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/r...cts/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 ?