RS 422/RS 485 Limitations On Number of Multi-dropped devices

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Thread Starter

Richard George

Can anyone advise what, if any,is the maximum number of devices which can be multidropped using either RS422/RS485 serial interfaces employing Modbus RTU protocol?
 
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Lynn August Linse

There are several answers.

The "legal" answer is "32-unit loads", with (on average) 1 unit load per slave or 32 devices. Some vendors sell chips with 1/4 unit load, so you could have 128 devices, but I haven't seen any field devices use these yet.

However, some vendors load the RS-485/422 wires with various bias and/or surge loads, meaning you may not reach the magic 32-device number before
your communications degrades.

In general, I say you have the following variables:
1) How fast are you talking (slower is "better").
2) How long are your drops between device & the bus (shorter is "better")
3) How good of quality is your cable (more-$$ is "better")
4) How long is your overall bus (shorter is "better")
5) How many devices are on your bus (fewer is "better")

Given these 5 variables, the more "better" you are in 3 or 4 of them, the less "better" you can be in the other 1 or 2.

For example if your baud rate is 9600, your drops are short (close to zero), your cable is average but only 300 meters long, then your risk to
assuming 32 devices on the bus will work is less.

If you do have a problem, but can drop the baud rate, that may overcome the problem. etc. Also, using isolating repeaters can segment your RS-485 and be a fall-back if your 32-devices don't work well.

Regards

Lynn A Linse, Senior Industrial Automation Apps Engineer
Lantronix, 15353 Barranca Parkway, Irvine CA 92618 (USA)
Mobile: (949)300-6337 Office (949)450-7272 Fax (949)453-7152
ly[email protected]
 
Modbus has a limit of 32 addresses per network. Its not a hardware limit just a legacy of the comm. protocol.

32 can be circumvented by subdividing into more networks. some versions support 64 -again having to do with parsing the address characters
out of the message sting composition in binary.
 
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Alex Pavloff

From the Modbus Protocol Reference -- Modicon document PI-MBUS-300 Rev J:

Valid slave device addresses are in the range of 0 - 247 decimal.
The individual slave devices are assigned addresses in the range of 1 - 247.

Alex Pavloff
Software Engineer
Eason Technology
 
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The general rule is 32 devices per RS485 network segment (including the master). Many driver chips now allow more, typically 64. More than this requires a segment repeater (for signal regeneration). 32 is always safe, as part of the RS485 spec: check with device vendors if you need to exceed this.

Strictly speaking, RS422 is point to point (like RS232 but using differential signalling to cancel noise) so the node count is 2. However if slave devices tristate their outputs when not transmitting (which they almost always do when using Modbus RTU), the Slave TX/Master RX channel becomes effectively an RS485 network, so the same node count as RS485 applies.

(Similarly by tristating the master TX/slave RX channel when not transmitting it is theoretically possible to create a multi-master system on RS422 using token passing or some such, though this is very infrequent in practise).

Cheers

Tim Linnell (Eurotherm)
 
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