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What Determines The Communication Speed? Is It The Physical Link's Speed?

02/15/2017 3:54 AM

Two IEDs (Intelligent Electronic Device),an electronic relay and a gateway, are connected with Modbus RTU protocol over RS232.

  • In the relay's manual it states that a speed of 9200 bauds can be selected (typical for modbus serial)
  • In the gateway's Manual, I found that the communication card is rated at 115 kbps (actually I was considering two gateways, one with communication speed 115kbps and the other with 38 kbps)

My question is at what speed these two IEDs will communicate? is it 9600 bauds, or 115kbps (or 38 kbps)?

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#1

Re: What determines the communication speed? is it the physical link's speed or the

02/15/2017 6:09 AM

The speed of the slowest link in the chain...
A chain is only as strong as the weakest link.
Del

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#2

Re: What Determines The Communication Speed? Is It The Physical Link's Speed?

02/15/2017 8:25 AM

i would say RS232 in the industrial environment is a hazard (unless coupled non resistively) , don't expect more than 9600 4800 or less reliable link speed

(if for any help) these guys show variety of connections ?? http://dl.mitsubishielectric.com/dl/fa/document/manual/got/sh080871eng/sh080871engu.pdf

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#3

Re: What Determines The Communication Speed? Is It The Physical Link's Speed?

02/15/2017 11:39 AM

Just because the uarts are capable of 115k baud it doesn't mean that you can achieve this communication speed. The problem will be the cabling. TIA-232-F (the ratified standard instead of the Recommended Standard) is a non-terminated, single ended instead of differential terminated form of digital communication. As such the cable capacitance and reflections can quickly make high speed communication impossible over anything but very short cable runs. 9600 baud is the nominal, albeit slow, rate that assures communication.

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#4
In reply to #3

Re: What Determines The Communication Speed? Is It The Physical Link's Speed?

02/15/2017 12:16 PM

True that. Most people forget about signal reflections due to impedance mis-matching.

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#5

Re: What Determines The Communication Speed? Is It The Physical Link's Speed?

02/15/2017 12:30 PM

Length of the cable affects maximum speed. Low capacitance cables can use longer runs for the given speed.

For a standard cable:

Table 11-1. Data rates and the maximum distances recommended in RS-232

Data rate (bps)Distance (m)
2400 60
4800 30
9600 15
19200 7.6
38400 3.7
56000 2.6

http://www.tldp.org/HOWTO/Remote-Serial-Console-HOWTO/serial-distance.html

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#7
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Re: What Determines The Communication Speed? Is It The Physical Link's Speed?

02/15/2017 2:41 PM

Can RS-485 go further?

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Re: What Determines The Communication Speed? Is It The Physical Link's Speed?

02/15/2017 3:13 PM

Yes, a balanced line (especially shielded) buys you a lot of noise immunity!

http://electronicdesign.com/what-s-difference-between/what-s-difference-between-rs-232-and-rs-485-serial-interfaces

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#6

Re: What Determines The Communication Speed? Is It The Physical Link's Speed?

02/15/2017 1:51 PM

Modbus RTU is normally sent over an RS485 physical link, so 115 kbps is easily within it's capability. RS232 is much less forgiving.

But the crucial thing here is that the protocol is asynchronous: you have to tell the elements at both ends what speed the the link is running at. Whatever you tell them, you may or may not get reliable comms. depending on speed and distance.

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#9

Re: What Determines The Communication Speed? Is It The Physical Link's Speed?

02/15/2017 3:32 PM

If you need high speed for long cable runs, you may wish to consider RS232-optical fibre.

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#10

Re: What Determines The Communication Speed? Is It The Physical Link's Speed?

02/15/2017 5:40 PM

GA to Del the Cat.

Explicitly, the communication is at 9600 baud plus latency.

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#11

Re: What Determines The Communication Speed? Is It The Physical Link's Speed?

05/22/2017 3:36 AM

Remember that the bit rate is meaningless if you have a high error rate!

The data throughput is what is really important.

A speed as fast as lightning with a 100% error rate = 0 data!

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