Modbus exchanges are limited to a maximum of 125 input words and 100 output words. If an application needs to exchange more than these limits for a device, more than one I/O Scanning line can be used: multiple lines for one device.
When the length is higher than the authorized limit for one Modbus exchange, the length is divided into 2 or more Modbus exchanges. New lines are created for each Modbus exchanges with the PLC.
The following I/O Scanning table is used for the multiple device lines example:
NOTE: This example shows an Advantys island, but DTM and PRA devices work the same way.
Multiple Line Length Configuration Example
In this example, the first (the main) I/O Scanning line 2 contains all the information for the exchanges with the device including the totals for the RD length and WR length.
The second line 2 contains the specific word lengths (125 and 100) needed so that it can also be used for part of the exchanges.
Line 2 needs a RD length of 300 word and a WR length of 110 words. How many extra lines are needed?
The larger of the 2 numbers is used:
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Three lines are needed to accommodate the RD length: 125 words, 125 words, 50 words for a total of 300 words
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The 3 lines for the WR length are: 100 words, 10 words, 0 words for a total of 110 words
The second line 2, line 3, and line 4 correspond to the Modbus exchange queries.
When multiple lines are used, only the RD length and WR length columns of these new lines can be edited. In the case of Advantys or DTM, the software supplies the RD length and the WR length, and they cannot be changed in the I/O Scanning table.
NOTE: It is not necessary to have a Device Name defined to use multiple lines.
The total number of words allowed in an I/O Scanning table is:
Line Length for Multiple Word Variables
When using variables with 2 or more words, adjust the RD and WR lengths so that a variable is not partly on one I/O Scanning line and partly on the next. Because the 2 newly created lines result in 2 independent Modbus exchanges that can be sent non-synchronized to the device. The variables can receive the wrong values (if the 2 parts are received at different times). It may be necessary to use a RD length < 125 and a WR length < 100 for some of the scanned lines, in order to get each variable on only one exchange line.
WARNING 