Block Transfer Throughput - Allen-Bradley 1747-SN User Manual

A–10 Specifications
Publication 1747 6.6 - July 1996

Block Transfer Throughput

Block transfer throughput is the time from when the BT is enabled
via the EN bit, until the DN bit is processed. The following BT
timing explanations are based on the directional continuous BT
example shown on page 5–29, where a BT is re–triggered
automatically upon each completion.
BT throughput is always slower than discrete data transfer.
Completing a BT is dependent on the time involved for the:
SLC control program to enable the BT via an M0 file write
scanner to detect that a BT has been requested
BT to be waiting in the queue due to another BT already being
processed on the same logical rack
scanner to schedule a pending bit
adapter to acknowledge the request
scanner to initiate the BT and transfer the data
SLC control program to detect that the BT has completed (DN
flag set)
The time to free up the BT buffer (by clearing the EN flag so another
BT can be performed) depends on the:
instruction time of the M0 file write which clears the EN flag
time for the scanner to detect that the EN flag has been cleared
time for SLC control program to detect that the DN flag has been
cleared
This is dependent on the SLC processor you are using.
Refer to the equations that follow.
The RIO network allows only one BT per logical rack (not logical device) per RIO scan. Therefore,
if multiple BTs are performed on devices within the same logical rack, BTs will have to wait in the
queue until any previously scheduled BTs for the same logical rack have been completed.
This is dependent on the RIO adapter.
The formula to calculate BT throughput is:
T + T (number of BTs + 1) + T
M0 SNo–bt
The equation for freeing up the BT buffer is:
T + T (number of BTs) + T
M0 SNo–bt
Substitute values for the variables in the formulas above. Locate
these values in the following documents:
+ 2T + 2T T
btwait RIO btx + adp–bt
ps
+ T
ps