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Principles of Operation
A short time later the SPU will instruct the LANIC to set its individual address. When this occurs, the
LANIC performs a duplicate address check, which is accomplished by transmitting 10 frames to the
network with a 500 msec separation between frames. The TX and the DO E LED will both come on for
each of the 10 frames. In addition, the CR E LED will indicate that the frames were sent to the coaxial
cable and caused the carrier to come on.
If
collisions are encountered, the frames will be retried up to 15
times each, with resultant activity indicated by the CL LEOs. The RX LED will not light during the
duplicate address check due to our own transmission, even though the duplicate address check frame is
addressed to the transmitting LANIC. If the RX LED lights during duplicate address checking, it is due
either to a duplicate station 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 the system software will close the link and clear the LANIC, forcing all the
LANIC MPU LEOs 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 be 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 pair of the AUI cable, and the CR E LED is lit. The CR L LED 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 encountered, the CR and DO E LEDs will go off after 6 msec, 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 the intensity of the
L
LEDs determined by frame length, number of
retransmissions required, and the time separation of the retransmissions. The CL LEDs 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 LEDs 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.
Feb 85
3-17
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