token

WHat is a Token ?

When u set an object master on a bus, it takes the token, what does that mean?

A "token" is an imaginary item.
Consider a room of people all talking at once. Complete confusion, nobody listening, etc. You could solve this by giving someone a stone and creating two rules -
1) Only the person with the stone is allowed to speak
2) The person with the stone must pass it on after 1 minute.

In this case the stone is the token. Some of the people in the room will be masters (capable of receiving the stone), others will be slaves (only listen to instructions, or answer questions from masters - never hold the stone).

Now imagine something like that in communications software. Obviously the time will be a lot quicker than 1 minute but the principle is the same. You also need other rules to decide what happens in unusual circumstances - what happens if the person holding the stone drops dead before they can pass it on etc?

Ken

Token passing is a method to allow multiple masters on a media which allows only one transmission at a time. The 'token' is a code transmitted by one unit to the next in addrtessing order. The 'token' informs the next unit that the previous unit is finished its transmissions and that the receiving unit now has permission to act as a temporary master. The unit which received the 'token' can then make an appropriate transmission/reception on the network. It then transmits the 'token' to the next unit in line.

Typically a unit powering up on such a network will wait an amount of time so that a token may be passed to it. If it never receives a token either the token has not been started (assuming there is no traffic taking place) or its presence in the addressing scheme has not been noted. The actions it takes then are dependent on the particular type of network.

The simplest communications networks are master slave. In this arrangement only one device, the master, intitiates the communications transactions and all other devices are slaves that only respond to data and enquiries from the master.

In a peer-to-peer network you need a method of communications flow control to keep several devices from talking at once and making gibberish out of the communications. One way to establish who can talk is with a "token ring" network. In this network each device gets the token in turn. The device with the token is the temporary master, and can read data, write data, or pass the token on to the next device without doing anything. If the device with the token initiates communications it "hangs on" to the token until it gets a response or times out, and then it passes the token.

DH-485 is an example of a token ring network.

hi i'm new here
how r u all
the token is good must be try it

Sir,
We are using siemens s-5 plc and already there are programmed by some professional. PLC is a very new system for me. So, sir i have some doubt..
Some time to bypass any or/ and logic we use F0.0 as logic 0 and F0.1 as logic 1. So, is the flag values contains constant values by default?.. And in many places the FD is called as FD200, FD 180 etc and these FDs are used to and with another DD values. So, sir these FD values are constant; like is FD200 contains a constant value and FD 180 is containing a different value?.. or the values in FDs are related to the program execution.. And if the FDs are constant then how to assign the value?

I can never think about tokens without remembering the conch shell from "Lord of the Flies". Never fails to depress me...

Thanks guys, it's very clear now