GB2303263A - Conversion of pulse stream into Fieldbus data stream. - Google Patents

Abstract

A stream of impulses comprising a preamble, a start delimiter and a data message is converted into an electrical fieldbus data-stream with automatic compensation for an inconsistent or undefined number of preamble pulses in order to create an output regenerated signal of the correct polarity. The compensation is effected by selectively delaying the effect of some impulses. Incoming impulses are assigned a signal value which is inverted with each received impulse until the start delimiter is detected. Then the signal value of the regenerated signal is examined, and, if incorrect, an adjustment is made.

Description

PULSE CONVERSION METHODS & APPARATUS This invention relates to methods of and apparatus for converting a stream of pulses into a Fieldbus datastream.

This invention is particularly concerned with circuits which convert a stream of impulses into an electrical Fieldbus data-stream. The circuit automatically compensates for an inconsistent or undefined number of preamble pulses to create an output signal of the correct polarity.

The Fieldbus Standard is a vehicle used in industrial control systems which defines a means of electrically connecting a number of devices, and a mechanism used to communicate data amongst the devices.

Document IEC 1158-2:1993 Fieldbus Standard for use in industrial control systems, Part 2: Physical layer specification and service definitions, defines the Standard.

The Fieldbus can be used to provide power to attached devices as well as for the purpose of communicating data. Data is transferred over the Fieldbus by means of signalling known as Manchester Encoding, superimposed on any supplied power.

Serial data is a logical stream of bits each having a value of "zero" or "one". . A device is required which converts serial data in the form of a stream of impulses into serial data in the form of a Manchester Encoded signal.

The stream of impulses is an attractive form of communication which features a low energy requirement for the transmission of data. Each impulse indicates a change of state for the Manchester Encoded signal.

Thus the Manchester Encoded signal can be synthesised directly from the impulse stream.

The document IEC 1158-2 referred to above describes the communication of data in messages which begin and end with a particular delimiter. The communication is asynchronous and therefore each message must be preceded by a preamble pattern which allows receiving devices to synchronise to the incoming data. Problems arise with this preamble due to a number of factors. The primary factor is that the total number of preamble bits is not fixed and noise or other extraneous events may therefore confuse the initial part of the preamble. This means that the correct state of the regenerated signal in the known systems cannot be resolved until the arrival of the start delimiter. It is not an acceptable solution to store incoming data until the start delimiter is received before relaying the information to the Fieldbus.

Therefore, in accordance with the invention, a starting signal value is assumed and is transmitted to the bus as soon as impulses are received. This signal value is changed with every received impulse until the start delimiter is detected. When the start delimiter is received the value of the regenerated signal is examined, and if it is correct the circuit simply continues to transmit signal changes as impulses arrive. However, if the signal value is incorrect an adjustment must be performed in order to make the message intelligible to Fieldbus devices.

The key feature of the present invention is the method of selectively delaying the occurrence of a signal state transition derived from an impulse stream in order to correctly polarise the Manchester Encoded signal transmitted to a Fieldbus.

The adjustment process involves delaying the application of some impulses to the regenerated signal by one-half bit time, depending upon what has previously been received.

A description will now be given of one presently preferred circuit for achieving this, with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of the circuit; and Fig. 2 is a schematic diagram to illustrate the signal timing.

The circuit of Fig. 1 can be considered to operate in one of four states. The first state is an idle state when nothing is happening. The second state is entered from the idle state when a preamble sequence is received. The device then enters state three or state four depending upon the state of the regenerated signal when the start delimiter is received. In state three the circuit transparently copies signal transitions to the Fieldbus. In state four the circuit modifies the timing of signal transitions in order to achieve the effect of inverting the sense of the regenerated Fieldbus signal. The circuit returns to state one (idle) when no impulses are received for a period greater than one and one half bit-times.

While in state two the time between impulses is measured in order to detect the beginning of the start delimiter. During this time the preamble is transmitted to the electrical interface, the signal sense being inverted at each received impulse.

