Class C can be quick and easy.

The LXI protocol offers a multitude of interfaces for communications with test equipment, including LAN Web pages, VISA sockets, and VISA VXI-11 interfaces. Converting an existing serial instrument to an LXI Class C device can be done quickly and economically using an off-the-shelf Ethernet-to-serial converter.

Choosing the Converter

Since many vendors offer off-the-shelf Ethernet-to-serial converters, make sure that the converter you choose meets the minimum requirements for buffer space, supports dynamic host configuration protocol (DHCP) and static IP addressing, and has in-the-field upgrade capability.

Although most available converters share the same space for code and data buffers, some do have separate areas for those two items. An evaluation of the requirements showed that an LXI Class C device needs a minimum of 24 kB of buffer space to support the required ports.

Kepco designed LXI devices by modifying off-the-shelf converters manufactured by Lantronix and Netburner. Our development cycle time was less than one man-year, and we used a subcontractor for the IVI driver. Our LXI devices have at least 256 kB of space allocated for the program code. With both the Lantronix and Netburner converters, we used 130 kB for the application with almost 70 kB of the space allocated to Web pages.

Ethernet Basics

An Ethernet device uses specific ports to send and receive commands. FIGURE 1 shows the ports required by an LXI device. Only the SCPI RAW port and VXI-11 port communicate with the Class C device because they receive messages for the device and send responses back to the host computer. The SCPI RAW port is a socket interface and provided on all converters. It usually has the capability to make one or two connections.

An instrumentation engineer usually has worked with GPIB and is accustomed to having unlimited connections to a device. However, the connections to an LXI device are limited because each one requires dedicated memory for its send and receive buffers.

If an LXI device has insufficient connections, program debug becomes complicated. At least four connections are needed to improve the first-time experience designing and debugging an LXI product.

Another feature helpful during user integration is the Telnet port. While not required by the specification, it provides a way to send commands to a device and get responses, like the features found in the National Instruments Measurement & Automation Explorer and the Agilent Technologies Connection Expert.

[FIGURE 1 OMITTED]

Converter Modifications

Although an off-the-shelf converter has many of the functions required to meet Class C compliance, it will not have support for several functions needed in LXI instruments, including a LAN LED display and LAN Reset, which ensure that all LXI devices start up in the same manner; discovery, which encompasses both VXI-11 support and Remote Procedure Calls (RPC) Get Port functionality; and an IVI driver for the device. In addition, many off-the-shelf converters do not support AutoIP and duplicate IP.

Let's take a look at implementation details of each of these requirements.

LAN LED

The simplest LAN-activity display can be a one-color LED with three states:

* Off to indicate the LAN detected an error.

* On to indicate the IP address is valid and the unit is able to communicate.

* Blinking when an operator activates the identify function via the Web interface.

If the converter supports AutoIP, then it accommodates duplicate IP detection as well. When an LXI device detects a duplicate IP, it must stop using that IP address. However, if the address in use was selected using AutoIP, device operation over the LAN can continue using a different IP address. Otherwise the LAN LED must be turned off and remain off until either the power is cycled or the LAN Reset is activated.

LAN RESET

In an LXI device, LAN Reset consists of two steps. First, enable DHCP and AutoIP; second, restart the converter to determine its IP address. A designer also can choose to implement many optional features that provide enable and disable capability through the Web interface such as ping, VXI-11 discovery, and multicast DNS (mDNS) support. If these capabilities are configurable, then they must be enabled after a LAN Reset.

DUPLICATE IP DETECTION

Duplicate IP detection is accomplished using the Address Resolution Protocol (ARP) over the network. ARP is the method for finding a host's hardware address when only its IP address is known.

ARP packets have a content that Wikipedia defines as, "On Ethernet networks, these packets use an EtherType of 0x0806 and are sent to the broadcast MAC address of FF:FF:FF:FF:FF:FF. Note that the packet structure shown in TABLE 1 has SHA, SPA, THA, and TPA as 32-bit words but this is just for convenience--their actual lengths are determined by the hardware and protocol length fields."

AUTOIP SUPPORT

The converter's ARP module will need modification if AutoIP is not supported. The modification must process each received ARP packet by comparing the IP address against the device's IP address. If the comparison is true, it is a duplicate IP address. The LXI device turns its LAN LED off and stops using the IP address.

An LXI device can use the ARP logic to ask another device to send its IP address. This is done using a special ARP packet called a Null Probe.

The Null Probe is used to ensure that either a static IP address, an address provided by the DHCP server in the system, or an AutoIP address is not already in use on the network. The Null Probe contains the sender's media access control (MAC) address, a 0 for the sender's protocol address, the target's hardware address, and the IP address being verified as the target protocol address.

ARP packets are broadcast to all devices on the network. If the target protocol address matches the device's IP address, the target responds with an ARP packet to the sender.

Broadcast messages sometimes are lost in large networks, but this can be overcome by sending the Null Probe packet multiple times. This code snippet shows the Null Probe packet is sent three times with different delays and the duplicate address flag is tested:

EthernetIP=ip_addr; // so duplicate address will be detected

Null_Probe(ip_addr); // send a probe

OSTimeDly(7);

if (Duplicate_address == 0){

Null_Probe(ip_addr); // send a probe--second required

OSTimeDly(5);

if(Duplicate_address==0)

{

Null_Probe(ip_addr); // send a probe--third required

OSTimeDly(8);

}

}

The transmission of three packets is a standard method as defined in RFC 3927. This time-delayed multiple transmission process provides adequate assurance that the packet will be received by all devices in the network. There are multiple attempts to detect the duplicate address, but if the address is duplicated, the routine exits to reduce network traffic.

AUTOIP

AutoIP automatically selects an IP address without using a DHCP server. It uses pseudorandom IP addresses to create an IP address and then the duplicate IP address logic to verify that another device on the network is not using the created IP address. If the address is in use, the LXI device probes for a new address.

Not all converters support AutoIP, but you can implement it by trying an address, detecting if it's in use, and if so, looping until a free IP address is found. AutoIP uses a specific address range from 169.254.000.000 through 169.254.254.254.

We recommend that AutoIP be done using a pseudorandom number generator and that the numbers be seeded using a MAC address. In this way, each device has a unique series of addresses that will be probed.

This method has been proven in networks of 1,800 devices; the number of attempts any device incurs is three to find a valid address. This yields a 30-s time period for all 1,800 units if they powered up at the same time.

Implementing the VXI-11 Protocol

The LXI device must support the VXI-11 protocol to provide a discovery method. The discovery method requires at least two port connections on the device: Port 110 to receive a Get Port command and another connection on a different port, normally 1024, to perform the VXI-11 commands.

The VXI-11 protocol mimics the IEEE 488.2 interface using Ethernet to make the connection between the host and the device. It has some advantages over a socket interface because it provides the capability to support an interrupt request such as the SRQ line of the GPIB interface and single-byte commands like Selected Device Clear.

The VXI-11 protocol is based on the standard for RPCs developed in the 1970s. The RPC relies on a tool called RPCGEN that creates all the code needed to implement this functionality in an LXI device. When working with a small embedded-processor instrument, this tool generates more code than is required to meet the LXI requirements.

Implementing VXI-11 LXI Discovery

There are two parts to the LXI discovery process: first, the RPC Portmap command, which is the port number of the VXI-11 protocol, is published, and second, the actual VXI-11 commands are sent. A documents archive found in the LXI Consortium's LAN and Web groups contains two Zip files that provide code for the two parts of the process.