CABLE TYPE
RG-62 Belden #86262
RG-59/U Belden #89108
RG-11/U Belden #89108
IBM Type 1* Belden #89688
IBM Type 3* Telephone Twisted
Pair Belden #1155A
COMCODE 26 AWG Twisted
Pair Part #105-064-703
Table 1
-
Typical Media
NOMINAL
ATTENUATION PER 1000 FT.
IMPEDANCE
AT 5 MHz
5.5dB
93!
75!
75!
150!
100!
105!
7.0dB
5.5dB
7.0dB
17.9dB
16.0dB
*Non-plenum-rated cables of this type are also available.
Note: For more detailed information on Cabling options including RS-485, transformer-coupled RS-485
and Fiber Optic interfaces, please refer to TN7-5 – Cabling Guidelines for the COM20020 ULANC,
available from Standard Microsystems Corporation.
FUNCTIONAL DESCRIPTION
MICROSEQUENCER
The
COM20020
contains
an
internal
microsequencer which performs all of the control
operations necessary to carry out the ARCNET
protocol. It consists of a clock generator, a 544 x
8 ROM, a program counter, two instruction
registers, an instruction decoder, a no-op
generator, jump logic, and reconfiguration logic.
The COM20020 derives a 10 MHz and a 5 MHz
clock from the output clock of the Clock Multiplier.
These clocks provide the rate at which the
instructions are executed within the COM20020.
The 10 MHz clock is the rate at which the program
counter operates, while the 5 MHz clock is the rate
at which the instructions are executed. The
microprogram
is stored in the ROM and the instructions are
fetched and then placed into the instruction
registers. One register holds the opcode, while
the other holds the immediate data. Once the
instruction is fetched, it is decoded by the internal
instruction decoder, at which point the COM20020
proceeds to execute the instruction. When a no-
op instruction is encountered, the microsequencer
enters a timed loop and 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. The COM20020 contains an
internal reconfiguration timer which interrupts the
microsequencer if it has timed out. At this point
the program counter is cleared and the
MYRECON bit of the Diagnostic Status Register is
set.
23
SMSC [ SMSC CORPORATION ]
