3 double word fragmentation, 4 sample programs – Rockwell Automation 57C445 AutoMax PC Link Interface Module User Manual

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D3Ć5

There are 16 slot maps (SLOT_MAPS) for a given drop. Each slot

map consists of two double words (each double word is 4 bytes).

The first double word is the output register map; the second double

word is the input register map. The double word register map is

defined in Table 5:

Table 5 - Double Word Register Map

Byte

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

#15

#14

#13

#12

#11

#10

#23

#22

#21

#20

#19

#18

#17

#16

#31

#30

#29

#28

#27

#26

#25

#24

If the bit for a given register is set in the output register map, the

input register map, the register is an input register. A register may not

be both an input and an output; therefore any bits set in the output

map for a given slot must not be set in the input map. All input and

output register blocks must be contiguous. A sample program which

demonstrates the creation of configuration packets, called RIOCNF.C,

is provided on the distribution diskette.

3.3

Double Word Fragmentation

Due to the 16Ćbit limitation of the PC Link module, the module is

accessed from the host one word at a time. When a double word

value register is read or written, it is actually read or written as two

words. If the PC Link module writes a double word value in between

the host's two accesses, the host will have one word which is from

the old double word value and one word which is from the new

double word value.
To protect from fragmentation during reads, each double word is

read twice in succession. The results of the two reads are compared.

If the results are the same, the value is ok. If the values are different,

the value must be read again, but his new read will not be

fragmented since information for this drop will not be updated for at

least 2.6 milliseconds.
For example, if the PC Link module was to change a double word

value between the host's two word reads for the double word, it will

not update that double word again until the next network scan which

is a minimum of 2.6 milliseconds later. If the values of the two host

double word reads (1 assignment and 1 compare) are different, we

know that the double word value will not be updated for at least 2.6

milliseconds. As long as the next access is within this time period, the

double word value read will not be fragmented.

3.4

Sample Programs

Three sample programs are provided with the RERIO software.

These programs are described in sections 3.4.1Ć3.4.3.