Rockwell Automation 8520 9/Series CNC Integration Maintenance Manual Documentation Set - Manual (Page 444)

Section 8
Communication Interface
8-11
The control transmits data to the peripheral in the following sequence. The
numbers correspond to those in Figure 8.11.
1. The control turns the CS and the DM signals ON to notify the
peripheral that there is data to be transmitted.
2. The peripheral turns the RS signal ON to notify the control that it is
ready to receive data.
3. The control begins data transmission to the peripheral.
4. If the peripheral buffer cannot accept the data, the peripheral turns the
RS signal OFF to notify the control to stop data transmission.
5. The control, upon recognizing the RS signal has been turned OFF,
stops transmitting data within two characters.
6. The peripheral turns the RS signal ON when it can receive data again.
7. The control, upon recognizing the RS signal has been turned ON,
begins data transmission .
8. After transmitting all data to the peripheral, the control turns the CS
and DM signals OFF to notify the peripheral that all data has been
transmitted.
9. The peripheral turns the RS signal OFF.
Level II protocol is used for the communication between the control and a
peripheral (tape reader/puncher) or a computer. Level II protocol differs
from Level I protocol in that data transmission/reception is controlled by
the control codes ( DC1, DC2, DC3, DC4, EOT, DLE ). The functions of
these control codes are shown in Table 8.E.
Devices where the protocol is labeled LEVEL 2* implement a slightly
modified version of the protocol. This serves to better match the specific
Level 2 device. The device type GENERIC LEVEL 2 implements the full
protocol as it is described here.
The 9/Series controls send and receive program data using the data format
appropriated for punched tape. This means all ASCII characters have even
parity in the eight bit position. EIA characters have their own parity
format which is recognized by the control. Characters which do not have
the proper parity are not stored in the control. In addition, lower case
ASCII data sent to the control is converted into upper case when stored.
RS-491 Level 2 control characters do not have eighth bit parity. The
control characters are used for device control only.
8.7.2
RS-491 Level II Protocol