9
9
E
Application
BIS C-6_2 Processor
The processor writes data from the host system to the data carrier or reads data from the
tag through the read/write head and prepares it for the host system. Host systems may
include:
–
a host computer (e.g. industrial PC) or
–
a programmable logic controller (PLC)
When sending data between the read/write head and the data carrier a procedure is
required for recognizing whether the data were correctly read or written.
The processor is supplied with standard Balluff procedure of double reading and compar-
ing. In addition to this procedure a second alternative is available: CRC_16 data checking.
Here a test code is written to the data carrier, allowing data to be checked for validity at
any time or location.
Advantages of CRC_16
Advantages of double reading
Data checking even during the non-active phase
(CT outside read/write head zone).
No bytes on the data carrier need to be
reserved for storing a check code.
Shorter read times since each page is read only
once.
Shorter write times since no CRC needs to be
written.
Since both variations have their advantages depending on the application, the user is free
to select which method of data checking he wishes to use (see Configuration on 14).
It is not permitted to operate the system using both check procedures!
Control Function
☞
Data checking
C6_2-019_641365_0506-e.p65
10
10
E
Application
BIS C-6_2 Processor
To use the CRC check method, the data carriers must be initialized. You use either data
carriers with the data map factory configured (all data are 0), or you must use the proces-
sor to write the special initialization command 'Z' to the data carriers (see
26/27).
If CRC_16 data checking is activated, a special error message is output to the interface
whenever a CRC error is detected.
If the error message is not caused by a failed write request, it may be assumed that one or
more memory cells on the data carrier is defective. That data carrier must then be re-
placed.
If the CRC error is however due to a failed write request, you must reinitialize the data
carrier in order to continue using it.
The checksum is written to the data carrier as a 2-byte wide datum. Two bytes per page
are 'lost', i.e., the page size becomes 30 bytes or 62 bytes depending on data carrier type
(see 18).
This means that the actual usable number of bytes is reduced!
Data carrier type
Usable bytes
128 bytes
=
120 bytes
256 bytes
=
240 bytes
511 bytes *)
=
450 bytes
1023 bytes *)
=
930 bytes
2047 bytes *)
=
1922 bytes
2048 bytes
=
1984 bytes
8192 bytes
=
7936 bytes
Data checking
(continued)
*) The last data carrier page for these EEPROM-
based data carriers is not available.
