C1120 S5 to II/O-Lightbus Interface
Beckhoff II/O-Lightbus System
Page 16 of 52
Version 2.1
Date 27.06.97
Indirect modules
To enlarge the address space of the PLC system, modules can be mapped "indirectly":
they are accessed via a list of "indirect" communications, being addressed by a dedicated base
address. These communications are processed in every PLC cycle. They can be used to
enlarge the address space of 128 I/O Bytes of the PLC by placing the base address of the
indirect communication list above 128. Another option is to change the contents of the I/O list
cycle by cycle: a part of the II/O-Lightbus modules are mapped in a round robin way to the
indirect I/O list to fit the total number of II/O Lightbus I/O bytes to the I/O address space of
the PLC, with the effect, that those modules are being served only at occurence in the indirect
communications list. One or more modules can be mapped to the indirect communications
list. A single address block consists of the module address (IDENT number) and four bytes of
I/O data. The reservation of memory space is done with the menu "Edit configuration": After
definition of the base address of the indirect I/O list it is necessary to allocate 5 x (number of
modules) Bytes list size: First byte contains the address number, following four bytes
containing the I/O data. The list ends with a "0" found in an address byte location. These
addresses are not available for standard I/O via PLC process image transfer. Address conflicts
have to be avoided by the PLC programmer.
Arbitration of indirect modules
Indirect modules are listed in the I/O setup list, coloumn "D0 .. D3" with IE or IA, with
address number 0. The choice of IE or IA is without use for the communication and only
indicates the configuration of the II/O-Lightbus modules for input (IE) or output (EA).
By use of the indirect communications list every module in the II/O Lightbus System can be
accessed, even the direct configured: Their IDENT- number has to be placed in the first
address block address location, following four bytes keep I/O data. By this, the module is
accessed via the indirect communication mechanism. By replacing the IDENT number in the
first location of the address block with "0", the indirect communication is skipped. If the
programmer decides to address a direct module via indirect communication, the module is
addressed twice by the II/O-Lightbus System.
Important!
If inputs are read via indirect communication, the programmer has to regard
that input informations are valid one cycle after the following. Outputs are
written during the following cycle . This effect can be managed by using the
statusbyte of the PLC (PB/PY 254): Bit 1 acts as life bit and changes status
every cycle. By reading the life bit the programmer can recognize and count
cycles (refer also to chapter "status register S5/C1120).
Example:
In above shown setup screen the modules 3 and 4 are mapped indirect. The base address for
indirect communications list is set up to 60, number of indirect modules is 2. By this, 10
Bytes of address block space are available for the programmer: Byte 60 contains the IDENT-
number of module 3 (3), the bytes 61 to 64 are I/O data for module 3. Address 65 keeps the
IDENT number of module 4 (4), bytes 66 to 69 are I/O data for module 4. Byte 70 is available
for standard or direct I/O.