When two impulses are received separated in time by half a bit-time this is interpreted as the leading edge of the N+ event in the start delimiter. At this instant the current state of the regenerated signal being transmitted to the electrical bus is examined.

If it is of the correct polarity then no further interpretation of data is necessary and the circuit enters state three.

If the polarity of the regenerated signal is wrong the circuit enters state four. In this state the effect of some impulses, including that which caused the transition from state two to state four, must be delayed according to the following rule: The effect of every impulse that occurs half a bit-time after a preceding impulse must be delayed by half a bit-time.

This rule is applicable to every impulse in the current message, including the impulse which was detected at the N+ event, and therefore the converter must be able to deal with remembering one delayed impulse while dealing with a previously delayed impulse.

Fig. 1 shows a series of three time delay elements T1, T2, T3 which when triggered become active for some known time. They are arranged such that when any element completes its timing function it triggers the next element in the series, the final element triggering an impulse generator I1. Additionally, element T2 controls the position of a changeover switch SW. The toggle element FF whose output is the Manchester Encoded signal changes state when it receives an impulse.

In the following description the delay-time yielded from each time delay element is referred to by the name of the element in the diagram, i.e. element T2 yields a delay T2. The values of the various delays are arranged to have the following relationships: T1+T2+T3 = one bit-time; T1+T2 > one half bit-time Tlcone half bit-time; T2 < T1 When the circuit of Fig. 1 is in a relaxed state, any incident impulse will pass through the switch SW and toggle the state of FF. It will also trigger the delay T1. If a second impulse occurs after half a bit time it will trigger the delay T1 but will not pass through the switch SW because T2 will be active and switch SW will be in the other position. After a time T1+T2 from the first impulse the element T3 will become active, and after Tl+half bit-time the element T2 will again become active, switching SW to the second position. The switch will remain in this position until after T3 expires, causing the impulse from I1 to toggle FF.

An examination of the resultant waveform (see Fig.

2) shows that this technique is equivalent to inserting one extra bit into the preamble before the start delimiter, i.e. a one-bit time delay in the transmitted data.

In Fig. 2 the broken vertical lines represent bit cell centres. The four signal patterns 1 to 4 represent the following: 1. Impulse pattern 2. Correct regenerated Fieldbus signal 3. Incorrect regenerated Fieldbus signal 4. Adjusted regenerated Fieldbus signal.

Claims (8)

CLAIMS:

1. A method of converting a stream of impulses into an electrical fieldbus data-stream, where the impulse stream comprises a preamble, a start delimiter and a data message, which comprises the steps of assigning a starting signal value and transmitting this value to the fieldbus as soon as impulses are received, inverting this signal value with each received impulse until the start delimiter is detected, then examining the value of the regenerated signal, and, depending on whether the regenerated signal is correct, either continuing to transmit signal changes as impulses arrive, or, if incorrect, adjusting the signal to make it intelligible to fieldbus devices.

2. A method according to claim 1, in which the adjustment comprises delaying the application of some impulses to the regenerated signal by one-half bit time.

3. A method according to claim 2, in which the effect of each impulse which occurs half a bit-time after a preceding impulse is delayed by half a bit-time.

4. Apparatus for converting a stream of impulses into an electrical fieldbus data-stream, comprising means selectively to delay the occurrence of a signal state transition derived from the impulse stream in order correctly to polarise an encoded signal transmitted to a fieldbus.

5. Apparatus according to claim 4, for an impulse stream comprising preamble bits and a start delimiter, which includes a circuit operable in one of four states, the first state being an idle state, the second state being entered from the idle state when a preamble sequence is received, and the third and fourth states being alternative states entered in accordance with the state of the output signal when the start delimiter is received.

6. Apparatus according to claim 5, in which while in the second state the time between impulses is measured to detect the beginning of the start delimiter, and during this time the preamble is transmitted with the signal sense being inverted at each received impulse.

7. A method substantially as hereinbefore described with reference to the accompanying drawings.

8. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.