Communication method and communication system

The present invention relates to a communication method and a communication system which perform mutual recognition among plural devices connected to a bus.

Hitherto, a system has been provided in which plural devices are connected via a bus and operate in a linked manner on the basis of data transmitted through the bus.

In such a system, it is common practice for each device to perform inquiries and to poll one another in order to recognize other devices. Polling refers to each device confirming (inquiring) with another device whether there is a data transmission request, and if there is a transmission request, the data is received.

For example, in a system formed of three devices as shown in Fig. 8, polling in a conventional system is performed mutually among the three devices.

More specifically, a first device 10₁ polls a second device 10₂ and a third device 10₃ ; the second device 10₂ polls the first device 10₁ and the third device 10₃; and the third device 10₃ polls the first device 10₁ and the second device 10₂.

However, if each device performs polling of another device in this manner, a substantial amount of the bus bandwidth is occupied for polling. Therefore, if the number of devices is increased, the occupied bus bandwidth will increase considerably.

Accordingly, it may be conceived that, for example, the first device 10₁, on behalf of the others, performs polling of other devices, and the result is transmitted as a message to the other devices. According to this scheme, since the devices other than the representative first device 10₁ do not perform polling, it is possible to reduce the bandwidth occupied.

However, when the second device 10₂ and the third device 10₃ are newly added to the system, the second device 10₂ and the third device 10₃ are not recognized until they are polled from the representative first device 10₁. Therefore, since the second device 10₂ and the third device 10₃ are isolated until they are polled from the representative first device 10₁, it is not possible to make full use of the mutual functions among devices, such as data exchange among devices.

Also, when the representative first device 10₁ does not perform polling for some reason or when the representative first device 10₁ is not present, the second device 10₂ and the third device 10₃ cannot be aware of the presence of each other. For this reason, it is not possible to realize functions which are possible only between the second device 10₂ and the third device 10₃.

The present invention has been proposed in view of the above-described circumstances. An object of at least preferred embodiments of the present invention is to provide a communication method and a communication system which are capable of reducing the amount of polling and reducing the bus transmission bandwidth without impairing the mutual functions among devices.

According to one aspect of the present invention, there is provided a communication method which uses a system in which plural devices are connected with one another by a communication line, one device performs inquiries to other devices, the result of the inquiries is transmitted to the other devices, allowing the devices to recognize one another, characterized in that the other devices perform inquiry more infrequently than the frequency of inquiry of the one device.

According to another aspect of the present invention, there is provided a communication system in which plural devices are connected with one another by a communication line, one device performs an inquiry to other devices, the result of the inquiries is transmitted to the other devices, allowing the devices to recognize one another, characterized in that the other devices perform an inquiry more infrequently that the frequency of inquiry of the one device.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: <ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 is a diagram showing the construction of a system in which plural devices are connected to a bus;</li><li>Fig. 2 is a diagram showing the structure of a header and data which follows the header;</li><li>Fig. 3 is a diagram showing a polling command and an acknowledgement;</li><li>Fig. 4 is a diagram showing the structure of a polling command;</li><li>Fig. 5 is a flowchart showing the flow of polling;</li><li>Fig. 6 is a flowchart showing the flow of polling at a different rate of frequency;</li><li>Fig. 7 is a diagram showing polling at a different rate of frequency; and</li><li>Fig. 8 is a diagram showing the construction of a conventional system.</li></ul>

An embodiment of the present invention will now be described below with reference to the accompanying drawings.

First, referring to Fig. 1, a description is given of a communication system, in which plural devices are connected to a bus of a communication line, and to which a communication method of this embodiment is applied.

This system consists of n devices: a first device 10₁ up to an n-th device 10_n. Here, n is an integer of 2 or more.

The first device 10₁ comprises a first control section 11₁ for controlling the entirety of this first device 10₁, and a first interface section 12₁ for connecting the control section 11₁ to a bus 21.

The first control section 11₁ performs control on each section of the first device 10₁. Also, the first control section 11₁ controls communication for other devices connected to the bus 21 via the first interface section 12₁.

This first control section 11₁ can be formed as a microcontroller having, for example, a CPU, a ROM, a RAM, etc.

The first interface section 12₁ connects the first control section 11₁ to the bus 21. The first interface section 12₁ performs conversion between an internal data format in the first device 10₁ and a predetermined protocol in the bus 21.

Since the construction from the second device 10₂ to the n-th device 10_n is the same as the construction of the first device 10₁, for the sake of simplicity, the same reference numerals are used, and descriptions thereof are omitted.

