com90c66 Standard Microsystems Corp., com90c66 Datasheet - Page 13

• Manufacturer Part Number: com90c66
• Description: Arcnet Controller/transceiver With At Interface And On-chip Ram Corporation
• Manufacturer: Standard Microsystems Corp.

TRANSMISSION MEDIA INTERFACE
The right half of Figure 2 illustrates the
COM90C66 interface to the transmission media
used to connect the node to the network. The
HYC9058/68/88 may be used to drive the
media.
transmits a logic "1" by generating two 100 nS
non-overlapping negative pulses, nPULSE1 and
nPULSE2. These signals are sent to the LAN
Driver, which in turn creates a 200 nS dipulse
signal on the media. A logic "0" is transmitted
by the absence of the two negative pulses, that
is, the nPULSE1 and nPULSE2 outputs remain
high, therefore there is an absence of a dipulse.
on the media is coupled through the RF
transformer of the LAN Driver. A positive pulse
at the RXIN pin of the COM90C66 is interpreted
as a logic "1". Again, if no dipulse is present,
the COM90C66 interprets a logic "0".
MICROSEQUENCER
The
microsequencer which performs all of the
control operation necessary to carry out the
ARCNET protocol.
generator, a 554 x 8 ROM, a program counter,
two instruction registers, an instruction decoder,
a
reconfiguration logic.
The COM90C66 derives a 5 MHz and a 2.5 MHz
clock from the external crystal. These clocks
provide the rate at which the instructions are
executed within the COM90C66.
clock is the rate at which the program counter
operates, while the 2.5 MHz clock is the rate at
which the instructions are executed. The
microprogram is stored in the ROM and the
During reception the 200 nS dipulse appearing
no-op
COM90C66
During transmission, the COM90C66
generator,
contains
It consists of a clock
jump
FUNCTIONAL DESCRIPTION
an
logic,
The 5 MHz
internal
and
13
Typically, RXIN pulse occur at multiples of 400
nS. The COM90C66 can tolerate distortion of
plus or minus 100 nS and still correctly capture
the RXIN pulses.
During Reset, the transmitter portion of the
COM90C66 is disabled and the nPULSE1 and
nPULSE2 pins are inactive high.
The COM90C66 includes the nTXLED and
nBSLED signals which, when tied to LEDs,
provide indication of transmit and board access
activity. In addition, it is possible for the user to
completely disable the transmitter through
software. These two unique features represent
two
diagnostics of the device.
Improved Diagnostics section of this document
for further detail.
instructions are fetched and then placed into the
instruction registers. One register holds the op
code, while the other holds the immediate data.
by the internal instruction decoder, at which
point the COM90C66 proceeds to execute the
instruction.
encountered, the microsequencer enters a timed
loop, in which case the program counter is
temporarily stopped until the loop is complete.
When a jump instruction is encountered, the
program counter is loaded with the jump
address from the ROM.
contains an internal reconfiguration timer which
interrupts the microsequencer if it has timed out.
after which the MYRECON bit of the Diagnostic
Status Register is set.
At this point the program counter is cleared,
Once the instruction is fetched, it is decoded
of
the
When a no-op instruction is
improvements
The COM90C66
Please see the
made
in
the