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2.
COM20020B pin description
Pin No.
Name
1
A0/MUX
A0/MUX
2
A1
A1
3
A2/ALE
A2/ALE
4
AD0
5
AD1
6
AD2
7
D3
8
D4
9
D5
10
D6
11
D7
12
GND
13
XTAL1
14
XTAL2
15
PULSE1
PULSE1
16
PULSE2
PULSE2
17
RXIN
18
TXEN
TXEN
19
RESET IN
RESET IN
In/Out
Status
Status
of OFF
of ON
No Token
In
L
L
In
L
L
In
L
L
In/Out
L
Pulse
In/Out
L
Pulse
In/Out
L
Pulse
In/Out
L
Pulse
In/Out
L
Pulse
In/Out
L
Pulse
In/Out
L
Pulse
In/Out
L
Pulse
Power
GND
GND
In
L
Pulse
In
L
Pulse
Out
L
H
Out
L
L
In
L
H
Out
L
L
In
L
H
— 9 —
Status
Description
of ON
Token
Input. On a non-multiplexed bus,
Pulse
these signals are dirctly connectoed to
L
the low bits of the host address bus.
L
On a multiplexed address/data bus,
A0/MUX is tied low, A1 is left open,
and A2 is tied to the address latch
enable signal of the host. A1 is
connectoed to an internal pull-up
resistor.
Input/Output. On a non-multiplexed
Pulse
bus, these signals are used as the
Pulse
data lines for the device. On a
Pulse
multiplexed address/data bus,
Pulse
AD0~AD2 act as the address lines
Pulse
(latched by ALE) and as the low data
Pulse
lines for the device. D3~D7 are
Pulse
always used for data only. These
Pulse
signals are connected to internal pull-
up resistors.
Signal ground.
GND
An external crystal shoul be connected
Pulse
to these pins. If an external TTL clock
Pulse
is used instead, it must be connected
to XTAL1 with a 390 ohms pull-up
resistor, and XTAL2 should be left
floating.
Output. In normal mode, these active
Pulse
low signals carry the transmit data
L
information, encoded in pulse format,
from the COM20020 to the media
driver circuitry. When the device is in
backplane mode, the PULSE1 signal
driver is programmable (push/pull or
open-drain), while the PULSE2 signal
provides a clock with frequency of
crystal/4. PULSE1 is connected to a
week internal pull-up resistor in
backplane mode.
Receive input. This signal carries the
Pulse
receive data information from the line
receiver circuitry to the COM20020.
Transmit Enable output. This signal
Pulse
used in backplane mode to enable the
line drivers for transmission. The
polarity of the signal is programmable
by grounding the PULSE2 pin. This
option is valid only in backplane mode.
Input. This active low signal issued by
H
the microcontroller executes a
hardware reset. It is used to activate
the internal reset circuitry within the
COM20020.
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