
Am79C965A
99
selection the T-MAU will be forced into the Link Fail
state. In the Link Fail state, XMT, RCV and COL are
inactive.
Collision Detect Function
Activity on both twisted pair signals RXD± and TXD±
constitutes a collision, thereby causing the COL signal
to be activated. (COL is used by the LED control
circuits) COL will remain active until one of the two
colliding signals changes from active to idle. However,
transmission attempt in Link Fail state results in LCAR
and CERR indication. COL stays active for 2-bit times
at the end of a collision.
Signal Quality Error (SQE) Test (Heartbeat)
Function
The SQE function is disabled when the 10BASE-T port
is selected.
Jabber Function
The Jabber function inhibits the twisted pair transmit
function of the T-MAU if the TXD± circuit is active for an
excessive period (20 ms
–
150 ms). This prevents any
one node from disrupting the network due to a 'stuck-
on' or faulty transmitter. If this maximum transmit time
is exceeded, the T-MAU transmitter circuitry is
disabled, the JAB bit is set (CSR4, bit1), the COL signal
is asserted. Once the transmit data stream to the
T-MAU is removed, an
“
unjab
”
time of 250 ms
–
750 ms
will elapse before the T-MAU COL and re-enables the
transmit circuitry.
Power Down
The T-MAU circuitry can be made to go into power sav-
ings mode. This feature is useful in battery powered or
low duty cycle systems. The T-MAU will go into power
down mode when H_RESET is active, coma mode is
active, or the T-MAU is not selected. Refer to the Power
Savings Modes section for descriptions of the various
power down modes.
Any of the three conditions listed above resets the
internal logic of the T-MAU and places the device into
power down mode. In this mode, the Twisted Pair driver
pins (TXD±,TXP±) are driven LOW, and the internal T-
MAU status signals (LNKST, RCVPOL, XMT, RCV and
COL) signals are inactive.
Once H_RESET ends, coma mode is disabled, and the
T-MAU is selected. The T-MAU will remain in the reset
state for up to 10 μs. Immediately after the reset condi-
tion is removed, the T-MAU will be forced into the Link
Fail state. The T-MAU will move to the Link Pass state
only after 5
–
6 link beat pulses and/or a single received
message is detected on the RD± pair.
In snooze mode, the T-MAU receive circuitry will remain
enabled even while the SLEEP pin is driven LOW.
The T-MAU circuitry will always go into power down
mode if H_RESET is asserted, coma mode is enabled,
or the T-MAU is not selected.
10BASE-T Interface Connection
Figure 32 shows the proper 10BASE-T network inter-
face design. Refer to the PCnet Family Technical Man-
ual (PID # 18216A) for more design details, and refer to
Appendix A for a list of compatible 10BASE-T filter/
transformer modules.
Note that the recommended resistor values and filter
and transformer modules are the same as those used
by the IMR (Am79C980) and the IMR+ (Am79C981).
Figure 32. 10BASE-T Interface Connection
XMT
Filter
RCV
Filter
RJ45
Connector
Filter &
Transformer
Module
PCnet-32
TD+
TD-
RD+
RD-
1
2
3
6
61.9
422
61.9
422
100
1.21 K
1:1
1:1
TXD+
TXP+
TXD-
TXP-
RXD+
RXD-
18219-39