The bus 21 connects the devices from the first device 10₁ to the n-th device 10_n so that data is transmitted among the devices. In this bus, communication is performed by a predetermined method in accordance with a predetermined protocol.

This embodiment has a feature in the method of communicating data transmitted among the devices via the bus 21. That is, in this embodiment, a device which is newly added to the bus due to a new connection or at the activation of power performs an inquiry, that is, polling, only once at that time, and confirms the presence of the device of the other party. After polling is performed once, a polling command from the above-mentioned representative device is received, and the newly connected device does not itself thereafter performs polling.

Also, in this embodiment, a device which cannot act as a device representing the system performs polling of the representative device more infrequently than the polling frequency of the representative device, and when the device is polled by the representative device, the device does not perform polling of other devices.

Furthermore, in this embodiment, when the representative device cannot be found by a device, polling is performed of other devices.

When polling is performed among devices, a client device does not usually perform polling, but performs polling only once at the set-up time of the device.

A communication method performed in this manner in a system having the construction such as that described above is described below in detail.

First, a common communication method, which is a basis of this embodiment, in a system in which plural devices are connected to a bus, is described below with reference to Fig. 2.

Here, a case, in which an n-th device 10_n is assumed to be a source device and a first device 10₁ is assumed to be a destination device, is taken as an example. Needless to say, a combination of a source device and a destination device is not limited to this example.

The n-th device 10_n transmits a header composed of a set of a source device address which is the address of the n-th apparatus 10_n and a destination address which is an address of the first device 10₁ which is the destination. This header is received by the first device 10₁ via the bus 21.

For the transmitted header, an acknowledgement is sent back from the first device 10₁ which is at a destination address. This makes it possible to confirm the fact that the header is received by the first device 10₁.

The n-th device 10_n transmits data following the header. This data is also received by the first device 10₁ via the bus 21, and an acknowledgement is sent back from the first device 10₁.

A case in which the n-th device 10_n performs polling to the first device 10₁ can also be performed in a manner similar to a case in which data is transmitted.

As shown in Fig. 3, in the case of polling, the n-th device 10_n transmits a polling command. As shown in Fig. 4, similarly to the above-mentioned header, the polling command is composed of a set of the source device address which is the address of the n-th apparatus 10_n and the destination address which is the address of the first device 10₁ which is the destination.

When the first device 10₁ receives the polling command sent from the n-th device 10_n, the first device 10₁ sends back an acknowledgement. This makes it possible to confirm the fact that the polling command is received by the first device 10₁.

As described above, in the case of polling, only the header part is transmitted as a polling command, and the destination device sends back an acknowledgement, making it possible to confirm the presence of the destination device.

In this embodiment, it is assumed that the basic state is one in which one device among plural devices making up the system performs polling as a representative and the other devices do not perform polling.

In the following, it is assumed that the first device 10₁ represents plural devices of the system. Needless to say, the representative device is not limited to the first device 10₁.

As a result of polling the other devices, the representative first device 10₁ obtains information on devices which are present and are connected to the bus 21. This presence information is transmitted from the first device 10₁ to the other devices. Specifically, the presence information is carried on the data part described with reference to Fig. 2, and is transmitted as a message for the other devices from the representative first device 10₁.

In this embodiment, either when a device is newly connected to the bus 21 or when the power is switched on, data is transmitted in accordance with a predetermined communication method. Accordingly, steps of such a communication method are described below with reference to Fig. 5.

This series of steps is started when, for example, an n-th device 10_n is newly connected to the bus 21, or when power to an n-th device 10_n which is connected to the bus 21, but is not switched on, is newly switched on.

In step S11, as shown in Fig. 3, an n-th control section 11_n of the n-th device 10_n outputs a polling command to the bus 21 via an n-th interface section 12_n. The devices, other than the n-th device 10_n, which are connected to the bus 21, know that the n-th device 10_n is connected to the bus 21 in accordance with this polling command.

In step S12, the process branches depending on whether or not the n-th device 10_n has received an acknowledgement from the first apparatus 10₁, for example, within a predetermined time. When the first device 10₁ is present, since the first device 10₁ sends back an acknowledgement, it is possible to know the presence of the representative first device 10₁ according to the presence or absence of the acknowledgement.

More specifically, when it is found that the representative first device 10₁ is present as a result of receiving the acknowledgement, assuming that the determination is "YES" the process proceeds to step S13.

Thereafter, the n-th device 10_n does not output a polling command, and only sends back an acknowledgement to the polling command from the representative device.

