- Buyers Guide
An Interface Providing Web-based Control of Microwave Power Amplifiers
The ubiquitous IEEE 488.2 interface has served the instrumentation industry well for many years, and will probably do so for many more to come. However, in order to offer considerably more customer choice for the mundane yet important task of instrument interfacing and to exploit emergent interface technologies, Milmega has developed the AC002 interface to complement its range of flexible, upgradeable amplifier topologies.
Rejecting the idea of a WiFi interface because of the numerous noise sensitive applications that power amplifiers are used in, the focus was put on exploiting the power of the Worldwide Web, and by doing so, providing a convenience not before available to power amplifier customers. The simple objective was to provide easy remote instrument control to individuals with appropriate access rights, without the need to add additional hardware to a standard PC.
Web-based remote control offers benefits to those in the engineering community who need to monitor, or control, equipment in limited access environments and for whom, to be continuously present on site, gathering data, monitoring progress and making periodic adjustments is both unnecessarily time consuming and cost prohibitive. Such convenient access capability would be an advantage, for example, during the extensive burn-in of a system where it is only necessary to interact sporadically with the equipment to check all is well.
The AC002 is integrated into the mechanical profiles of the company’s Series 2000 range of amplifiers. Figure 1 shows the board fitted into an amplifier. These amplifiers deliver solid-state power in applications up to 14 GHz. The interface offers the flexibility to communicate via three remote interfacing standards: RS232, 10BaseT Ethernet and USB 2.0. The characteristics of each of these interfaces are summarized in Table 1.
The core of the design is based around a programmable integrated circuit (PIC) microcontroller and a dedicated network interface chip (NIC). Both were chosen for the breadth of integrated functions they possess, reducing development time, time to market and the resultant product cost.
The microcontroller has a number of useful built-in peripherals such as UART (universal asynchronous receiver transmitter), I2C, ADC and a timer that make the chip particularly suitable for this application. External 20 MHz crystals provide the microcontroller and NIC clocks, the controller implementing a divide by four function, resulting in a minimum resolution for any timing function of 200 ns.
The versatility of the I/O lines is put to good use in the new interface design. All are software configurable as either inputs or outputs and are used internally to simulate existing Milmega interface units, ensuring backward compatibility for existing users. In addition, these I/O lines double up to provide the connection for the In Circuit Serial Programming (ICSP) interface, a function that enables the programming and debugging of the firmware in situ. The key building blocks are outlined in Figure 2.
Remote Interface Implementation
The microcontroller, via its UART interface, provides an elegant solution to the provision of a RS232 interface. A simple level converter chip is all that is required between the PIC and the outside world. The baud rate for RS232 operation is user selectable from rates of 9600, 38,400 and 57,600, with the factory set standard of 19,200.
A suitable interface device converts the USB protocols to RS232 for communication with the second UART port of the microcontroller. The USB interface provides the ‘plug and play’ facility of many of today’s computer peripherals, underlining the fact that a user need purchase no additional PC hardware to take advantage of the AC002.
The primary goal behind the new remote interface development was the provision of user-friendly LAN/Ethernet interfacing. This is implemented using a dedicated NIC, the device performing all the timing, frame gathering and sending for the Ethernet protocols. The chosen LAN standard is 10baseT, selected for its ease of interfacing with existing PCs and the simple fact that the data handling requirements of an amplifier are not that complex. Physical connection is via a RJ45 connector, which has integrated status LEDs and a line transformer.
LAN Protocol and Control Methods
While the LAN interface hardware facilitates a generic connection to a network, a number of software and packet protocols have to be provided to ensure effective communication between the interface and a controlling computer.
A summary of these protocols follows:
TCP/IP – Transmission Control Protocol/Internet Protocol: used by many higher level protocols such as HTTP and Telnet. TCP provides a guaranteed flow of data in the correct sequence in a similar style to reading data sequentially from a file. This effectively frees the application from having to police the lower level activities of sending and receiving data packets, error correction and the reception of packets in the correct order.
ARP – Address Resolution Protocol: provides the low level message services, which allows a NIC to join a network.
ICMP – Internet Control Message Protocol: provides for messaging related to network operation or misoperation. PING, a common network management tool, is based on a common function of ICMP.
DHCP – Dynamic Host Configuration Protocol: provides the facility for the NIC’s IP address to be set dynamically by a server system when it first attaches to a LAN.
The new interface provides for several methods of control via a command set, which is a simplified subset of the Standard Communications Protocol defined for GPIB-controlled instruments (SCPI) and IEEE488.2. The command set is the same whether programming over a LAN socket connection or via RS232 or USB.
The LAN interface delivers further flexibility for customers, providing three options for remotely controlling the amplifier:
- An option for reading and writing directly to TCP sockets via programs written in Visual Basic, C++ or Delphi.
- An option for programming the amplifier via a Telnet interface.
- An option for controlling the amplifier via a Web browser, each Milmega amplifier having a unique IP address and an embedded Web page with stored calibration data and command and control functions.
All of these options make good use of the data sharing capabilities that are inherent to the Web. Figure 3 shows a screen shot of the Web page embedded in the amplifier with an example of the type of status data fed back to an operator.
An array of options for interfacing with the company’s amplifiers brings flexibility to customers in the defense and commercial markets for whom remote control and monitoring is seen as a desirable advantage. In particular, troubleshooting systems deployed remotely is made easier. It is perfectly possible for a production manager in Stockholm, for example, to monitor the performance of a bank of amplifiers in a company subsidiary in Shanghai. In such a situation this sort of set up provides the ability for making decisions in real time regarding maintenance scheduling, or even making adjustments to the performance of individual amplifiers via the Web page embedded in that amplifier’s AC002 interface.
The ability to interface with the instrument over an internal company Intranet, or via the Internet, opens up new possibilities for interacting with equipment and, for ATE, there is now a choice, not only of low cost, readily available connections but numerous control software options as well.