COM20020D datasheet(22/72 Pages) SMSC

22 / 72 page

Table 1 - Typical Media

CABLE TYPE

NOMINAL

IMPEDANCE

ATTENUATION PER 1000 FT.

AT 5 MHz

RG-62 Belden #86262

Ω

5.5dB

RG-59/U Belden #89108

Ω

7.0dB

RG-11/U Belden #89108

Ω

5.5dB

IBM Type 1* Belden #89688

150

Ω

7.0dB

IBM Type 3* Telephone Twisted

Pair Belden #1155A

100

Ω

17.9dB

COMCODE 26 AWG Twisted

Pair Part #105-064-703

105

Ω

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

COM20020D

contains

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,

instruction

decoder,

no-op

generator, jump logic, and reconfiguration logic.

The COM20020D 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 COM20020D.

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

COM20020D 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

COM20020D 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.

COM20020D датащи(PDF) 22 Page - SMSC Corporation

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