RS 485 network

[mychangl]

I want to use 75176 for RS 485 network with over 50 devices. However it is mentioned that the maximum devices i can connect in a network is 32. Is it possible to increase the number to more than 32 ?

Thanks

[ralfmayr]

We use as address header in our protocol one byte, so you have a range of 256 devices.
Normaly you specify the high-nibble for the device class, the lower-nibble for the device address range.
For example unit-type 1 has A0h - AFh, unit-type 2 has B0h-BFh and so on.
Regards,
Ralf

[dhouston]

Older RS485 driver ICs (e.g. 75176) can only do 32. Newer drivers (e.g. ST485) can do as many as 256.

[Ioannis]

check the data sheets of the device you will use. If the input of the RS-485 receiver puts high load on the lines then node number is limited (up to 32 for the RS-485 specs).

Newer devices like form analog devices (http://www.analog.com/en/interface/rs-485/products/index.html) present minimum load on the lines so up to 256 nodes can be implemented.

Ioannis

[ralfmayr]

Ah, sorry, was a misunderstanding from me,
thoght you have restrictions with the addresses...

[mychangl]

Thanks for the information. Get better understanding.
1. If i still use 75176, do you think the attach circuit will work ?

2. Do you think if should isolate the line as in the attachment if the wiring goes so long ? ( could have trouble with terminal resistance or isolation of fault )

3. Most of the articles i gone through does not mention the actual type of cable to be used except twisted pair. In order to determine terminal resistor value, i need to know the cable impedance. What if i do not have the information ? Will 120ohm resistor still helps. Or it is better to use 120 ohm with shorter distant cable but i can add isolator to extend the communication line ?

[mychangl]

Ooops, i missed out the diagram..

[Ioannis]

How long is "long"?

Do you really need isolation?

As for cable, I always use UTP cable. It is twisted and has characteristic impedance very close to 120 ohms.

If you don't have a copy of the Serial Port Complete by Jan Axelson, you must get it soon.

Lastly for short distances and low baud rate (less than 9600), termination is not really necessary.

Ioannis

[mychangl]

I will use baud rate of 9600. Total distance is estimated 500 meters since i cannot use star topology.

You use UTP, it is unshield - is it okay ? how about other noise ?

I am thinking that, may be isolation can localize the fault in case it happen.

Regards

[Ioannis]

OK. 500 meter is almost half the max distance of a RS-485 network. So termination is better to be used. The UTP is indeed unshielded. If there are no very strong interferences I see no reason to use other cable. The twisted pair has the property of self cancelate any noise presented on both the wires at the same time.

The opto isolation is really necessary if the two nodes are at different voltage referenced to (common) ground. If this is the case the do use isolation. Otherwise connect directly and the ground with 100 ohm resistor to be safe.

For the role of this resistor I direct you again to Axelson's book.

Ioannis

[dhouston]

Your diagram doesn't provide sufficient detail - you should do a Google search on "RS485 repeater" which is what you really need where your diagram shows an optoisolator.

[b1arrk5]

I've seen networks using 75176 chips with up to 99 nodes. The data rate was slowed down to 1200 baud, and they were wired with good quality twisted pair wire, properly terminated. There are newer chips that will allow 128 nodes at much faster data rates, however if your data packets are small you might find slow baud rates are just fine.

Jerry.

[Ioannis]

As always, do choose the right tool for the job. 75176 chips are for 32 nodes and nobody will take the blame if your data are lost on an overloaded network.

As shown on a previous post there are more recent chips that do 256 nodes. Price is very good, so why challenge your luck?

If you insist on using the ancient (but reliable I have to admit) 75176 then do place a repeater after 30 nodes an extentd the network another 30 nodes and 1200 meters. I would place a dedicated chip for transmit and another one for receiver, forgeting the auto switch between the Tx/Rx lines. You are going to place a UTP/FTP cable, so there are free pairs to use as dedicated Rx/Tx lines.

Ioannis