Decoding: Lower 16–20 address bits decoded by Adaptor; upper address bits supplied by programmable Adaptor register (modifiable via I/O write)

– Key Features

• Eliminates bandwidth contention between interconnected systems
• Treats dual-port RAM as global memory; accessed identically to standard bus memory
• Independent address spaces per system—target addresses need not match
• Arbitrated simultaneous access prevents memory conflicts
• Programmable window configuration for flexible memory mapping

– Compatibility & Integration

• VMEbus Systems: No additional card slots required in VMEbus chassis
• Q22-bus Installations: Compatible with specialized Grant Continuity cards
• System Support: 32-bit data access on most architectures; limited to 16-bit on PC/AT, MULTIBUS, and Q22-bus platforms
• Interface: Memory-mapped interconnection via Bit 3 Adaptor cards