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Application Notes – PLCs and Devices
54
SIEMENS S7-200 PLCS
Registers Accessed
The DN-3000 will only access registers in the V-Memory area of PLC memory (VB0 to VB9999) which
are 8-bit registers. Because the DN-3000 will only read and write 16-bit values, it will use two
consecutive 8-bit registers in Siemens S7 PLCs to form a 16-bit value. The first of these consecutive
registers must be located on an even boundary (i.e. VB0, VB2, etc.). The low order byte of the 16-bit
value is located in the lowest numbered register and the high order byte is located in the highest
numbered register. These registers can also be accessed in the PLC program or in Charts as 16-bit
registers VW0, VW2, VW4, etc.
When entering register numbers, only the offset into the V-Memory area (0-9998) should be entered. For
example, to access register number VW0 (VB0 and VB1), the value 0 should be entered. To access
register number VW9998 (VB9998 and VB9999), the value 9998 should be entered.
Protocol Information
The DN-3000 follows the Point-to-Point (PPI) protocol specification for token passing masters used with
Siemens S7-200 PLCs. It will operate on a PPI network along with other masters.
The fully implemented, multi-master version of the PPI protocol is very slow because of the token
passing, Therefore a unique feature has been implemented on the DN-3000 which allows it to run as a
single master with no token passing. This feature can only be used when no other masters are present on
the PPI network. To activate this feature, enter the value 125 into the DN-3000 Address field on the PLC
Port configuration screen in the DN-3000 Configuration Software. Whenever the DN-3000 Address
(address which it resides at on the PPI network) is set to 125, it will act as a single master.
Pass-Through Port
For Siemens S7-200 PLCs, there is an additional advantage to the “Pass-Through” port of the DN-3000.
It allows a non-token passing master such as a personal computer running Micro/DOS to have access to a
multiple-master token passing network without the requirement for an MPI card in the personal computer.
his feature can only be used when the DN-3000 Address is not set to 125 (see above).
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