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from the Automation List department...
Number of devices I can connect to MODBUS systems
Communications systems and equipment. topic
Posted by Edgar Marino on 22 March, 2004 - 5:20 pm
I would like to know what are the different types of Modbus available, what is the maximun number of devices I can connect, what is the baud rate and the maximum distance. Also is there a ASI to Modbus gateway? if there is one how will that affect the ASI scan rate on 2.0 or 2.1 compatible systems?


Posted by Max Power on 25 March, 2004 - 12:23 pm
1. Modbus is a simplex protocol and does not specify a physical layer. A half-duplex physical layer is all that is necessary.

2. Modbus has an ASCII version that is designed more for terminal queries and an RTU version that is more robust for automation and control.

3. The maximum number of slave devices per net that can be individually addressed is 247. However, with multiplexing, you will be able to address more.

4. The maximum total number of devices also depends on your physical layer.

For RS485: Some say "32" for RS485, but that is based on a single design done years ago. If you were to design a system today, you can get "1/8u" transceivers that basically load the net down by 1/8 of the original design example. So you could have 32x8 = 256 devices.

5. Your distance depends on the physical layer design.
For example, I use RS485. My transmission line has a characteristic impedance of Zo=60 ohms. (Note: I tried to get 100 ohm like CAT5, but the UL flammability rating was not available for that type of wire.) The frequency components of the transmission are very important and play a factor in the transmission line design. Since logic levels are used in RS485, limited slew rate drivers are better because they reduce the higher frequency components - thereby increasing your possible distance or data rate. A pulse sent down an unproperly terminated line may cause reflections if that line is greater than 1/4 wavelength in that medium (v ~ 0.66C).

Max Power
superalpha@yahoo.com


Posted by Santhosh on 28 March, 2004 - 12:01 pm
Hi,
Basically MODBUS is divided in to two modes - RTU and ASCII modes. The networks can be used were either serial or TCP/IP. MODBUS devices were identified by it's Slave Address and it varies from 1 to 247 and 0 is assigned for Broadcast.

We do have ANSI C compliant MODBUS Master and Slave protocol stacks and protocol converters/gateways. If you are interested in knowing more, mail me or check out our website www.sunlux-india.com

Cheers,
Santhosh
santhosh@sunlux-india.com

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