am79c961 Advanced Micro Devices, am79c961 Datasheet - Page 74

• Manufacturer Part Number: am79c961
• Description: Pcnettm-isa Jumperless Single-chip Ethernet Controller For Isa
• Manufacturer: Advanced Micro Devices

1-548
Since a 10BASE-T hub does not normally feed the sta-
tion’s transmitter outputs back into the station’s receiver
inputs, the use of external loopback in a 10BASE-T sys-
tem usually requires some sort of external hardware that
connects the outputs of the 10BASE-T MAU to
its inputs.
LEDs
The PCnet-ISA
programming of the status signals, which are displayed
on 3 LED outputs. One LED (LED0) is dedicated to dis-
playing 10BASE-T Link Status. The status signals
available are Collision, Jabber, Receive, Receive Polar-
ity (active when receive polarity is okay), and Transmit.
If more than one status signal is enabled, they are ORed
together. An optional pulse stretcher is available for
each programmable output. This allows emulation of the
TPEX (Am79C98) and TPEX+ (Am79C100) LED
outputs.
RCVADDM Active during Receive with Address Match
RVPOL
LNKST
Signal
RCV
XMT
AMD
Active during Link OK
Not active during Link Down
Active while receiving data
Active during receive polarity is OK
Not active during reverse receive polarity
Active while transmitting data
+
controller’s LED control logic allows
Behavior
P R E L I M I N A R Y
Am79C961
Each status signal is ANDed with its corresponding
enable signal. The enabled status signals run to a com-
mon OR gate:
The output from the OR gate is run through a pulse
stretcher, which consists of a 3-bit shift register clocked
at 38 Hz. The data input of the shift register is at logic 0.
The OR gate output asynchronously sets all three bits of
the shift register when its output goes active. The output
of the shift register controls the associated LEDx pin.
Thus, the pulse stretcher provides an LED output of
52 ms to 78 ms.
Refer to the section “ISA Bus Configuration Registers”
for information on LED control via the ISACSRs.
RCVADDM E
RCVADDM
RCV E
XMT E
COL E
RVP E
JAB E
RCV
XMT
COL
RVP
JAB
LED Control Logic
18183B-21
16907B-14