Overview
IEC 60821:1991/AMD1:1999 is Amendment 1 to the IEC 60821 international standard for the VMEbus - microprocessor system bus for 1 byte to 4 byte data. Published in January 1999, this amendment updates signalling definitions, data-transfer rules, timing and arbitration language, mechanical board requirements, and various tables and mnemonics used to describe master/slave capabilities. It clarifies behaviours and tightens mandatory/optional language to improve interoperability of VMEbus modules.
Key topics and technical requirements
- Signal semantics and timing
- Revised definition of WRITE*: a level-significant line strobed by the falling edge of the first data strobe (DSA*), valid while any data strobe (DSA* or DSB*) is low.
- Corrections to interrupt and acknowledge signal names (e.g., IACK), and changes in arbitration signal identifiers (BRO* → BR0*, BGOIN* → BG0IN*).
- Data-transfer capabilities and mnemonics
- Stronger normative language: “can accept/monitor” replaced by “must accept/must monitor” in multiple tables for basic, block, and read-modify-write capabilities.
- Specific rules for D08(O) SLAVES forbidding DTACK* during certain byte-location access cycles and clarifying unaligned transfer (UAT) behaviour and permitted cycles.
- Corrections in byte-group mapping and data-line groupings used in multi-byte transfer tables.
- Arbitration and requester rules
- Fair arbiter behaviour clarified: after grant, a requester must monitor its request line and must not issue a new request until that line has been high once.
- Priority interrupt bus
- Updated titles and usage for IRQ1–IRQ7** and IACK, and clarified permissions for interrupt-driving lines.
- Mechanical specifications
- Mandatory board thickness: 1.6 mm ± 0.2 mm in the guide area; updated rules and figures reflecting this requirement.
- Informative and normative annexes
- Appendices updated: some marked normative (connector/pin descriptions, permissible subsets), others informative (metastability and resynchronisation). Metastability guidance and arbitration timing (MTBF considerations) were revised.
Applications and who uses it
- VMEbus hardware designers and module manufacturers implementing 1–4 byte data transfers.
- System integrators and test engineers ensuring interoperability and correct timing/handshaking.
- Standards committees and firmware developers referencing precise signal and arbitration semantics.
- Useful for legacy system maintenance, upgrades, and compliance verification of VMEbus-based embedded and industrial control systems.
Related standards
- Original IEC 60821:1991 (base standard) - this document is Amendment 1 (1999).
- Relevant ISO/IEC JTC1/SC26 working documents referenced for voting and technical basis.
Keywords: IEC 60821 amendment, VMEbus, microprocessor system bus, data transfer, arbitration, interrupts, WRITE*, DSA*, DSB*, D08(O), unaligned transfer, board thickness, metastability.