Rockwell Automation 1747-SN Remote I/O Scanner - Manual (Page 17)

Publication 1747-UM013B-EN-P - January 2005
Overview 1-9
Transferring Data with RIO Discrete and Block Transfers
Input and output image data and command information are quickly
exchanged between a scanner and adapter using RIO discrete
transfers. RIO discrete transfers are the simplest and fastest way a
scanner and adapter communicate with each other. RIO discrete
transfers, which are transparent to the user, consist of the scanner
sending the output image data to the adapter, and the adapter
transmitting input data to the scanner. Each RIO discrete transfer also
contains scanner commands for the adapter.
Through your control program, you command the SLC processor to
initiate RIO block transfers, which directs the scanner to exchange
large amounts of data to/from an adapter. Block Transfers (BTs) use
the basic RIO discrete transfer mechanism of the RIO link. However,
the actual transfer of data occurs asynchronous to the discrete
transfers. It is possible for several discrete transfers to occur before the
scanner processes a block transfer. Refer to Chapter 5, RIO Block
Transfer for more details.
Physical and Logical RIO Link Specifications
The maximum number of adapters with which your scanner can
communicate is determined by the scanner’s and adapter’s physical
and logical specifications, as described below:
• Physical Specifications are the maximum number of adapters
that can be connected to the scanner. For more information, see
Extended Node Capability below.
• Logical Specifications for the scanner are the maximum number
of logical racks the scanner can address, how the logical racks
can be assigned, and whether the scanner can perform BTs.
Extended Node Capability
Extended node functionality allows you to connect up to 32 physical
devices on an RIO link. You must use 82 Ohm RIO link resistors in an
extended node configuration. You can only use extended node if all
RIO link devices have extended node capability. (Refer to the
Compatible Devices table at the end of this chapter, or to the
specifications of your device.) The 1747-SN Series B Scanner has
extended node capability. However, the smallest logical rack division
is 1/4 logical rack and the scanner image size is 4 logical racks.
Therefore, the scanner is limited to 16 devices unless complementary