# Characterizing Serial Interfaces

May 1, 2000

There are many, many serial interface standards. So many, in fact, that there isn’t anything standard among the standards. For example, there are different waveforms, different frequencies, different voltages, different jitter tolerances, and different transmission orders. And there are new proprietary designs using creative engineering to get more bits through limited bandwidth in a noisy channel.

Unfortunately, test equipment hasn’t kept pace with technology. Today, design and test engineers spend valuable time developing custom interfaces to connect to their device under test. The answer would be a general-purpose solution—one that could account for a number of variations.

Electrical Formatting Schemes

What is a bit? Logically, a 1 is a 1 and a 0 is a 0, but not so electrically. Some common format or encoding methods are described in Table 1 and illustrated in Figure 1.

Table 1.

 Encoding Method Description AMI A pulse indicates a 1. No pulse indicates a 0. Each pulse is the opposite polarity of the previous. NRZ Actually, this is not encoded, just a confusing way of saying the binary data is sent as is. NRZI A transition at the start of each bit-time indicates a 0. No transition at the start of a bit-time indicates a 1. The data must have enough 0s to keep synchronization (for example, HDLC’s zero-bit stuffing ensures this). Manchester or Bi-Phase-L The complement of the data is sent during the first half of each bit-time, followed by a transition to the opposite polarity (uncomplemented data) at the center of each bit-time. This transition is used for the clock. Another way of saying this is that the direction of the transition at the center of the bit-time indicates the data value. A rising edge is a 1, a falling edge is a 0. Another transition will be needed at the start of each bit-time if two consecutive bits are the same. Differential Manchester or DBi-Phase-M A transition at the start of each bit-time indicates a 0. No transition at the start of a bit-time indicates a 1. A transition at the center of each bit-time is used for the clock.

Different serial interface standards use different schemes such as:

• MIL-STD-1553 uses Bi-Phase-L or Manchester.

• The Universal Serial Bus (USB) uses Non-Return to Zero Inverse (NRZI).

• T1 carrier systems often use Alternate Mark Inversion (AMI).

And then there are the custom or proprietary schemes.

Many of the schemes are self-clocking so a phase-locked loop (PLL) will be required to lock to the data transitions. There also are speed variations from kilobits per second to hundreds of megabits per second as well as differences in amplitudes and rise/fall times. A general-purpose serial-interface instrument must support these different encodings as well as custom schemes.

Word or Byte Encoding

What is a word? Is it 8 bits, 16 bits, 20 bits, or 32 bits? Is it framed by start/stop bits? Is there a parity (even or odd) bit? These are the simple questions.

The transmission order is not standard. Some interfaces transmit the least significant bit (LSB) first, and others send the most significant bit (MSB) first. Is a fixed translation such as 8 bits to 10 bits used? Is bit stuffing used, where a string of 1s or 0s is stuffed with an extra bit of opposite polarity to ensure that adequate transitions occur?

Sometimes bit stuffing is used to ensure a start/stop code cannot be mistakenly encoded in the data stream. Block 8 Zero Substitution (B8ZS) actually replaces a block of eight 0s with a specific line-code violation of the AMI format as shown in Figure 2.

So what’s a bit and what’s a word? A general-purpose serial-interface instrument must accommodate user specifications independent of the data itself.

Packet or Data Structure

What is data? The obvious role of bits and bytes is to transmit data. This takes a synchronization scheme. When confronted with a stream of 1s and 0s, where does the data start and end? There are fixed and variable data or packet lengths. MIL-STD-1553 has a fixed length and signals the start of data with an invalid Manchester waveform as shown in Figure 3.

The USB uses a sync pattern of 0x80 [note a 0x80(hex) in the NRZI format equals a 0x54 in NRZ format transmitting LSB first] with a nominal packet length of 12,000 bits. The Synchronous Optical NETwork (SONET) interface actually allows the position of the data or payload to float within the packet structure (using an offset pointer) after an initial sync code of 0xF628. The High-Level Data Link Control (HDLC) has variable length data and frames it with a start and stop code of 0x7E.

A general-purpose serial-interface emulator must generate and respond to these various synchronization schemes as well as numerous others.

Error Injection

In a design environment, proper operation under error-free conditions often is the simplest thing to verify. The more time-consuming effort verifies correct operation under various error scenarios. The need to test for error detection and rejection requires that a general-purpose serial-interface instrument allows you to program various error conditions. These conditions would include bit errors such as invalid parity or cyclic redundancy checks, bit-stuff failures, line-code violations, forbidden voltage levels, induced jitter, and out-of-sync sync pulses.

The Data Stream

Finally, after going from bits to bytes to data packets, you must be able to assemble a data steam representing realistic data transmissions. Some interfaces such as ARINC 429 require an intermessage gap time of at least 4 bit-times. Other interfaces use an idle packet to maintain 100% traffic for synchronization. Of course, each packet or series of packets may be repeated or looped. Automatic data generation such as pseudorandom bit sequences also must be provided.

The Result

The Talon VXI 2108 is designed to meet these challenging characteristics for a general-purpose serial emulator. It can be used by design and test engineers alike. It contains many unique features to simplify serial data generation and recording.

James A. Stroot is the marketing manager at Talon Instruments. Before joining the company 10 years ago, he was a manager of software engineering at Honeywell for 10 years and an engineering manager at Interface Technology for six years. Mr. Stroot has a B.S. in mathematics. Talon Instruments, 150 E. Arrow Hwy., San Dimas, CA 91773, (909) 599-0690, e-mail: [email protected].

May 2000

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