15.5.2Physical encoding of bits

Telegraph systems were Boolean in nature: representing “dots” and “dashes” by one electrical state of the telegraph line, and pauses by another. When manually-actuated keyswitches were abandoned in favor of teletype machines, and Morse code abandoned in favor of the Baudot (5-bit) code for representing alphanumeric characters, the electrical nature of the telegraph (at least initially25) remained the same. The telegraph line would either be energized or not, corresponding to marks or spaces made on the teletype paper.

Many modern digital communication standards represent binary “1” and “0” values in exactly this way: a “1” is represented by a “mark” state and a “0” is represented by a “space” state. “Marks” and “spaces” in turn correspond to di erent voltage levels between the conductors of the network circuit. For example, the very common EIA/TIA-232 serial communications standard (once the most popular way of connecting peripheral devices to personal computers, formerly called RS-232) defines a “mark” (1) state as −3 volts between the data wire and ground, and a “space” (0) state as +3 volts between the data wire and ground. This is referred to as Non-Return-to-Zero26 or NRZ encoding:

Non-Return-to-Zero (NRZ) encoding

space

mark

An easy way to remember the di erence between a “mark” and a “space” in this scheme is to recall the operation of old telegraph printing units, specifically how they created marks and spaces on moving paper strip. When the printing unit was energized (i.e. the transmitting key was pressed, sending current through the solenoid coil of the printer, corresponding to a “1” state), the printer’s iron armature would be pulled down to draw a mark on the paper strip. When de-energized (transmitting key released, stopping current in the telegraph line, corresponding to a “0” state), the printer’s armature would spring-return up from the paper to leave a blank space.

25Later versions of teletype systems employed audio tones instead of discrete electrical pulses so that many di erent channels of communication could be funneled along one telegraph line, each channel having its own unique audio tone frequency which could be filtered from other channels’ tones.

26This simply refers to the fact that the signal never settles at 0 volts.

This is not the only way to represent binary bits, though. An alternative method is to use an oscillating (square-wave) signal, counting up and down transitions (pulse edges) at specific times to represent 1 and 0 states. This is called Manchester encoding, and it is used in the 10 Mbps (10 million bits per second) version of Ethernet and in both the FOUNDATION Fieldbus “H1” and Profibus “PA” instrumentation network standards:

Note how each binary bit (0 or 1) is represented by the direction of the voltage signal’s transition. A low-to-high transition represents a “1” state while a high-to-low transition represents a “0” state. Extra “reversal” transitions appear in the pulse stream only to set up the voltage level as needed for the next bit-representing transitions. The representation of bits by transitions rather than by static voltage levels guarantees the receiving device can naturally detect the clock frequency of the transmitted signal27. Manchester data is therefore referred to as self-clocking.

Interpreting a Manchester waveform is easier than it first appears. The key is identifying which transitions represent “clocked” bits and which transitions represent “reversals” prior to bits. If we identify the widest periods of the waveform, we know the transitions in these periods must represent real bits because there are no reversals. Another way to say this is the greatest time period found between successive transitions in a Manchester waveform is the clock period. Once we identify this clock period, we may step along the waveform at that same period width to distinguish clocked bits from reversals.

27This is most definitely not the case with NRZ encoding. To see the di erence for yourself, imagine a continuous string of either “0” or “1” bits transmitted in NRZ encoding: it would be nothing but a straight-line DC signal. In Manchester encoding, it is impossible to have a straight-line DC signal for an indefinite length of time. Manchester signals must oscillate at a minimum frequency equal to the clock speed, thereby guaranteeing all receiving devices the ability to detect that clock speed and thereby synchronize themselves with it.

Yet another method for encoding binary 1 and 0 states is to use sine waves of di erent frequencies (“tone bursts”). This is referred to as Frequency Shift Keying, or FSK, and it is the method of encoding used in the HART “smart” instrument communications standard.

Frequency Shift Key (FSK) encoding

In the HART standard – based on the Bell 202 standard used in telephone data exchange – two complete cycles at 2200 Hz represents a “0” bit (space), while one complete cycle at 1200 Hz represents a “1” bit (mark). This standard was invented as a way to exchange digital data over telephone networks, which were built to communicate audio-frequency28 AC signals and thus could not reliably communicate the square-wave signals associated with direct digital data. By assigning digital values to di erent audio frequencies, serial data could be communicated over telephone channels as a series of sine-wave tones.

The same principle of FSK allows HART-compatible instruments to communicate digital data over cables simultaneously carrying DC current (4 to 20 mA) signals representing control system data. Since each bit of FSK-encoded data consists of complete AC cycles (one full positive swing for every full negative swing), the superposition of AC tones does not a ect the time-averaged value of the DC milliamp signal.

Other methods exist as well for encoding digital data along network cables, but these three are the most popular in industrial networks.

28It is relatively easy to build an apparatus that makes HART tone signals audible: simply connect a small audio speaker to the low-impedance side of an audio transformer (8 ohms) and then connect the high-impedance side of that transformer (typically 1000 ohms) to the HART signal source through a coupling capacitor (a few microfarads is su cient). When HART communications are taking place, you can hear the FSK tones reproduced by the speaker, which sound something like the noises made by a fax machine as it communicates over a telephone line.