Register vs. Message-based Interfaces on the VXIbus Register-based Devices These are the simplest type of VXIbus devices. They are programmed at a very low level using direct read and writes to data registers. This enables register-based devices to communicate at the speed of the hardware. Another characteristic of register-based devices is that the VXIbus interface is small and low cost. This interface is ideal for modules that must move large volumes of data across the VXIbus backplane, e.g. high-speed digital I/O modules. The protocols for controlling register-based devices are completely device dependent. The VXIbus specification makes no attempt to address this. The instrument designer must specify all register interaction and control protocols required to operate the instrument. This is a low-level task which requires attention to detail and documentation explaining the manipulation of the device registers. This method of controlling sophisticated instruments is different and requires manipulation of registers to perform complicated control sequences. Typical programming string A16 #h4305, #hA502 This command writes A502h to VXIbus A16 address space, address 4305h. The result of writing this word to this register could be to close a relay. Message-based Devices Message-based devices support the VXIbus defined configuration and communication protocols. They communicate with each other via a well-defined set of rules known as Word Serial Protocol. This is an asynchronous protocol which defines the handshaking necessary to move commands and data between instruments. Examples of instruments that would use this type of interface are controllers, digital multimeters and counter/timers. They have local intelligence and require a higher level of communication to control them effectively. The message-based type of interface is more complex, larger and costs more. The communication speed is slower. The communication speed is usually not a problem since these instruments have on-board processing and may only need to transfer final results back to the controller. Often, with more sophisticated instruments, the instrument's speed is limited more by the measurement cycle time than the data transfer rate of the communication bus. Message-based devices are easy to integrate into VXIbus systems since they improve compatibility for the user, ensuring communication inter-operability. Programming is a simple case of sending and receiving high level ASCII characters, as with GPIB devices. The tedious task of determining module-specific, register read and write sequences is handled by the module interface hardware. A much more sophisticated level of control is available, allowing the user to put the system in operation faster. Typical programming string Wsstr 8, "Meas" This command writes the string "Meas" to a device at logical address 8, the logical address being defined by a switch setting on the device. The result of writing this string to the device could be to cause the device to take a measurement. Software From the sample commands given for each type of interface, it is clear that a message-based device is simpler to program. Each message-based command can perform an equivalent of many register reads and/or writes. The format of the message-based commands conforms to IEEE 488.2 or more recently, SCPI (Standard Commands for Programmable Instruments). IEEE 488.2 commands are a subset of the SCPI command set. Conclusion Each interface has its advantages and disadvantages. Register-based is fast and easy to implement for the instrument designer. However, for the systems integrator, it is more difficult to integrate, more documentation is required and the user needs to be very familiar with the hardware involved, both on the instrument and within the VXIbus. Message-based instruments are slower, require more design effort and cost more. However, they are far easier to integrate into a system and provide a more sophisticated level of instrument control. This is a necessary feature with complex instruments. Some examples include: Model 2351 Time Interval Analyzer. This is a very sophisticated instrument with a high level of local intelligence. The complex set up and arming of the instrument is achieved with ease through a message-based interface. To transfer large amounts of raw data back to the controller, a register-based interface is also implemented on the instrument, thus taking advantage of a register-based interface's speed of data transfer. Model 1260 Series Switch System. This is a modular electro-mechanical switch system in which the message-based interface is housed on a small plug-in subassembly. A single message-based interface controls additional modules via direct register communications over the local bus. This provides the capability of sophisticated high-level control of the switch system, while saving on the cost of multiple message interfaces. Within a switching system the operating speed of the electro-mechanical relays is relatively slow, several milliseconds due to contact bounce. The speed of a register-based interface would not be taken advantage of. A message-based interface can take advantage of high level control features. Some examples include: non-volatile memory for storing settings, confidence checking to confirm relay closures, and exclude lists to ensure certain relay paths cannot close simultaneously. TOC 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13