Control.com - Nerds in Control

Hi,
Recently, I was told that generic limitation of data transfer for PLC is 19800bps. If I'm not mistaken, this is limitation of serial port connectivity i.e. MODBUS RTU on RS485 connection. I think that this is too slow.

In my project, there will be 2 PLCs communicating via digital microwave radio. The protocol for PLC communication will be MODBUS TCP/IP. Hence, I presume that the data transfer should be as per Ethernet, which is 10/100Mbps.

The primary PLC will extract all the data from secondary/slave PLC to enable remote monitoring and control.
Please advise the correct data transfer rate.

Thanking in advance.

It is reaaly a limitationof the bandwidth. For instance at 9600BPS it takes approximately 1 msec to transfer 1 byte. At 19.2 kbaud it takes appriximately 1msec to transfer 2 bytes, at 36kbaud 4 bytes/msec and so on.

Many people are fooled into believing that if they place a serial to ethernet converter onthe line it will increase the transfer rate. To quote my daughter "Not!!" You are stalling talking to the device serially so that will always be the bottle neck.

Next, the modbus protocol limits packet size to 256 bytes, well a little less then that the most you can request from a device at ontime is 240 bytes of contiguous data.

Hope that helps to explain it.
Fred Loveless
Senior Applicaiton Engineer
Kepware Technoligies
http://www.kepware.com

"digital microwave". Caveat Emptor.

Many wireless devices advertise serial port rates of up to 115k baud, but the radio link can be as slow as 9600 baud between the radios.

So the serial/radio modules hold while they piecemeal forward over the radio, but not necessarily at the serial port data rates.

I would advise being very persistent in determining what the serial throughput rate is for whatever radio you are using (and then realize that such is under ideal conditions).

Because even Ethernet radios do not necessarily transmit at wireless rates that their wired ports can support. For example freq hoppers can be as slow as 150K baud throughput.

The inherent lower-power-per-bit at the higher data rates of DSSS technology also make DSSS far more prone to noise disruption, with the inevitable renegotiation and resending of packets.

Sure, shovel it over "digital microwave". Shovels are good for digging graves, too.

David

10/100 M are common speeds for Ethernet. However, note that is *Ethernet*. TCP/IP can operate over other hardware besides Ethernet. Your radio link could operate at some entirely different speed.

Modbus/TCP is a protocol, not a hardware layer. It can take advantage of whatever speed is available. That means for example that if you had newer 1 Gig Ethernet hardware, Modbus/TCP could travel at that speed.

The data transfer rate however will be limited by whatever is the slowest connection between two points. You need to look at the speed of each component in the communications link to see where the bottleneck will be. That includes both PLCs, the radio link, and any other hardware in between (e.g. switches, hubs, etc.).

I wonder if your Digital Microwave is shared or dedicated? If shared, you'll find there is no answer since it depends what other things are moving over the link.

You will hit CPU limitations in the PLC long before you hit the limit of even old-fashioned 10M Ethernet. Many PLC only move a limited number of transactions per second, and faster data links won't improve throughput.

You should have the slave/PLC carefully concentrate important data in a block of 125 registers or less - thus you can move all required data in 1 poll.

Trying to read 1000 registers fast or 5 registers here & 10 there and 7 over there in 3 reads will ultimately be slower due to network/transaction overheads.