Read Device Identification: range of regular and extended categories

  • Thread starter Michael Teichgräber
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Michael Teichgräber


in the MODBUS Application Protocol Specification v1.1b, on page 44,
for the Read Device Identification function three categories of
objects are listed: basic, regular, and extended.
The description for regular and extended categories makes use of
the term "in addition to", which suggests to me, that

- the regular category <i>also contains</i> basic objects
(i.e. extends from object ID <b>0</b> to 0x7F), and

- the extended category <i>also contains</i> objects of basic,
and regular categories (i.e. extends from ID <b>0</b> to 0xFF).

This view is backed by e.g. :
Extended data objects <i>include</i> regular objects, which include basic objects.

Though, the categories could be understood differently: "in addition
to" the basic category, there <i>also exist</i> the regular, and extended categories, but objects only belong to <i>one</i> category, the categories are <i>orthogonal</i> to each other. This would result in the following ranges:
- basic object IDs: 0 to 2
- regular object IDs: <b>3</b> to 0x7F (as opposed to <b>0</b> to 0x7F)
- extended object IDs: <b>0x80</b> to 0xFF (as opposed to 0 to 0xFF)

I know of a manufacturer, that apparently has come to this conclusion.
Additionally, in this forum, I found that lists orthogonal category ranges.

Which understanding is the one intended by the specification?
As I'm not a native speaker of English, I'm not sure whether the
wording in the specification, p. 44, leaves room for interpretation
or not. I'm inclined to the first version, also because in the
specification, Object ID 0 is being used as kind of a default, which
suggests that regardless of whether the requested category is basic,
regular or extended, one should expect to get Object 0 as the first
object of the list.