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Transmitter/Communications Characteristics

Page 8 34-ST-25-20 MC Toolkit User Manual Release 0

7/03

CAUTION:

Although it in not necessary to put a control loop in manual before communicating with a

transmitter operating in DE mode, caution is required if there is any potential for error in

identifying operating mode.

Honeywell HART Transmitters

Transmitters with HART capability have features that vary among manufacturers and with the

characteristics of specific devices. The MC Toolkit supports the HART Universal, Common Practice, and

Device Specific Commands that are implemented in Honeywell HART transmitters.

As the diagram in Figure 6 shows, the Honeywell HART Transmitter is virtually identical to non-HART

transmitters, except that the HART version includes a Digital I/O Modulator/Demodulator block (key

number ¯ ) instead of the Honeywell DE communications block.

As indicated in Figure 7, the output of the HART includes two primary modes:

· Point-to-Point Mode, in which one transmitter is connected via a two-conductor, 4-20 mA current loop

to one receiver.

· Multi-Drop Mode, in which several transmitters are connected via a two-conductor network to a

multiplexed receiver device.

In point-to-point mode, the value of the primary PV is represented by a 4-20 mA current loop, almost

identical to that of the Honeywell Transmitter operating in analog mode. In this case however, the analog

signal is modulated by Frequency Shift Keying (FSK) methods, using frequencies and a current amplitude

that do not affect analog sensing at the receiver.

Note that the accuracy of the analog level must be precisely controlled for accurate sensing, but that HART

communications will not "bump" the process variables.

In multi-drop mode, up to 16 transmitters (addresses 0-15) can exist on the two-conductor network, which

precludes analog transmission methods. In this case, the same FSK modulation method is used for

conveying levels of PV (and other variables) and also for communications.

CAUTION:

Before connecting to a HART transmitter, ensure that the MC Toolkit is not set up for DE

communications, whose current amplitude can "bump" process variables in either point-to-

point mode or in multi-drop mode.

Other HART Transmitters

HART-capable transmitters from any manufacturer and for any specific purpose are designed to common-

agreement standards that provide for inter-operability.

Guidelines published by the HART Communication Foundation enables manufacturers to design devices

that communicate via a set of standard commands and responses.

The standard set of commands is an integral component of the Honeywell MC Toolkit that enables

communication with many HART transmitters from other manufacturers.

The MC Toolkit supports the HART Revision 5.0 Universal Commands and HART Revision 5.0 Common

Practice Commands. However, the MC Toolkit supports only a recommended number of Common

Practice commands. The MC Toolkit does not support device-specific commands for non-Honeywell

transmitters. For more information, refer to tables in the Reference Data section of this manual.

1 2 ... 11 12 13 14 15 16 17 18 19 20 21 ... 109 110

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