Packets, Handshaking, And Fifo Buffers - National Instruments 7344 User Manual

Packets, Handshaking, and FIFO Buffers

© National Instruments Corporation
At address offsets from the controller's base address are two read-only
status registers. The communications status register (CSR) provides bits for
communications handshaking as well as real-time error reporting and
general status feedback to the host PC. The move complete status (MCS)
register provides instantaneous motion status of all axes.
This section briefly describes how commands and data are passed between
the host computer and the 7344 controller. This information is provided for
reference purposes. The FlexMotion software that ships with the controller
provides drivers, DLLs, and C function libraries that handle the host to
controller communications automatically, with built-in error reporting.
Data passed to or from the 7344 controller is handled in a packet format. A
packet consists of a packet identifier word, command and data content, and
a packet terminator word. This approach to communications enhances the
accuracy of data communications, speeds the processing of the transferred
command and data, and organizes operation into powerful, high-level
motion functions.
Each word in a packet is sent over the host PC bus after checking the
Ready-to-Receive (RTR) handshaking bit in the CSR. Refer to the Read
Communication Status (
Reference Manual for the Status bitmap and more information on the status
reported in the CSR.
Command and data packets are checked for packet format errors as the
controller receives the packets. If a packet error is detected, it is
immediately reported by setting an error bit in the CSR. Once the packet is
received without error, the command and data is stored in a FIFO buffer.
This FIFO can hold up to 16 commands. The 7344 controller RTOS will
process commands whenever it is not busy with higher priority tasks. In the
unlikely occurrence that the FIFO fills up before any commands can be
processed, the host detects a Not-Ready-to-Receive condition and waits for
available room in the FIFO before sending additional packets.
Each command is processed and a determination is made whether to
execute the command immediately, or store it away in an onboard program
to be executed later. Commands are also checked for data and modal
(sequence) errors at this time. A modal error is flagged by setting the error
message bit in the CSR. A modal error is functionally different from the
packet communication error described above. See the
later in this chapter for additional information on modal errors.
) function in the FlexMotion Software
read_csr
4-11
Chapter 4 Functional Overview
Error Stack section
7344 Hardware User Manual