dp83261 National Semiconductor Corporation, dp83261 Datasheet - Page 95

• Manufacturer Part Number: dp83261
• Description: Bmac Device Fddi Media Access Controller
• Manufacturer: National Semiconductor Corporation

7 0 Signal Descriptions
7 4 3 Operation
The MAC Request Interface has three logical states as determined by TXRDY and TXPASS The interface state machine is
shown in Figure 7-11 followed by a description of the conditions states and transitions
Conditions
Send Frame
Service Opportunity
Continue Service Opportunity
End of Service Opportunity
Table 7-4 shows the timer thresholds used to determine if a Service Opportunity is possible for each service class
MR0 Not Ready
MR1 Ready
MR2 Sending
State
Request Service Class
All Requests
All Requests with THT Enabled
Priority Asynchronous Requests
TABLE 7-4 Thresholds Used to Determine Service Opportunities
A frame can be sent from the interface when at least 8 bytes of preamble have been transmit-
ted TXRDY RQRDY and RQSEND are asserted and RQFINAL has not yet been asserted for
this request
A Service Opportunity occurs when it is possible to service the current request as defined by
the current service parameters (RQRCLS RQCLM and RQBCN) The rules for servicing re-
quests are described in Section 5 2
A Service Opportunity is continued after the current frame if valid service parameters continue
to be presented during the frame and the timer(s) used for the (next) requested service class
have not reached their threshold
The end of a Service Opportunity occurs when it is no longer necessary or possible to continue
the Service Opportunity The service parameters are continuously compared with the current
state of the Transmitter If an unserviceable request is presented or any timer threshold is
reached the Service Opportunity will not continue after the current frame (if any)
Ring Engine is not ready to service a request
Ring Engine is ready to transmit a frame
Ring Engine is sending a frame
FIGURE 7-11 MAC Request Interface State Machine
(Continued)
Description
95
Threshold
TRT Expiration
THT Expiration
Asynchronous Priority Threshold
TXRDY
0
1
0
TXPASS
TL F 10387 – 18
1
0
0