General concept of the ABC- Computer Network

Fuhse GmbH, Uferstraße 38, D- 26135 Oldenburg, ABC- computer network

The picture shows the principle way of connecting the ABC- network nodes to the bus cable. The double arrow thereby stands for the general bus cable, onto which all units are simply connected "in parallel". There is no central point, no "token generator", no network server. Every unit is master and slave at any time, and - in principle - every unit can handle the whole bus- system.

This principle makes the whole system very easy indeed. You may start with just one network node, connected to a standard PC, and a second network node, say, connected to a simple switch. This second unit directly transfers any change of state to the first unit. So, the computer does not have to bother about the state of the switch, because it will automatically be given the information, if anything changes. The computer can concentrate on other tasks.

This exemple indicates already, why the ABC- computer network is so uniquely suitable for plant monitoring. In fact, You may add a third unit, equal to the second one. Now the computer will be given any change of two switches. It may display the states of the switches in one window of the screen. And You may add even more units, ending up with a complete plant monitor, where You have all essential states on the screen of the simple computer. Still, the computer is not at all tied up with cyclic checks of the switches, because it will automaticalle be informed about every change in the plant.

In fact, if You want to asure that all network nodes are working perfectly, the computer may perform a cyclic check at any time, since the network nodes generally can be checked. But this check may only be done once in a while, whereas other system tie up the computer with cyclic checks, just in order to get the information. (You may even employ another computer with a separate network node. This other computer may do the checking, thus completely freeing the first computer of doing donkey work. This first computer may entirely be free for evaluating strategies, to make the best out of the plant.

The ABC- Computer Network allows for many units to be connected to the same cable. Theoretically, up to 65000 units may be on the same strand. Practical application empoyed some 200 units in one system, with distances up to 4000 meters.

Every unit is - by implanting the programmed network operating system - extremely skilled for the network dialog:

If one unit wants to send a message - say "the limit level of a process state in the plant is reached" - to the central computer, it first observes the state of the bus line. It patiently waits, till the bus is free, and, of course, the other units are polite enough, to terminate their dialog as quickly as possible. So, when the bus is free, the unit puts a message on the bus: "My priority is P, I want to call unit AB, my name is XY, and I want to send a standard message". This message, of course, is coded. It reads "PABXYN", where P stands for priority, AB for the address of the computer, XY for the own address, and N for normal message.

This message is codes serially and asynchronously. The coding follows just standard RS-232-C encoding, except of the fact, that the signal is inverted (this has important practical reasons due to increasing the system reliabilty).

When the unit "AB" is listening to the bus, it will send an "R"- message (Ready) back. Only then, the message will be delivered, flanked by a number of checks to encertain that all data are correct.

Generally, this whole transfer of date takes place within 100 milli- seconds. This is a long time as compared to todays standards in office communication. But please consider, during this time, many checks have been made. And, most importantly, the system allows to garantee that the message passes within this time. The whole system is a "real time system".

The reason for this extreme network safety and reliability of the ABC computer network is, that the units observe the bus, while the data transfer takes place. Even the two units, who actually do the communication, observe very carefully, what is happenen on the bus, while putting the data on the bus. E. g. if two units are - by accident - sending simultaneously, the unit with the lower priority will realize that a high priority message is on the bus; it therefore will retreat without having disturbed the high priority message.

This is a very important item of the ABC- system: in fact, two or even more units, who happen to send a message at the same time, will sort out, who is going to proceed, without having disturbed the message of this one unit. Therefore: if there is a collision of data, there is always one unit left, which may proceed, - even, if one of the other units has the same priority. And the system makes shure that this one message will not be disturbed an can reach the addressee freely.

This is one of the most important particularities of the ABC- computer network, and it makes the ABC- computer network unique as compared to all other field busses. It allows for perfect data transmission, even if two or more units try to access to the bus simultaneously. Of course, further methods, as to randomly accessing the bus after the end of another dialog, etc. ease data traffic even more.

Our motto is:

We are one step nearer to the future.

Please, look at more information on the ABC- computer network and our products at the following page:

Network nodes with computer interface:

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(c) Fuhse GmbH 1997
date of last issue: 11. 3. 1997