FR2636447A1 - Method of managing a message memory in a station of a data transmission network and station designed for implementing the method - Google Patents

Abstract

The invention relates to a method of managing a message memory or the like in a station of a data transmission network including a bus contention handler governing accesses to the bus by the various stations, in which the sending of messages by a station involves the sending to the contention handler of a corresponding request and the storage by the latter, keeping them in order, of all the requests received from the station, then subsequently the sending by the contention handler of a sending authorisation to the station in question. The method is characterised by the steps consisting in: - storing, in succession, in a memory, the information to be sent and keeping them in order; - each time a request is sent to the contention handler, altering the value of an index y for access to the said memory; and - each time a sending authorisation is received from the contention handler, sending the information designated by a pointer n + y indexed by the said index. The invention also relates to a station implementing this method.

Description

 The present invention relates to a method and a device for managing a buffer memory, and more particularly to a first-to-first memory type. outputted for storing data to be transmitted over a broadcast type communication network.

 Such a communication network, used in particular in an industrial environment (so-called field network) conventionally comprises a bus-type transmission line, a set of stations that must transmit and receive data, such as parameter values, and possibly messages on the other hand. the network and an arbitrator responsible for managing access to the network by the various stations.

 Typically, the data exchanged on the network are values of industrial parameters, called "objects", determined once and for all and well identified. However, certain types of networks offer a non-priority mode of service "messaging", allowing under certain conditions the transmission of messages between the various stations. When this is the case, each station creating messages intended for the other stations must store these messages in a buffer memory while waiting for the referee, who determines the periods of time reserved for this type of transmission, to give him the authorization to cancel. a first message, then one after another the following messages.

 The present invention proposes to optimize the management of such a memory by minimizing the movement of data therein and by minimizing the storage of information other than the messages properly so-called.

For this purpose, the present invention firstly relates to a method of managing an information memory in a station of a data transmission network comprising a plurality of stations connected to each other by a bus, as well as a bus arbitrator governing the accesses to the bus by the various stations, a network of the type in which the transmission by a station of at least one type of information involves the sending to the arbitrator of a corresponding request and the storage by the latter, with conservation of the order, of all the requests received from the station, then subsequently the sending by the arbitrator to the station in question of an authorization to issue this type of information, characterized in that that he understands the steps of
- store in succession in a memory the information to be issued, with preservation of their order;
each time a request is sent to the arbitrator, modifying the value of an access index to said memory; and
- Whenever a transmission authorization is received from the arbitrator, issue the information designated by a pointer indexed by said index.

The invention also relates to a station of a data transmission network, for implementing the method defined above, of the type comprising a central unit, a random access memory and an acquisition and / or retrieval unit. of data, characterized in that at least a part of the memory is a message memory whose access by the central unit is governed by a position pointer of the first message, an index of the number of messages stored, and a index of the number of stored messages for which a message transmission request has been sent over the network, the pointer being incremented or decremented modulo
N, N being the number of message slots in said portion of the memory.

Other aspects, objects and advantages of the present invention will become more apparent upon reading the following detailed description of a preferred embodiment thereof, given by way of example and with reference to the accompanying drawings, in which:
FIG. 1 is a block diagram of a so-called ground network to which the present invention applies,
FIG. 2 is a diagram of the organization of a memory of a station of the field network of FIG. 1, managed in accordance with the present invention, and
- Figure 3 is a flow chart of the operation of the management unit of this memory.

 Referring firstly to FIG. 1, a field network for communication in an industrial environment comprises a set of stations 1, 1 ', t ", ..., such as microprocessor-controlled PLCs, interconnected via a network 2 of the broadcast type, constituted in practice by a simple bus A bus arbitrator 3 is responsible for managing the various accesses to the bus for exchanging data, messages, etc., between the various stations. here in detail all the architecture of such a network.For more information refer to the document <FIP>.

 The present invention is particularly aimed, in the context of such a network, with the exchange of data of the "message" type, a function which is inserted in the category of the normal and cyclical operation of such a network for the exchange of data. object values between stations.

It is necessary first of all to specify that the first roll of the bus arbiter 3 is to allow the cyclic and automatic exchange of data or values of objects on the bus 2, by sending in succession on this ci of a plurality of identifers previously and bi-univocally associated with each of the objects, in the form of noted frames
ID DAT (i), where i is the identifier. Each identifier, received simultaneously by the totality of the connected stations, gives the initiative to one of them to emit on the bus a frame containing the value v of the considered object, denoted RP
DAT (v), and one or more others to listen to the bus, to receive the frame in question and thus complete the exchange of the object value considered.

