Powering the Next Generation of ATE
Power instruments are unique because of their large size and power requirements, which restrict them from being plugged into a VXI or PXI chassis. Consequently, power test instruments have been relegated to being standalone instruments or instrument chassis controlled via an analog signal, serial bus or general-purpose interface bus (GPIB). Each bus has advantages and disadvantages, making bus and chassis architecture a hot topic in the power automated test equipment (ATE) world.
Although GPIB (or IEEE-488) is still king of the bench, it has significant limitations. The plug-in cards are expensive and occupy a valuable expansion slot in the computer. In addition, the cables are large and the connectors bulky. Each bus is limited to 15 devices with a maximum cable length of 2 m (times the number of devices) up to a total of 20 m. Furthermore, each instrument must have its own unique address, which must be set manually.
Data transfer rates up to 500 kbytes/s are possible, but only if the total bus cable length is limited to 15 m or less. This communication limitation is one of the reasons complex ATE systems moved to the proprietary VME, VXI and PXI chassis. The introduction of these chassis hasn't been beneficial for power supplies and power test equipment. First, power test equipment typically doesn't require high-speed data transfers. Second, all standard chassis have limits on the amount of power (typically <100 W) that can be drawn from the backplane or dissipated by the cooling fans. Thus, most dc power supplies, ac sources and electronic loads are still controlled via GPIB or analog signals.
Change is Coming
Because of these limitations, ATE companies are looking to transition to other communication buses. USB, Ethernet and, to a lesser extent, Firewire (IEEE 1394), which are standard or readily available on most computers and embedded single board computers (SBCs), have become an option on many test instruments. Table 1 compares each bus being considered for test applications.
The arguments for and against each bus reflect many discussions that non-test engineers have been having with these buses — USB for ease-of-use, Ethernet for connectivity and IEEE 1394 for high speed. These discussions, which parallel the VXI, VME, PXI debate, pit proponents of each new bus against its detractors. However, proponents of the new buses agree that GPIB lacks the scalability, remote control and higher data transfers of the three other buses (Table 1). It's also more expensive to implement (Table 2).
GPIB's key advantage is the number of instruments that use it as the communication bus. However, this is rapidly changing as manufacturers start supporting the other buses, such as Agilent's USB and LAN test instruments and National Instruments' USB and Ethernet to GPIB bridge products. Other examples include Keithley's Ethernet DMMs and data acquisition systems, and American Reliance's ePower and eLoad series of Ethernet-based electronic loads and programmable dc power supplies.
As the tables show, all three alternatives have advantages over GPIB and are less expensive. While no leader has emerged among these three, several differences are worth noting. Compare the bus alternatives on the following desired performance criteria:
- Faster data transmission rates
- Remote capabilities
- More manageable network topology
- Expandable, inexpensive architecture.
Faster Communications
Ethernet, USB and IEEE 1394 are exponentially faster than GPIB. This is important when moving large data blocks. For example, in power-supply test, the initial setup of the instruments and data collection will be much faster. The common Ethernet interface 10/100 Mbps can be eight times faster than USB1.1 but four times slower than USB 2.0 and Firewire V400. Meanwhile, Firewire V800 is eight times faster than 10/100 Mbps Ethernet. Nevertheless, Ethernet 1000 (Gigabit Ethernet) is the fastest, albeit the most expensive, option.
Remote Control
The leading desire for engineers is the ability to sit at their desks and dial up the lab to check a program or instrument status. Being able to incorporate the instruments into a local or wide area network would satisfy this demand for remote connectivity.
Management
Ethernet LAN topology is the established network standard for enterprise management. Most management software and hardware devices support it, making it easy to manage and inexpensive to maintain.
Ethernet-based instruments have the added advantage of increased reporting capabilities. For example, by monitoring failure rates, you can provide more-accurate data and streamline the ATE processes. Another benefit of advanced reporting capabilities is the ability to provide long-term cost management controls through the capturing and storing of more-reliable resource allocation data.
Currently, this direct data collection capability is only available for Ethernet-based instruments, because the management software vendors don't directly support direct data collection from USB and IEEE 1394 buses. Therefore, data from USB and IEEE 1394 instruments must be processed on the local computer, uploaded to a separate storage medium and then downloaded onto the main server. This method is slow, lacks direct remote access to disparate data via a LAN, and creates redundancy of stored data on the server and on multiple ATE stations.
Scalable
An Ethernet network is the common design for large networks. You can add additional hubs and switches, and increase the number of devices without adding more bus cards. Because Ethernet is also the industry standard for local area network topologies, many diagnostic tools are available. USB and IEEE 1394 both have maximum address limitations per bus card. While USB is easily and inexpensively expanded, each additional USB device halves the maximum data rate.
