By Roland Gamper, LeCroy Corporation

Regardless of the system topology and application domain, the development, maintenance and monitoring of electronic system architectures using mixed protocols requires tools able to straddle several busses, at various speeds and with different line characteristics. Today’s oscilloscopes, with flexible input channels and advanced toolsets, are ideally suited to this task.

Why Would a System Need Several Protocols?

click to enlarge Figure 1. Figure 1. An example of a MIL-STD-1553 (1 Mb/s) bus on Channel 2, inducing a UART message (400 Kb/s) on Channel 4. The approximate translation latency of 3.404 microseconds is read out using the cursors.

The need to incorporate several protocols can be due to historical, technical, commercial or compatibility reasons. Modern system architectures often exhibit a backbone transmission using a high speed protocol from node to node over several tens or hundreds of meters. Each node on the backbone distributes selected information to local nodes using a lower speed, less expensive protocol to neighboring devices. In this case the oscilloscope would be used to monitor the nodes input (in one protocol) and output (in another protocol) and verify their coherency, both in time and contents. As an example, a vehicle could have a FlexRay (fast, extremely robust but expensive) backbone, and a LIN bus (slow, nearly free and safe) in each seat and door. As another example, the Airbus A380 combines a CAN bus and ARINC 429. In this case the reason is historical: many of the radio communication and navigation systems are manufactured with an interface to the ARINC 429 bus only. Examples like this abound in other industries, such as automation, textiles, printing, medical field, bottling machines, construction equipment, mining and more.

Protocol Analyzer vs. Oscilloscope

Engineers working on serial busses of any kind have long been limited to two classes of tools: Protocol analyzers and traditional digital oscilloscopes. Each instrument alone could not show the type of view useful to both hardware and software engineers. The protocol analyzer displayed a good application layer view but did not provide any useful physical layer details. The oscilloscope properly displayed the physical layer waveform but did not accurately present the application layer.

Current advances in oscilloscope technology provide an alternative that combines the power of a dedicated protocol analyzer with the versatility of an oscilloscope. New oscilloscope trigger and decode packages, enabled by increased memory length, processing power, and display technology, provide advanced analysis functions for various serial protocols. The protocols supported vary from very common (RS232 , I2C, SPI) , to specialized (MIL-STD-1553, ARINC 429, CAN, FlexRay, LIN), to applications such as audio (I2S, TDM) and cellular technology (MIPI), or to high-speed busses (USB1.x/2.0/3.0, SATA, PCI Express, 8b/10b).

click to enlarge Figure 2. Shown here is a zoom of Figure 1. The synchronous acquisition on all channels permits observation and measurement of the time difference between the end of the MIL-STD-1553 message (end of the parity bit) to the beginning of the UART message (beginning of the Start Bit). The time lag is measured as 3.196 microseconds.

Increased memory length permits oscilloscopes to capture serial streams over useful time spans at the appropriate sampling rate for each protocol. The processing power lets the unit decode the serial stream into its most fundamental components, such as bits, bytes, words, packets, messages, transactions at high rate, on several channels. Brighter displays provide brilliant, crisp and clear annotations, in real time, of the decoded elements. Bigger displays offer the real estate necessary to monitor and understand several digital streams and possibly other related analog signals at the same time.

There are many advantages to these new Serial Data Trigger and Decode tools and one limitation. The oscilloscope does not acquire and store continuously, as does a protocol analyzer. However, during the “open eye” time the oscilloscope sees everything, from the glitches on the signal, all the way to corrupted transactions. The protocol analyzer, on the other hand, can stream data to disk for hours or even days, but with a one-dimensional view of the transmission since the signal data is not stored along with the decoded contents.

One of the many advantages of using an oscilloscope for protocol analysis is the ability to monitor different protocols at the same time, which is impossible with a protocol analyzer, dedicated to one single protocol. The ability to observe several serial streams of different nature simultaneously is very useful on systems which inherently incorporate several protocols.

An oscilloscope is a generic tool that does not contain special hardware dedicated to analyzing one particular protocol. In some ways, one could say that the hardware is protocol neutral. It is only when equipped with the appropriate software options that it can process different types of protocols. Fundamentally, the analog signals are all acquired using the same acquisition hardware.

A digital oscilloscope acquires signals of nearly any nature on different channels simultaneously. Thanks to the extreme flexibility of a channel (in terms of sensitivity and bandwidth) and the attached probes, each channel can be tuned to optimally record signals carrying different types of protocols. Decades of development have taught oscilloscope engineers to build versatile front ends, able to amplify or attenuate nearly every possible signal and to bring it to the very heart of the oscilloscope: the Analog to Digital Converter (ADC). The precise time alignment between the channels can be acquired because the channel’s ADCs operate synchronously. In other words, an event occurring first on a given channel will be recorded before an event occurring later on another channel.

Multiple Protocol Time Alignment

This time, consistency across channels is the great power of the oscilloscope. The importance of the time alignment cannot be understated since systems combining several data streams have strict cause-consequence relationships between the packets of information transiting on the networks. When a packet is received from the backbone link (cause), it has to be passed on to the lower speed link (consequence). The consequence is expected to happen within a well-specified amount of time after the cause. This latency, or time delay, can easily be observed with an oscilloscope equipped for multiple protocol monitoring. Not only can the latency be observed visually on the oscilloscope screen, but it can also be quantified statistically. For example the latency of a gateway translating messages from MIL-STD-1553 to UART could be characterized as 2 to 10 microseconds, with 80 percent of the packets being translated in less than 5 microseconds. This indication would help the avionics engineer in tracking errors on the aircraft.

To conclude, we can say that relentless technological advances in the digital oscilloscope arena in the past decade have fostered a new generation of tools aimed at visualizing, triggering and decoding serial data streams. These tools help system engineers at all levels to understand the complex events occurring on single or multiple data busses, in different domains of industry.