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Chapter 5. About the SI7 I/O Driver 61

Feature: Block Writes

Block writes let you send data to various registers in the hardware at one time by

using a special “send” command. This command instructs the driver to send all

outstanding writes in a single protocol message. Block writes are useful in batch

situations where multiple setup parameters are required by the hardware at one time.

The SI7 driver supports block writes. The maximum length of a datablock with the

Block Write option enabled is 10 bytes less than the maximum length of the datablock

with the option disabled. For example, if the maximum datablock length without the

Block Writes option is 119 bytes, then the maximum length with the option enabled is

109 bytes.

If more than the maximum number of bytes need to be written, the write fails. To

correct this, configure the datablock to be 10 bytes less than the maximum or smaller.

When the driver sends its outstanding writes, it also sends any unmodified values in

the datablocks that are changing using the last known value. Consequently, we

recommend relatively short poll times for datablocks you are writing to so that the

driver uses the most up-to-date data.

You can write analog or digital values. When writing digital values use datablocks

configured with BYTE data types. Each digital register is interpreted by the PLC as

eight data items. The bits are written out with the last known values of the other bits

in the same register, just as analog values are written.

Datablock 1

Address: M0

10 20 30 40 50

Before the block write

If you write 60 to address

M0 and 70 to M2, the

driver sends the datablock

at the right when the

!SEND command

executes.

After the block write

Datablock 1

Address: M0

60 20 70 40 50

1 2 ... 66 67 68 69 70 71 72 73 74 75 76 ... 267 268

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Siemens SIMATIC NET Spécifications Page 71