AST 5251/11 Enhanced Twinax Emulator ISA Card 202139-002 Rev X4 IBM AS/400

AST 5251/11 Enhanced Twinax Emulator ISA Card 202139-002 Rev X4 IBM AS/400

The AST 5251/11 is an 8-bit ISA expansion card that enables a DOS-based PC to emulate an IBM 5251 twinax terminal, allowing the host machine to communicate directly with IBM AS/400, System/36, and System/38 midrange systems over a twinax cable network. The board is labeled "Enhanced" and carries part number 202139-002 at hardware revision X4, visible on the label affixed to the PCB.

Specifications & What's Included

• Manufacturer: AST
• Model: 5251/11 Enhanced
• Part Number (MPN): 202139-002
• Hardware Revision: Rev X4
• Bus Interface: 8-bit ISA (full-length card)
• Protocol / Emulation: IBM 5250 twinax terminal emulation
• Bracket Connector: DB-15 twinax port with TERM / THRU selector switch on bracket
• On-board ICs visible: MC74F08N logic ICs (multiple), Kappa-Taiwan IC, additional TTL logic array
• Board finish: green PCB, full-length ISA form factor
• Included: card only — no cables, software, or documentation

Applications

The AST 5251/11 was designed for the 1980s and early 1990s PC-to-IBM-midrange connectivity market. IBM's AS/400 (introduced 1988), System/36, and System/38 used a proprietary twinax cabling scheme — a two-wire, multi-drop network that connected up to seven terminals or workstations to a single port on the host. Rather than use a dedicated IBM 5251 or 5291 display station, businesses could install a twinax emulator card like the AST 5251/11 into an ISA-bus PC and run emulation software, giving the PC full 5250 terminal functionality at a lower cost and with the added flexibility of a general-purpose computer. The TERM/THRU selector on the bracket allowed the card to be placed anywhere in the daisy-chain: TERM terminated the cable at the last device, while THRU passed the signal down to the next station. AST Research was one of several third-party vendors — alongside Arlan, Attachmate, and IBM itself — producing these cards during the height of IBM midrange installations in corporate environments. Today these cards are sought by collectors and by organizations still maintaining legacy AS/400 or System/36 infrastructure.