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being detected or to an ordinary frame being address to the LANIC. If a reply
to the duplicate address check is received, this will cause the address check to
fail. no further check frames will be sent, and t.he system software will close the
link and clear the LANIC, forcing all the LANIC MPU LEDs to come on and
stay on.
If the duplicate address check passes, the link is opened. and frame transmission
and reception will commence. The LEDs will indicate activity as it occurs.
During normal network operation. frame transmission causes the LEDs to
operate in the following manner, assuming that the network and the LANIC
were both idle before the transmit request arrived at the LANIC from the SPU:
While idle. the VP. Q. and IT LEDs are on.
When the MPU begins processing the transmit command. the
Q
and IT LEDs
go off, and the TX LED comes on. The LANIC begins the transmit process
by reading the frame from the system to the on-card memory.
Once the frame is in LANIC local memory. and the network is free, the serial
transmission process begins. This causes the DO E LED to light. The DO L
LED will also be turned on for the duration of the frame transmission. but
this mayor may not he visible, depending upon the length of the individual
frame being sent.
The serial data reaches the MAU and is transmitted to the coaxial cable. The
MAU begins to receive its own signal from the coax. and sends it back down
the AUI cable. The LANIC detects data arriving on the DI pairof the AUI
cable, and the CR E LED is lit.. The CR L LEO will also be lit for the
duration of the frame, but this mayor may not be visible. If the DO L LED is
visible, the CR L LED will also be visible for approximately the same length
of time.
If no collision is occurs. the CR and DO E LEDs will go off after 6
millisecond, followed quickly by the transmit LED going off, and the
Q
and
IT LEOs coming on. If a collision is encountered. the CL E LED will come
on. and the frame will be retransmitted up to 15 times. The retransmissions
will cause the DO and CR E LEDs to appear to be on. and the DO and CR L
LEDs will probably appear to be partially lit, with t.he intensity of the L LEOs
determined by frame length. number of retransmissions required, and the time
separation of the retransmissions. The CL LEOs will also display behavior
similar to the CR and DO LEDs if multiple retransmissions are required before
the frame is successfully transmitted. In the collision case, it must be
remembered that other network activity will also cause the CL and CR LEOs
to light, and the activity caused by the LANIC will be superimposed on the
network activity being displayed by the CR and CL LEDs. A little experience
at observing the LEOs will allow the occurrence of single or multiple
collisions to be easily distinguished.
Principles of Operation
3-21
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