Rockwell Automation 1771-QA Stepper Positioning Assembly User Manual - Manual (Page 57)

3–35 Programming and Operation
Publication 1771-UM002A–EN–P – May 2000
Important: Increasing the size of the data table by 128-word
sections reduces the amount of words for user program
by the same amount.
Handshaking
Handshaking is the exchange of commands and/or data with status
information between the PC processor and stepper controller
module. Handshaking is normally used to ensure successful block
transfers independent of data transfer times. The stepper positioning
assembly uses any one of three handshaking cycles depending on the
content and purpose of the transfer. The steps of the handshaking
cycles are summarized below.
CYCLE 1 Transferring a command bit (start, jog forward, jog
reverse, override, initialization preset, load jog) with or without
accompanying data.
1. Enable the transfer containing the command bit (with or without
accompanying data).
2. Observe that the command verify bit is set in the status word by
the stepper controller module.
3. Clear the command bit.
4. Observe that the command verify bit is then cleared in the status
word by the stepper controller module.
CYCLE 2 Transferring a reset or stop command.
1. Transfer the reset or stop command.
2. Observe that the reset or stop bit is set in the status word.
3. Clear the command bit. The reset or stop status bit will not be
cleared until the first move of the next moveset begins.
CYCLE 3 Transferring data with no command bit.
1. Observe whether the data receive (toggle) bit in the status word is
set or reset.
2. Enable the transfer of data.
3. Observe that the data receive toggle bit changes to the opposite
state.
Command bits are contained in the moveset control word, MS preset
word and single move control word. Status bits are contained in the
status word. Command bits which directly correspond to bits in the
status word during the handshaking cycle are presented in Table 3.B.