 When a station, having created and stored in memory a message intended to be sent to one or more other stations, wishes to carry out the transmission thereof, then it sends a corresponding request to the bus arbitrator 3. Being given that a station never has the initiative of exchanging information on the bus, be it values of objects or messages, such a request is here transmitted to the referee on the occasion of the broadcasting on the bus by the station in question an RP DAT (v) frame containing an object value, by supplementing this frame with a specific field denoted MSG, dedicated to this function. The sent frame is thus denoted RP DAT MSG (v). One of the roles of the referee is to identify the various requests of this type received by the various stations, to store these requests in a stack (memory) of the type first-in, first-out, and, at each time slot reserved for the transmission of data type "message", broadcast on the bus a message identifier frame, denoted ID MSG (i), which authorizes the transmitting station considered , as a result of his request, to broadcast the message m on the bus, in the form of an RP frame MSG (m), to the stations concerned by the message, according to the first-in first-out basis considered.

 As can be seen, each station mainly comprises a central unit 12, for example a microprocessor, a random access memory 10 connected to the central unit by an address bus A and a data bus D, as well as a unit interface 14 with an industrial process, and in particular with appropriate sensors and actuators.

The set is controlled conventionally by a program contained in a read-only memory (not shown).

 The object of the present invention is, in association with such a network organization, a method and a memory management device which avoids redundancy with the management of message identifiers by bus arbiter and simplifies the processing of messages to be transmitted. in each station of the network, with in particular a minimized movement of messages in the memory and a memory size also mini- mised.

 Referring now to FIG. 2, there is shown a portion 10a of the RAM 10 of a station of the network, part which is intended to temporarily store messages to be transmitted to other stations and which comprises a set of N locations or "boxes" for these messages. The memory constitutes a first-in, first-out (FIFO) stack, that is, it is the message that was created and stored first in the memory that will be sent over the network in the first place. to an authorization ID MSG (i) sent by the referee, as will be seen in detail below.

 As illustrated, the memory is managed in a loop, that is to say that the pointers Sxi Ss or not selecting the various locations thereof are counted "modulo N".

 Referring simultaneously to Figures 2 and 3, will now be described in detail the different steps of the management of messages in the station 1 and the corresponding steps of the management of the memory 10a storing these messages.

We will adopt the following conventions
N is the total number of boots for messages in the memory;
n is a pointer designating the number of the first box occupied in the memory; with OXn <N
- x is an index indicating the total number of boots occupied by messages, with O <= x <= N
- y is an index indicating the number of messages contained in the memory and associated with identifiers, with O <-y <x.

 The pointer n and any indexed pointer involving it are counted "modulo N".

 When the station has created a message intended to be sent over the network to one or more other determined stations, the processor first checks that the memory 10a is not full, ie that one if x <N. If yes, the message is populated in the memory. Otherwise the stacking is refused and a corresponding warning can be sent to the user.

 It can be noted here that the message simply stacked as described above is not associated, that is to say that there is not yet in the network an identifier associated with i this specific message.

 Moreover, as soon as there exists in the memory 10a at least one message, recently created or not, which has not yet been the subject of an association operation as will be detailed below, that is to say, as soon as we have <x, then such an association is carried out, as follows: as soon as it has received an identifier of object value, denoted ID DAT (i), the station returns as indicated above an RP DAT response frame MSG (v) which not only returns the value (v) of the object associated with this identifier but also will indicate to the bus arbiter that a message in the stack is waiting for an association for its subsequent transmission. Once this response frame is sent and taken into account by the referee, there exists in the network an additional identifier i available for use. sending messages from the station.

 The next step is then to complete the association process by transcribing this new situation within the station, which is done by incrementing the index y by one unit, ie y = y + 1. other words, the number of messages contained in the memory and already associated is incremented by one unit.

 In the case where all the messages are already associated, that is to say if y = x, then a simple RP DAT (v) frame is discarded.

 Thus, it can already be seen that the stack of message identifiers located in the arbiter and the message stack located in the station under consideration cooperate remotely in order to manage the transmission of messages.

More specifically, the index is a sufficient trace of the association of the messages one after the other, that is to say, the stack of corresponding message identifiers in the stack of the referee, which allows advantageously to avoid creating a concrete association of each message with its identifier in a common special memory.

 Now, when, in a time window reserved for sending messages, the referee broadcasts on the bus the identifier of the message ID MSG (i) which is in the first position in its stack, the station then detects this identifier, which constitutes an authorization to send a message. It then checks that there exists in the stack 10a at least one associated message, that is to say that the condition y> O is fulfilled, and it then emits on the bus the message which is located in first position among all the associated messages, that is to say the message that occupies the box n, in the form of a frame of general type RP MSG (m).

Otherwise, no message is sent.

After transmission, two cases may occur, depending on whether the message transfer is of the acknowledgment type (MSG PRG frame ACK (m)) -or not (MSG RP frame
NOACK) (m)). In the first case, the station is waiting for the corresponding acknowledgment (RP ACK) from the station or stations that were recipients of the message, according to a conventional process that will not be described in detail here.

