Mutiple clients with one Microcontroller unit

That's a very vague specification. From your attitude in that post, I'm assuming you don't want to tell us much about the application. This is a problem because it wastes everyone's time. So if you would describe the overall application, that would help a lot. Otherwise, please give us some basic information such as number of clients, data rate, typical message size, typical message frequency.

If you have multiple clients communicating through multiple async serial paths, you will either need a UART (serial transmitter/receiver) for each client, or some kind of retry mechanism in the protocol, because your 8051 will not be able to monitor multiple serial streams, unless the bit rate is very low and UARTs can be implemented in software.

8051s are available with several UARTs, but not more than 4 AFAIK. You can add external UARTs if you have an external address and data bus, or you can use the Maxim MAX3108 which has a 128-byte FIFO and is addressed through I2C or SPI to give you more serial ports.

As I said, a thorough description of the project would help a lot. Also you could post your design, because it will also contain clues about your requirements.

 

I see. Thanks for explaining the application; it makes a big difference.

So you have an 8051 "slave" connected to each RFID reader, and you want to use RS-485 to communicate between the central processor and the slave processors?

Are the slave processors physically distant from each other?

I think you'll need to use a round-robin polling arrangement. The central device polls each of the slave microcontrollers in turn, and the slave microcontroller responds with a message that means "nothing to report", or a message containing data from its connected RFID reader.

The central controller either validates the information immediately and replies to that slave micro before continuing to the next, or queues the request and replies later, depending on how long it will take to validate the request.

Each slave will have to be addressed independently and you'll have to ensure that the polling loop runs fast enough to get around all of them in an acceptable amount of time.

Another approach might be to use an SPI connection to the slave devices instead of RS-485 and chain the devices together so data is shifted through them all like a long shift register. I haven't done this before; it's just a vague thought. Probably not workable if the slaves are at separate locations.

How many slaves do you expect to have? Minimum, typical, and maximum?

How much memory do you need in the central processor? What microcontroller families are you familiar with, or have you been considering?

How much data needs to be transferred from a slave to the central micro and vice versa on each authorisation?

What is the required time scale for the authorisation process?

 

OK. RS-485 is a multi-drop bus. This means that every device on the bus will see every transmission. So you will need to design your protocol so that each device on the bus can identify messages that relate to it, and ignore messages that don't. This means that messages must include an address. Since there is only one master, the address can always be the slave micro's address, for messages in both directions.

Some kind of message framing is also needed, so devices can determine when a new message is starting, and when a message is finished. Some kind of checksum is also a good idea, so the receiving end can check that the message has not been corrrupted.

Luckily for you, I have a lot of experience with multi-drop RS-485 communication of this type - it was used extensively by the company I used to work for. So I can give you some very specific suggestions.

This is probably best done through some kind of instant messaging system. I will send you a PM with some contact information.