In this case, the devices other than the first device 10₁ know that an acknowledgement is returned from the first device 10₁ to the n-th device 10_n and the first device 10₁ is a representative device.

On the other hand, when an acknowledgement is not received and it is found that the representative first device 10₁ is not present, assuming that the determination is "NO", the process returns to step S11. Then, in step S11, a polling command is transmitted again to the representative first device 10₁.

In step S13, the process branches depending on whether or not the n-th device 10_n has received a polling command from the representative first device 10₁. Then, when the polling command is received, for example, within a predetermined time, assuming that the determination is "YES", the process proceeds to step S14, and when the polling command is not received, assuming that the determination is "NO", the process returns to step S13.

The n-th device 10_n waits until a polling command from the representative first device 10₁ occurs in this step S13.

In step S14, since the n-th device 10_n has received the polling command in step S13, an acknowledgement is sent back.

Next, another communication method of this embodiment is described with reference to Fig. 6. In this communication method, when polling is performed among devices, the devices other than the representative first device 10₁ perform polling to the representative first device 10₁ more infrequently. The first device 10₁ and the n-th device 10_n perform polling at different levels of frequency.

More specifically, as shown in Fig. 7, the representative first device 10₁ performs polling at an interval of a first period T₁, and the n-th device 10_n performs polling at an interval of a second period T₂. The second period T₂ is longer than the first period T₁:<maths id="math0001"><math display="block"><mrow><msub><mrow><mtext>T</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext> &gt; T</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><mtext>.</mtext></mrow></math><img file="EP1096733A2_D0001.tif" /></maths>

Also, when a polling command is received once, the devices themselves other than the representative first device 10₁ stop polling.

In the initial step S21, the process of the n-th device 10_n branches depending on whether or not the second period T₂ has elapsed from when the polling command was received previously at the time the polling command is newly received.

The n-th device 10_n has contained therein an interval timer for measuring the elapse of time. Whether or not the second period T₂ has elapsed is determined according to whether or not this interval timer is 0.

Then, when the second period T₂ has elapsed, assuming that the determination is "YES", the process proceeds to step S22. When the second period T₂ has not elapsed, assuming that the determination is "NO", the process proceeds to step S25.

When the representative first device 10₁ is present, since the first device 10₁ issues a polling command at an interval of the first period T₁. the second period T₂, which is longer than the first period T₁, never elapses. Therefore, when the first device 10₁ is present, the polling command is transmitted only from the representative first device 10₁.

Step S22 corresponds to a case in which it is determined that the representative first device 10₁ is not present in step S21. This is a state in which polling cannot be performed, for example, as a result of the first device 10₁ being disconnected from the bus 21 or the power being switched off.

In such a case, the n-th device 10_n starts polling at the timing when the interval timer reaches the second period T₂.

In step S23, the process branches depending on whether or not the n-th device 10_n has received an acknowledgement, for example, within a predetermined time. When the acknowledgement has been received, a device of the other party which sent back the acknowledgement is present. When the acknowledgement has not been received, a device of the other party which sends back the acknowledgement is not present.

When the n-th device 10_n has received the acknowledgement, assuming that the determination is "YES", the process proceeds to step S24. In step S24, the second period T₂ is set to the interval timer. Then, the process returns to step S21.

When the n-th device 10_n has not received the acknowledgement, assuming that the determination is "NO", the process returns to step S22. In step S22, the polling command is transmitted again.

On the other hand, step S25 corresponds to a case in which it is determined in step S21 that the first device 10₁ is present. In this case, since the n-th device 10_n has received the polling command from the first device 10₁, the n-th device 10_n recognizes the presence of the first device 10₁ and starts the counting in the interval timer from that time.

In step S26, the process branches whether or not the n-th device 10_n has received a polling command, for example, within a predetermined time. When the polling command has been received, a device of the other party which transmitted the polling command is present. When the polling command has not been received, a device of the other party which transmits the polling command is not present.

When the n-th device 10_n has received the polling command, assuming that the determination is "YES", the process proceeds to step S27.

In step S27, the second period T₂ is set to the interval timer. In step S28, the n-th device 10_n transmits an acknowledgement. Then, the process returns to step S21.

When the n-th device 10_n has not received the polling command, assuming that the determination is "NO", the process returns to step S21.

As has thus been described, in at least preferred embodiments of the present invention, a device which is newly connected to the system performs an inquiry once at the time of connection. Also, a device other than a device representing the system performs an inquiry more infrequently than does the representative device.

Therefore, according to at least preferred embodiments of the present invention, it is possible to reduce the bandwidth occupied in the bus and to reliably start up mutual functions among devices.