Software is Key
Since the late 1980s, almost every GPIB instrument has had some type of virtual instrument driver that supports National Instruments' LabVIEW, LabWindows or similar platforms, as well as providing C++, Visual Basic, VISA or SICL library support. Manufacturers of Ethernet, USB or Firewire-based test and measurement instruments must be just as compatible with these virtual interfaces. They also must be easy to install, diagnose, control and maintain.
20 m (<500 kbytes/s)
V800: 100 m
V800: 800 MB/s
V2.0: 480 MB/s
V100: 100 MB/s
V1000: 1,000 MB/s
2. If not built-in, PCI cards available
2. If not built-in, PCI cards available
2. If not built-in, PC adaptors available
1. PCI card: $550
2. Cables: $100
2. Firewire cable: less than $30
3. Hubs: $50 to $100
2. USB cable: $8 to $30
3. Hubs: $25 to $100
2. Ethernet cable: less than $10
3. Hubs: $25 to $100
2. No restriction on master/slave
3. Needs users to physically assign address
2. Plug-n-play
3. Automatically assigns address
2. Only one fixed master (PC), instruments can only be slave
3. Automatically assigns address
2. No restriction on Master/Slave
3. Need to assign address /DHCP provides auto configurations
2. One unit failure does not affect the entire bus
3. Troubleshooting easy (no need to disconnect all the units)
2. One unit failure does not affect the entire bus
3. Troubleshooting easy (no need to disconnect all the units)
2. One unit failure does not affect the entire system
3. Remote trouble-shooting available
2. AMREL provides four e-Tools to work with any interface
2. May need a lockout feature
2. Needs a lockout feature
The challenge will be developing software drivers that integrate the IEEE 488.2 SCPI programming while maintaining compatibility with the bus standards. For Ethernet, that means supporting IEEE 802.3 Ethernet standards and TCP/IP. USB drivers will need to support versions 1.1 and 2.0, while Firewire IEEE 1394 must support versions A and B.
This compatibility to the SCPI protocol is easily stated, but the coding behind it is more complex. For example, to ensure quality Ethernet-based power instruments, the product must be a combination of configuration software and drivers that allow interface with a remote PC. To make this possible, the power product must generate its own unique address — a typical occurrence on most LAN networks. Developing drivers that assign IP addresses for each instrument at the COM port can accomplish this, making identifying each product on a complex network environment easy.
These instrument drivers also will comply with the globally accepted transport standard TCP/IP. This standard can bind to many types of physical media from wired to wireless or LAN to WAN. At an opposing end of a connection, it allows encapsulation of virtually any type of data for transport. Uses of TCP/IP are limitless. TCP/IP can adapt to the physical layer (cables and transport media) and the application layer (end-user application and communication software) in almost any method or application.
Another beneficial software tool is a bus analyzer. These diagnostic software support packages ease the integration of instruments in an ATE system. The software makes it possible to diagnose, control and maintain power products and other instruments either on an Ethernet network or closed loop. These products also provide an interface with popular test and measurement software platforms, such as LabWindow and LabVIEW, and C/C++ test applications running on Windows, making it a powerful system debugging tool.
Card (if req.) $20
Card (if req.) $20
Card $50 to $80
Card $50 to $80
The Case for Ethernet
GPIB has been king of the power supply instrument bus for more than 20 years, and remains so by sheer numbers of instrument choices that support the bus. But today, other buses common in the lab, office and test benches are vying to replace GPIB as the bus of choice for power-supply and other ATE systems.
USB, Firewire and Ethernet could all replace GPIB. Regardless of which bus is used in the future, software compatibility and integration with LabVIEW, LabWindows or similar platforms as well as providing C++, Visual Basic, VISA or SICL library support is crucial for any manufacturer's or engineer's success in a changing I/O world.
USB, Firewire and Ethernet each have advantages; however, as Table 1 shows, Ethernet has an edge over the other buses by providing:
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Unlimited remote communication (wireless, global connectivity and control) as long as an Internet connection is present
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Familiar interfacing with IEEE 488.2/SCPI command sets
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Easy plug-in implementation with unlimited multiple-channel control
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Scalable technology
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Isolated communications reducing potential ground loops that are inherent in power supply testing
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Fast data transfer rates and more complex and data-intensive communications
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More efficient remote-servicing methods (for firmware upgrades and field services)
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Lower cost than GPIB
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Support for a variety of cost-management software packages.
For power-supply ATE engineers, Ethernet-equipped programmable power supplies and electronic loads are easily configured alongside other Ethernet-based measurement instruments and bridge products. These products provide a single standard-based test network in which all equipment can connect to common multi-port hubs using readily available cables and protocols.
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