how to increase number of node on RS485

[mehulraninga]

increase devices in rs485

Dear all,
I want to connect 120 nodes on RS485.one is master and others are slave..but since RS485 is having limitation of 32 nodes,how can i connect 120 nodes???i am thinking for repeaters but i havent use repeaters..so if anybody is having idea,kindly guide me...
Thanks in advance........

 

[laktronics]

rs485 transorbs

Hi,
Use MAX487 like devices, without repeaters.

Regards,
Laktronics

 

[svicent]

6n137 rs485

When using the default RS485 receivers with an input resistance of 12 kΩ (unit-load) it is possible to connect 32 devices to the network. MAX487 has a 1/4 unit-load receiver input impedance, allowing up to 128 MAX487 transceivers on the bus. Currently available high-resistance RS485 inputs (1/8 unit-load) allow this number to be expanded to 256.

 

[mehulraninga]

max1482 termination

Thanks all...
i am using 89v51 controller for my application...for RS485 interface i had choose MAX485 chip....now if i want to use RS487 then what should i use....i mean is there any chip available that can give directly 487 conversion...Kindly guide me..
Thanks

 

[laktronics]

max487 connection

Hi,
MAX 487 is just a part number for an RS485 compatible IC. It is an improved IC which can handle up to 128 devices on a line. Please refer MAXIM site for further details.

Regards,
Laktronics

 

[mehulraninga]

how to increase nodes in rs485

thanks...
I am going for MAX487..
By the way,one more idea is there in my mind is to use MAX1482...as per datasheet of MAX487,we can connected maximum of 128 nodes....these are enough for me...but i am doubt that when application is onload then will it be able to handle exact 128 nodes or not....In case of MAX1482/83 ,we can have 256 nodes...Which one is worth going for...
Regards
Mehul

 

[laktronics]

rs485 4n35

Hi,
Yes, you can use MAX 1482/1483 to take care of any future requirements. With so many devices on line, you may also have to consider optoisolation at the input of the 485 transceivers and a lot of planning in cable selection, routing and connections.

Regards,
Laktronics

 

[mehulraninga]

how to increase nodes

Thanks a lot....

As such i am going to connect 120 nodes on the bus,I want to know that such type of cable are directly available in market or we have to design manually...I want to use twisted and shielded pair... And what other care should i take as far as hardware is consern..i mean cabling and all those...Main thing i want to ask is that such cable which can connect 120 nodes are available or not???If yes then give me some reference..

Regards
Mehul

 

[laktronics]

how to increase number of nodes using rs485

Hi,
Yes, they are very easily available, look for twisted pair data cables from any standard cable manufacuterers/ suppliers in your area. Use as thick a wire guage as possible to reduce the signal drop on the line. Use only cables with known characteristic impedance for determining termination resistor values.Try to terminate the RS485 signals on a pair of parallely connected connectors at the back of your nodes as 485 IN/OUT and loop the cable through these to reduce the stub length. The first and the last of these connectors can be used to provide line terminating plug-ins. Opto isolation is strongly recommended with so many nodes connected. Also route the cable in underground trenches rather than over head. Use also low voltage transorbs on the bus at each node, very near to the 485 In/Out connectors

Regards,
Laktronics

 

[mehulraninga]

rs485 6n137 isolated

thanks..
By the way for opto isolation 4N35 opto coupler will work or other i should use...and using opto coupler should i change voltage level of bus ???if yes then how much?

regards
Mehul

 

[laktronics]

max487 and opto

Hi,
No need to change the voltage level of the bus, keep it at 5V only, but an isolated voltage for the bus is required and keep the bus ground connected to safety earth preferably at one point near the master. At other nodes, you can connect the earth through a low voltage transorb.

4N35 is very slow, 4N22 is better and I recommend 6N137.

Regards,
Laktronics

 

[mehulraninga]

Hi,
Is it required to make common ground for all the nodes???will there be 2 line running among all nodes or 3 lines???
Second thing we required two opto channels for A+ and B- ...Am i right no????

 

[laktronics]

Hi,
You should use optos on the TTL side and allow 485 driver/receiver to hang on the isolated bus. You can use three optos, two 6N137s for TX/RX path and one 4N22 for the direction control signal.

Since you are using twisted pair wires, use one twisted pair for ground connection for all nodes.

Regards,
Laktronics

 

[mehulraninga]

thanks..
Now i am very cleared about my tasks...

once again thanks...
Regards
Mehul

 

[laktronics]

Hi,
I have a few more points to tell you:

1. For TX enabling for 485 driver, use active low port bit to ensure that when one of the swiched off slave in a working system when switched on, does not turn on its driver to the bus during its RESET period when all port pins are in high state.

2.Mount the RS485 In/OUT connectors along with 485 IC and opto isolators on a small PCB and mount it on the node back panel. The three TTL signals and nonisolated Vcc and ground from the controller board may be connected to this board using a 5pin single row cable connector. Using another two pin connector you can feed the Isolated supply to this board.

3. On the PCB, ensure there is physical isolation between the opto's nonisolated pins and its isolated signals and power supply circuits.

4. Also make a through connection of the cable shield of cables connected to the IN/OUT connectors.

5. After the installation, remember to check the continuity of shield and its connection with safety earth at one point.

Regards,
Laktronics

 

[mehulraninga]

Thanks..
One more think i want to ask is that which type of connectors are used for 485 bus..DB9 connector is enough or any special connectors are available?
Kindly guide me...

Regards
Mehul

 

[laktronics]

Hi,
I have used DB9 only for applications using 19.2k baud. I am not quite sure whether some other connectors are required for higher frequencies.

Regards,
Laktronics.