Is There Life After HART?

4800 BPS HART:  A Proposal

by Analog Services, Inc.

Characteristics of the FASTHART Channel
and Proposed Specification Changes

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    In this section we propose two changes to the HART Physical Layer Specification and then summarize the environment in which the proposed method must work.

Changes From Existing Spec

    We assume that the HART Physical Layer specification can be changed as follows:

1.    The maximum network resistance is 600 ohm rather than 1100 ohm.   The network resistance consists mainly of the current sense resistance in a conventional current loop.   In some cases intrinsic safety barriers are also present and barrier resistance adds to the current sense resistance.  But these combinations typically result in a total of about 550 ohm.  A total resistance of 1100 ohm is rarely seen and would prevent many loops from operating due to excessive DC voltage drop.  The proposed 600 ohm therefore seems a reasonable upper limit.  The reduction of the network resistance to 600 ohm has two beneficial effects as will be seen.

2.    The transmit signal into a test load is a minimum of 500 mV pp rather
than 400 mV pp.    An increase in minimum transmit amplitude is desired to improve signal-to-noise ratio.  This amounts to a tightening of the transmit amplitude spec, which should be possible because of improved component precision and generating the transmit signal using a D/A converter and voltage reference.   Notice that the upper limit is left as is because an increase in the upper limit would cut into available operating current for 2-wire devices.

 

Minimum Received Signal

    The existing spec for minimum received signal is 120 mV pp.  This is presumed to result from the minimum transmit signal and from the various sources of attenuation that are consistent with a properly constructed network.  The increase in minimum transmit signal will increase the minimum received signal to 150 mV pp.

Interference

    The existing interference spec. is 22 volt pp at 25 Hz and drops at 40 dB/decade above 25 Hz.   The 22 volt is calculated from 16 mA pp * R where R is a combined network and cable resistance of 1375 ohm.  If 1100 ohm of this is network resistance, then 275 ohm is cable resistance.  Using 600 ohm of network resistance, the new value of R should be 875 ohm.  Then 16 mA * 875 ohm = 14 volt pp.  This is the maximum interference at 25 Hz and drops at 40 dB/decade above 25 Hz.

    Assume that the interference should be reduced to about 0.2 of the signal.  The interference and signal are developed across the same resistance and will tend to track; with the ratio of maximum interference to minimum signal being 16.  Relative to the signal, the interference should then be attenuated by a factor of 16/0.2 = 80 or 38 dB.  If the passband gain is 1, then the gain at 25 Hz should be 1/80 or -38 dB.  This gain can rise with frequency at a rate of 40 dB/decade so that gain becomes 1 at 224 Hz.   Thus, the highpass filtering needed to remove the 4-20 mA interference is as illustrated in figure 1 below.

Figure 1 -- Required Highpass Receive Filtering

    The combined interference and signal can be as large as 14 volt pp * (1+1/16) = 14.9 volt pp.   Since the modem chip will typically have an input range of 0 to 3 volt, the combined signal and interference is too large to apply directly to a modem pin.  At present, the way around this is to do part of the highpass filtering with passive components.  If the interference is reduced by passive filtering to about 2 volt pp, then the signal and the remaining interference are 2.9 volt pp.  The requirement for a single-pole passive highpass filter is, therefore, that it attenuate by a factor of 14/2 = 7 at 25 Hz.  The implied corner frequency is 175 Hz.   The required off-chip filter is illustrated in figure 6.

Figure 2 -- Minimum Off-Chip Filter

HighPass Effect in HART Master Coupling

    A low-impedance HART Master is represented by an ideal voltage source in series with a capacitor.  This capacitor and the network resistance create a single-pole highpass filter.  The network resistance ranges from 170 ohm to 600 ohm and various influences dictate a capacitor size on the order of 2 to 5 ufd.  Thus, the highpass corner frequency due to this coupling ranges from 53 Hz to 468 Hz.

Lowpass Corner Due to Network

    The network capacitance and network resistance form a lowpass filter.  Existing HART specifications keep this corner frequency at or above 2.5 kHz.

Noise And Bit Error Rate

    A study of noise on process loops by Rosemount Inc. found noise as high as 174 microvolt/(root Hz).   This, however, is too high for existing HART and is probably not representative of an usable loop.  (Our on-line book on HART indicates an error rate of 0.314 for existing HART with this noise density.)  A reasonable level of noise is more like 100 microvolt/(root Hz), which would include all but one of the measurements in the Rosemount study.  This level of noise is assumed to exist at the receiver regardless of the filtering that the signal may have experienced ahead of the receiver.

    A measure of the effectiveness of error control is the rate of undetected message errors (UMEs).   A rate of about 10 UME per year of continuous operation is the generally accepted number for HART.  For messages of 40 bytes each at 4800 bps, it takes 92 millisecond to send the message.  This is followed by a 73 millisecond gap for a total of 165 millisecond.  The number of messages per year is 191e6.  And the required probability of UME =  Pume = 5.2e-8.

Need For Constant Envelope

    Many existing devices operate in such a way that they cannot tolerate much variation in the signal envelope.  For example, suppose a device is putting out an average value of   4 mA and the superimposed communication has a peak amplitude of 1.5 mA.  The result is that the operating current available to the device during the communication peak is only 2.5 mA.  This may not be possible in some devices because of the need to draw an operating current of at least 3 mA.

    Existing HART has a very constant envelope.  A higher speed HART doesn't have to be this good, but should at least be close.

Transmit Shaping

    HART communication is subject to crosstalk because networks are single-ended.  Crosstalk is controlled by preventing high-frequency components of the transmitted signal.   Existing HART devices use either sinusoidal or trapezoidal shaping of the transmit signal.

HART Channel Summary

Table 1 -- Summary of HART Channel

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