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ReadIDN_S command for IDN 100 using a Quantum/Premium SERCOS Controller and a Lexium Drive

Goals and Symptoms

The purpose of this resolution is to inform the user on the behavior of IDN 100 ( Ascii equivalent GV) when using the command ReadIDN_S (opcode 1556) from a Quantum (or) a Premium SERCOS Controller.

Facts and Changes

The data length of IDN 100, as described in the IDN user manual (version 2.0, June 2003) is 2 bytes. The PLC register can hence accommodate values from -32767 to + 32767 in signed format. Although the IDN document lists the maximum value for IDN 100 as 200, GV can have values higher and calls for data length greater than 2 bytes in the PLC register, due to its scaling ( GV*100). However the 2 byte limitation is in compliance with the SERCOS standard for this IDN.
Due to the above reason, values for IDN 100 set higher than 32767 (GV = 327) when read, will be returned as follows:

(i) 32768 -> -32768
(ii) 32769-> -32767
(iii) 35000 -> -30536 in signed format. Unsigned format will result in a huge number.

Causes and Fixes

When ReadIDN_S returns a negative value, it may be converted to its positive equivalent by adding 65536.
For example, -30536 + 65536 = 35000 and so on. There is no implication on using the WriteIDN_S command for the same IDN which is Unsigned decimal format.

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