TechArchive

This article was originally written in the period 1995-2000

You’ve installed the flowmeter. Now how do you handle the signal? Advice from John Kelly of Fischer & Porter.

The primary task of the converter connected to a primary sensor is to produce an output signal which is proportional to the measured value and which can be used by other secondary instruments connected to the converter (indicators, recorders, controllers). Standardised output signals are essential because secondary instruments are universally installed.

With the advent of the so-called intelligent converter, electrical data systems have forged to the forefront. Analogue information transmission is still preferred but with the increasing number of signals transmitted the costs per signal is increasing. Digital technology offers appreciable advantages:

The number of electrical interconnections decreases dramatically because not every output requires its own leads.

Communication is not limited to the transmission of measurement signals. The data exchange occurs in both directions, so that the access and setting of parameters can be effected from the outside. Error diagnosis is possible at any time.

The digitally operating process computers do not require an A/D converter, which could cause transmission errors.

The complete system can be expanded without great effort.

Insensitivity to external interferences on the communication lines.

Instruments and systems using an air supply are no longer used extensively. The reason is the large expense incurred of the air supply and installations The standardised signal is the pressure range between 0.2 and 1 bar.

The most common standard electrical signal is a direct current of 0-20 mA or 4-20 mA. It is called an impressed direct current because it is independent of the electrical resistance up to a maximum load. The maximum load for flow rate converters is 750 or 1000 ohms which means that the total resistance in the secondary circuit can lie between 0 and 1000 ohms. The 0-10, 2-10 mA or 0-5 mA signals are used less frequently.

There are two types of output contacts. The continuous contact is used for status signals with which an alarm or limit condition can be indicated.

The pulse output has a completely different function. Short term pulses transmit data, in fact, volume proportional signals can be integrated by a totaliser. In this manner a volume signal is generated from the flow rate. The totaliser indicates the total amount that has flowed through the meter during a specific time interval.

Electrically there are a number of methods to effect an output contact. Passive methods are a relay or optocoupler contact. Current and voltage pulses are known as active outputs.

The data traffic with a process computer is in digital form. The data link has the task to convert all the signals so that a faultless communication is possible. All command characters and values are transmitted as data words. Their structure and timing are determined by a transmission protocol which is designed by the manufacturer.

Bailey-Fischer & Porter utilises the ASCII Code. a communication is always initiated from a personal computer (PC), a memory programmable control (SPS) or from the lead computer set up as the master, which means that the metering instrument only transmits data upon request.

The serial data link, RS232C (or also V.24) operates with a square wave signal level of 15 V.

The data link, with three connecting leads, functions only in a direct connection with the PC or other devices. The cable length is limited to 15m because of the asymmetrical square wave and its sensitivity to interference.

With a similar but symmetrical square wave signal conducted on two leads the RS485 operates with considerably less sensitivity to interference. Therefore the cable length can be as long as 1200m. Bailey-Fischer & Porter installs four conductors instead of the usual two and forgoes the cross cabling.

A bus system can be utilised with the RS485 data link with up to 32 instrument participants. This means that up to 32 instruments can be connected in parallel on a single cable. They are only activated when they are addressed by the master.

The world wide standardised signal in the measurement industry is the 4 to 20 mA current. Since this analogue signal can only transmit one parameter a system has been developed with which additional information can be transmitted using an alternating current signal superimposed on this current. With the HART Protocol (Highway Addressable Remote Transducer) processes the various parameters from a variety of manufacturers. Because the average value of the frequency is zero the digital communication does not affect the analogue signals.

The modulation of the communication signals is provided by a Bell 202 modem as the link between the PC or master computer and the 4-20 mA line, which can be interconnected at any arbitrary location along the current line.

Another communication means is with a hand held terminal which contains a modem as well as a PC. It is certainly an elegant and problem-free procedure when a hand held terminal is connected at any location on the signal line to access or change the instrument parameters. And for a variety of instruments, one after the other.

The HART Protocol allows operation from two master instruments. When a multiplexer is installed by Bailey-Fischer & Porter utilising the HART Protocol, up to 1024 field instruments can be monitored and programmed centrally.

Field bus technology is the logical extension of the path established by the HART Protocol. The goal is the standardisation of digital data processing as is the case for the 4-20 mA system in the analogue technology and will eventually replace it.

The field bus is a two conductor line to which each participant can connect. It includes an energy source, that is, the bus functions as the power supply for all the field instruments connected to it. As opposed to the HART Protocol which operates at 1200 bits/s the bus will operate appreciably faster at 1 Mbits/s. The functions will be greatly improved, as the master does not have to be turned on for every data exchange. Direct exchange from slave to slave is possible and because of the speed, preferred.

The user need no longer be concerned with the details of the communication technology, because the instrument manufacturer will implement all his parameters in the protocol so that similar products can be operated with similar schemes.

• Fischer & Porter