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MODBUS RS485 Node Limit
Local and wide area networking in factory automation. topic
Posted by Landau@attbi.com on 6 October, 2002 - 9:53 am
I am engineering a system using Momentum CPU to talk to Fuji Loop Controllers w/ Modbus.

Modicon Documentation notes max node limit is 254? while Loop controller limit is 32 per network (but allows address #'s up to 255)

Which is it?

S. Landau
SPEC


Posted by Konrad Heidrich on 7 October, 2002 - 12:27 pm
Modbus can handle 254 nodes (remember: 255 is common for broadcasts in other networks, perhapes it is also possible in Modbus).

At the other side a common rs485 driver can handle only 32 nodes - for solving this problem use repeater after each 31st node

hope that helps,
Konni


Posted by Ekinox2 on 7 October, 2002 - 12:28 pm
If you use Modbus+ the limit is 32 nodes per network else the limit is 254


Posted by narendrakumar on 7 October, 2002 - 2:01 pm
Maximum limit is 32 only but you can safely configure for 20 for optimum speed. And the speed of responce also based on Number of register for read and write.


Posted by Brian Cervi on 7 October, 2002 - 1:42 pm
Do not confuse the addressable node limit with the allowable node load (limit) on the electrical interface.

The Modbus Protocol supports addresses between 0 and 255 (8 address bits). Since "0" is a special (broadcast) address, then there are 254 permittable addresses left.

The RS485 electrial interface typically permits a loading of 32 devices for a network segment. This is a design characteristic of the chip set and is not always the same. A network is also influenced by the length, speed of transmission and such.
Check the specifications of the RS485 devices and any RS232/485 convertors to determine the number of devices (nodes).


Posted by Michael T Mellish on 8 October, 2002 - 3:30 pm
Perhaps the confusion is in your terminology. Modbus is a protocol, and the standard implementation is via RS232C ports (standard on all Schneider PLC systems). This protocol allows up to 254 nodes to be addressed from a single host (master) connection. Obviously, the RS232C host port cannot directly connect up 254 other RS232C serial ports, so a media conversion must be made. This can be RS422, RS485 or a variety of other modem technologies. The selection of the media conversion will create a new limit which will generally be smaller than 254. This is likely where your limit of 32 comes from.

This is a separate discussion from Modbus Plus. Modbus plus uses an RS485 media, but has a proprietary access controller chip to provide a multiple master network. It uses a 5 byte addressing scheme, and has a limite of 32 Modbus Plus nodes per network segment, 64 per sub network (join two segents with a repeater to get to 64). Sub networks are joined via bridges so and a message can pass through up to four bridges so a MB+ network could be 5 x 64 nodes total. There are tricks to extend that furthur. Modbus Plus is not Modbus RS485, so perhaps that is your confusion.


Posted by Daniel Scott on 9 October, 2002 - 3:34 pm
Both. The Modbus protocol handles up to 255 addresses, 0 through 255. 0 is the broadcast address so in effect you can have 254 different slave addresses.

The RS485 protocol allows a maximum of 32 devices. An RS485 network will have it's own address space so you can use any address between 1 and 255 for each node.

If you need to talk to more than 32 devices with a Momentum then you will need the optional top hat with a second Modbus port. This will allow you to address 64 nodes. As each network will be independent you could use the same addresses on each network or you could choose to keep them different.

If you need to talk to more than 64 devices, or if you can only use one Modbus port then you will have to go with something like a bridge mux which will allow you to have up to 4 networks.

Danny Scott
Fredericton, N.B.
Canada
d.o.scott@UNB.ca


Posted by Malcolm Moorhouse on 10 October, 2002 - 5:04 pm
As far as I'm aware, the original Modbus is able to address 254 nodes. That is 1 to 254, on the other hand the original spec for RS-485 has a limit of 32 nodes per network. With the original RS-485, should you need to exceed 32 nodes than a repeater or hub is needed. Recently companies like Maxim Semiconductor have produced RS-495 chips that support 128 nodes per network. Take a look at rs485.com for some good info.

Malcolm


Posted by Henrik W Maier on 18 October, 2002 - 11:27 am
It is not quite right to say that RS485 is limited to 32 nodes. The standard limits the number of nodes to 32 on one wire segment. If the network is extended by repeaters into multiple wire segments each segment can have up to 32 nodes. There is nothing said how many segments
you can connect but of course there will be a practical limitation somewhere.

Considering the use off repeaters it makes now sense to have more than 32 nodes on one Modbus network. If this would be practical by means of poll times is another issue.

Also bear in mind that RS485 is not the only physical layer for Modbus. Some applications use optical fibres. The 32 node limit does not apply to other physical layers.

The Modbus documentation I retrieved a couple of years ago from Modicon's web site lists valid slave numbers from 1 to 247. 0 is the broadcast address. It appears that 248 to 255 is a kind of reserved address space. But I know that a lot of Modbus devices and stack implementations allow addressing up to node 255. There is also other documentation floating around which says the upper
limit is 254.

Henrik W Maier
_____________________________________

FOCUS Software Engineering
Brisbane, Australia
http://www.focus-sw.com
______________________________________

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