Once the corresponding acknowledgment (s) have been received, then the station may discard the transmitted message, i.e., unpack the message from the stack. This operation first requires verification that there exists in the memory 10a at least one associated message, that is to say y> O (as for the transmission). In the affirmative, the CPU simply performs the following operations on indexes and address pointers
x = x - 1,
y = y - 1, and
n = n + 1 (mod N).

 It is understood that this amounts to releasing the first location of the memory (formerly box n), and decreasing by one unit the total number of messages x and the total number of associated messages y. It is thus the box formerly n + 1 which becomes the first box containing a message, and it is the latter which will be broadcast on the bus during the next time window reserved for sending messages.

 In the case where the transmission of the message is carried out without acknowledgment, the forwarding of the message sent, as described above, is carried out without any other condition immediately after the transmission of the message on the bus.

 FIG. 3 illustrates by a flow chart the various main steps of the memory management method as described above. It should be considered as part of this description. It should be noted that this flowchart is only a subprogram of the operation of the station, the complete interactions with the main program not being illustrated for the sake of clarity.

In addition, in Figure 3, optional or minor operations from the standpoint of the present invention have been illustrated in dashed lines.

 Table I below summarizes the essential aspects of memory management in accordance with the invention.

TABLE I
Action Condition Satisfied? Execution
Stack x <N no stacking refused
yes stack the message
in the boot n + x
then do x = x + 1
Association y <x no association refused
yes associate the message
occupying the box
n + y then do
y = y + i
Emission y> O no emission refused
yes send the message
occupying the box n Scroll y> O no stripping refused
yes do x = x
y = y
n = n + 1 (mod N)
It is understood that each station of a data transmission network of the type considered will include the features as described above.

The present invention finds advantageous application in any type of broadcast network offering a messaging service and in which the initiative of access to the network, whether for data transmission or messages, is reserved for the sole arbitrator, the condition the main reason being that the referee keeps on his side, in a memory, a first-in-first-out (FIFO) stack of the different identifiers of messages received from the stations, and therefore keeps in this manner the order in which the requests for sending messages have been received from the respective stations of the network, by restoring the corresponding authorizations in the same order
Furthermore, through the use of loop-managed memory, the movements of message-type information in the memory are minimized. It also avoids the need for a memory organization with
N boxes for messages associated with N boxes for the corresponding identifiers. The capacity of the memory is thus also minimized.

 Of course, the present invention is in no way limited to the embodiment described above and shown in the drawings, but those skilled in the art will be able to make any variant or modification in accordance with his spirit.

Claims (8)

1. A method for managing an information memory in a station of a data transmission network comprising a plurality of stations connected to each other by a bus, and a bus arbitrator governing access to the bus by the various stations, a network of the type in which the transmission by a station of at least one type of information involves the sending to the arbitrator of a corresponding request and the storage by the latter with the preservation of the order, of all the requests received from the station, and subsequently the sending by the referee to the station in question of an authorization to issue this type of information, characterized in that it comprises the steps of  - Store in succession in a memory the information to be issued, with conservation of their order;  each time a request is sent to the arbitrator, modifying the value of an index (y) of access to said memory; and  - Whenever a transmission authorization is received from the referee, issue the information designated by a pointer (n + y) indexed by said index. 2. Method according to claim 1, characterized in that it further comprises the step of erasing information from said memory after it has been validly issued. 3. Method according to one of claims 1 and 2, characterized in that the information is stored in a memory "first in first out" type and in that said index (y) is modified incrementally. 4. Method according to claim 3, characterized in that the memory is managed in a loop. 5. Method according to claim 4, characterized in that, following storage in memory of information, an index (x) representative of the number of information in the memory is incremented. 6. Method according to claim 5, characterized in that when information has been validly transmitted on the network, a pointer (n) representative of the position of the first piece of information in the memory is incremented modulo N and said indexes (x, y) are decremented to thereby erase said transmitted information from said memory. 7. Method according to one of the preceding claims, characterized in that the network is a field network for cyclic and automatic transmission of parameter values (v) between a set of industrial controllers (1, 11 i "... ), that said information is messages (m), and that said message transmission requests are included in frames (RP DAT MSG (v)) for sending said parameter values.  8. Station of a data transmission network, for carrying out the method according to claim 1, the type comprising a central unit (1 2), a random access memory (1o) and an acquisition unit and / or data recovery device (14), characterized in that at least one part (10a) of the memory is a message memory whose access by the central unit is governed by a position pointer (n) of the first message , an index (x) of the number of stored messages, and an index (y) of the number of stored messages for which a message transmission request (RP DAT MSG (v)) has been sent on the network, the pointer being incremented or decremented modulo N, where N is the number of message slots in said portion of the memory.