Real-time multitasking of instruments in the VXIbus command modules - VMEbus Extensions for Instrumentation, Standard Commands for Programmable Instruments - Technical

Hewlett-Packard Journal, April, 1992 by Christopher P. Kelly

The operating system in HP's command modules uses two reentrant processes to handle communication between the user and instruments on the VXIbus.

The ROM-based program (firmware) in the HP VXIbus controllers provides the "personality" that users interact with when operating the instruments in the VXIbus cardcage. The factors that affected the design of the firmware in these products include: The VXIbus standard, which was evolving during firmware development, describes certain required functions. The SCPI instrument language was chosen as the standard language for all HP VXIbus products. The IEEE 488.2 language standard (upon which SCPI is based) defines certain instrument behavior and some language constructs.

Support had to be provided for both message-based and register-based VXIbus instruments with no programming differences visible from the HP-IB interface. The firmware was targeted to run on the on the CPUs for both the B-size HP E1300 and the C-size HP E1400 product families.

Since ongoing development of instruments for the VXIbus is expected, the firmware had to allow the addition of new instrument drivers to the existing products with minimum development and upgrade effort.

Software development for many register-based instruments from different divisions had to be coordinated.

This article explores the design of HP's VXIbus multitasking real-time operating system, or VXI-OS, and the system configuration firmware called the resource manager. A significant portion of the VXI-OS provides support for register-based instruments. To use the high-level SCPI language, register-based instruments with less on-card intelligence require more support from the host CPU than message-based instruments. Because of this fact most of the discussion in this article will be specific to VXI-OS support for register-based instruments.

The VXIbus Instrument Model

The VXIbus instrument model can be described as an "instrument on a card," meaning that each card acts as an independent instrument. This contrasts with some previous generations of HP card-based instruments. The older products use languages in which all the instruments in the cardcage operate as a single instrument. Also, the older architecture provides the integrated services of many different cards with a single, massive command language. The new model treats each card as a single instrument, but also allows combining several cards into a more complex virtual instrument.*

By adopting the new model, VXIbus instruments allow the use of a standard command language shared by both card and rack-and-stack instruments. For example, a VXIbus DMM and a rack-and-stack DMM use the same SCPI language in the same way. This provides some hardware independence to the user, who can now choose between performance, cost, and space without requiring changes in the software driving the instruments. The SCPI language standard protects the sizeable software investment the typical customer has made in the software for an instrument system.

The two major classes of VXIbus instruments are message-based devices and register-based devices. Message-based devices usually include all language processing and a large well-defined register set on the instrument card. Protocols for communications between a commander and a message-based instrument are well-defined and aimed at multivendor compatibility. By contrast, register-based devices have a smaller required register set and a simpler programming interface. These devices are usually smaller and less expensive than equivalent message-based instruments, but may require more support from the command module. When programmed at the binary register level, register-based devices can be orders of magnitude faster than equivalent message-based devices that use a higher-level language.

One goal of the VXI-OS firmware team was to provide the same language and programming interface for both register-based and message-based instruments. This means that both types of instrument interfaces should answer the HP-IB communications bus in exactly the same way, and should operate using exactly the same SCPI language regardless of the instrument type.

Register-based instruments, with little or no intelligence on each card, require the services of a microprocessor to use the SCPI language. This microprocessor can be shared by several instruments, since in most cases an instrument does not require full and continuous use of the microprocessor's power. This sharing of expensive, intelligent hardware can result in lower system cost than in alternative designs. In the HP VXIbus systems, the command module provides this shared microprocessor, which processes commands and data for many register-based instrument cards.

The HP Series 75000 models E1300 and E1405 VXIbus command modules are based on the Motorola 68000 microprocessor. The B-size E1300 and the C-size E1405 provide an HP-IB interface to many HP register-based devices. The E1405 additionally provides full VXIbus slot 0 command module and resource manager functions for message-based devices (see the article on page 6 for more about slot 0 functions). The design of the firmware in these modules permits power-on system configuration based on the instrument card mix. It also supports independent operation of multiple instruments in the cardcage, and the addition of new instrument drivers. Both modules are also able to use HP Instrument BASIC (IBASIC) to control the instruments.