Rockwell Automation 1395 Node Adapter Board - Manual (Page 25)

Chapter 3
Functional Description
3-13
Data File – is the block transfer message illustrated in Figure 3.5 which
contains both the header and the data portions of the message. The number
of words required for the data file is dependent on the size of the message
being sent. In Figure 3.7, N111:5 is the first word in the data file for the
BTW block and N111:50 is the first word for the BTR block.
Length – specifies the length of the block transfer message in words. The
BTR must always be set to 40.
IMPORTANT: The low byte of Word 4 in the message itself contains the
length of the message in bytes. The length specified on the length line of
the block transfer block is the same as specified in Word 4 of the message
itself. The only difference is in units used to specify the length. In
Figure 3.7, the length is 6 words for BTW (40 for BTR), which will appear
as 12 bytes (80 bytes for BTR), in Word 4 low byte.
Continuous – specifies whether the block transfer block is to be executed
on a continuous update or only when enabled. Normally, because of the
nature of the data being transferred to the Drive using the block transfer
function, this is set to N representing non-continuous execution.
PLC-5 Controller Block Transfer Rung Example
Figure 3.8 is used to sequence a block transfer operation between the PLC5
and the 1395. The node adapter status (I:010 for this example) and the
block transfer Write/Read control bits are used to control the order of
execution. The block transfer control bits are contained in the upper byte of
the first word of the control block. When the rung is examined, it may
seem that some of the bits are not needed. It is important to remember the
block transfers are executed asynchronously to the program scan, so the
bits are used to cover all possibilities. To start the block transfer operation,
the enable bit (N7 : 3/1) must be set and communications between the
PLC5 and the 1395 must be established. If the two are communicating, bit
10 (octal) will be set in the node adapter status word.
The first BTR instruction is used to discard erroneous data. This instruction
is executed when a block transfer Read is available and the program has
not executed a block transfer Write. This condition is detected by using the
examine off instruction with both enable (15) and the done (13) bits of the
BTW. This reasoning can be used because the block transfer instructions
are normally executed in pairs with the BTW being the first. Therefore, if
the drive has a read available before a write is done, the data may be
discarded. If the first BTR is executed, the masked move on the next
branch resets the upper byte of its control word. The byte is reset whether
the BTR has faulted or has been completed successfully.