rs485 or CAN or ?? industerial network

[7rots51]

Hi silvio
Thanks for your help on RS485.

I want to use a 2000 meter cable for transmit data at 128kbit/sec but I do not know that I can send data at this rate at this lenght?!!(without repeater)

Is there another physical layer except RS485 for this task?

 

[silvio]

Hi rots51,

I know it's human nature to cover our back. But you must have a little bit of confidence in RS485. My opinion it's : give it a try.
A more robust way to solve the problem it's to buy a ready made device,say a proven isolated RS232 - RS485 convertor. Do some tests with this one and if everything works OK, try to build one yourself.
Why I give you this advice ? Because in your system are three source of problems. One is the length of the cable and choosen a proper one it's not such easy. Even if someone could tell you he succceed to manage with 2000 meter legth cable you have to ask him what kind of cable did he use. Not every twisted pair cable is working. I told you from my experience for even less than 1000 meter, that I encountered some problems.
The second problem is the match between the cable and the driver itself. Even the same RS485 driver could work different on different topology.
If these two could be overcome it still exist the third one. Even if you'll be able in the end(and I don't have any doubt about that) to send at 128 kbits/second at 2000 meter without repeater, you must solve the upper layer. From this level problems are coming more often than expected.
If you want only a master-slave system, well that's easy. You can close the third chapter. But if you want a multimaster system where each device it's talking with each other (a true network), well it's not so easy. You must to provide a very good collision detect system without increasing turn around delay and scheduling retransmision according to some algorithm (i.e. truncated binary exponential backoff algorithm inherited from ethernet physical layer, even someone could claim it's too complicated for your requirement and don't worth bothering). Due to the legth of cable, the signal will take some time to travel across, even at 128kbits/second and you'll have to increase the preamble and jam sequence too. And what's the big deal if you can send burst of data at 128kbits/sec and wait among frames.
But don't be intimidated. Try and learn from your own mistakes. And beyound all of these, don't loose your confidence in RS485. With Pre-Emphasis technique mentioned in Maxim site, your requirement could be solved.

You can ask trace_ru why did he switched from RS485 (read special software + driver 485) to CAN (hope with RS485 driver, too). I think he is more excited about the features provided by CAN protocol rather than the benefits got from changing the physical interface.

Silvio

 

[7rots51]

hi silvio

Thanks for your detailed help.
I have done several project with RS485 as physical layer and MODBUS protocol (master/slave architecture ).But the network lenght was about 100 meter and the baud rate was 19200.My current application is simple I have only two device at both ends and I want to send data from one to another (unidirectional only) at 128kbit/sec at 2000 meter lenght without repeater,I think RS485 can do it but I am not sure.If I buy industerial data cable with 2$ /meter I must pay 4000$ for test at this lenght,I want to know that can I send data without problem before starting my test!

Is there any other physical layer that be better than RS485 for this task?

 

[silvio]

Hi 7rots51,

If the cost of cable is a real concern and your customer could pay $4000 only for cable, I believe it could support the cost of a repeater, too ($150). Thus, less headache for you.
If your customer has already the cable I don't see any reasons why would not allow you to test the application on line (cables stored not in coils, otherwise the test it's not conclusive)
A pair of terminator resistors and a scope it's only things you needs.
Come on man, give it a try. If I'm telling you that a fiberoptics cable it's much suitable for your application, would you believe me ?.
Or if I give you a solution with 3 cents / meter and $2000 for interface, would you find it much suitable than the one above ?
I'm not telling you fibs ! The only thing I'm trying to do it's to convince you could solve the problem on RS485 basis.

Silvio

 

[7rots51]

hi silvio
I can not use fiber optic ,because they do not accept it.They also do not want to use repeater (the are some limitation in that environment).

I want to use RS485 and 2000 data cable.

BTW: please suggest a good RS485 driver that is fail safe and reliable and have good ESD protection.If you have any suggestion I will be glad to hear it.
bye

 

[silvio]

Hi 7rots51,

A ready made device I used on testing was ADAM 4520 from Advantech.
That was available in my company at that time. Unfortunatelly it can't goes beyond 115.2 Kbps.

https://www.telebyteusa.com/catalog/products/245.htm up to 128K
**broken link removed** up to 921.6 Kbps

BTW, why do you need 128Kbps ? You need this rate continuously during let's say 1 minute or maybe less in short burst, in which case the 128K it's only a peak ?
Or the total amount of bytes you have to send from master to slave it's compulsory to be sent within specific amount of time ?
What are two process that needs to be synchronized at 2000 meter distance between them and at 128Kbits ?
What happens with the slave if he didn't receive the data in required time ?
How it's sent the feedback from slave to master (i.e. please resend, command done...)? Do you need that ?
Since the BIAS resistor it's fitted on receiver side (in your case the slave) how the system will manage with broken cables ?
Or I wrong understand and the master is some kind of sensor (sending continuously samples inside the process) and the slave is the embedded system ?
Even so, what actions are taking place at cables edges, that need to be so quickly syncronized ?

My last question: Why they don't want repeater ? What are the limitations in customer environment ?
Do you agreed with me that the repeater must be total transparent for customer ?
Why customer should be awarned of his presence on system ? As long as the system works as required, what it's so important that the customer to be informed of what solutions has been chosen for system integration ?
I don't get it ! It's true that our moto it's CUSTOMER TOTAL CARE ! But still don't get it !

Silvio

 

[refman]

use CAN or ethernet for your network.

 

[7rots51]

hi silvio
The communication is full duplex.There is the same link from slave to master at that speed.
I use a 4 wire cable to communicate between slave and master,2 for send and 2 for receive.(both at 128kbps).
There is a protocol between them.(HDLC).there are many sensors in slave that their data must be send to master.
The environment is hazardous area and there are danger of explosion.The cost of cabling is this areas is too high and there is no power cable like as 110VAC in that area for repeater ,and the system must be portable(slave is not fixed!)

Because of this conditions using repeater is not possible.

bye

 

[silvio]

Hi 7rots51,

OK, I got the point with repeater.
How about 128K ? How often need the master to access the slaves and how long it's the frame during one complete transfer from slave to master ? How many slaves are in system ?

silvio

 

[trace_ru]

You can ask trace_ru why did he switched from RS485 (read special software + driver 485) to CAN (hope with RS485 driver, too). I think he is more excited about the features provided by CAN protocol rather than the benefits got from changing the physical interface.
Silvio

You're right. It is the protocol rather than anything else. I've done several projects with RS485 as the transport link and I can tell that quite a large part of the programs was dedicated to the RS485 protocol. Also, I've had to copy the protocol part into the nodes in order to maintain the compatibility. By using CAN I have a choice of perepheral ICs with the imbedded protocol. It's absolutely priceless.
trace

 

[7rots51]

hi
the communication link is simplified to one master and one slave (point to point) but the dark point for me is here,Is it possible transfer data at rate of 128 kbps(bit) by a rs485 physical layer network for 2000 meter?


bye