COM20019IP datasheet(23/81 Pages) SMSC

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Table 1 - Typical Media

CABLE TYPE

NOMINAL

IMPEDANCE

ATTENUATION PER 1000 FT.

AT 5 MHz

RG-62 Belden #86262

5.5dB

RG-59/U Belden #89108

7.0dB

RG-11/U Belden #89108

5.5dB

IBM Type 1* Belden #89688

150

7.0dB

IBM Type 3* Telephone Twisted

Pair Belden #1155A

100

17.9dB

COMCODE 26 AWG Twisted

Pair Part #105-064-703

105

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

COM20019

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 COM20019 derives a 625 kHz and a 312.5

kHz clock from the output clock of the Clock

Multiplier. These clocks provide the rate at which

the

instructions

are

executed

within

the

COM20019.

The 625 kHz clock is the rate at

which the program counter operates, while the

312.5 kHz 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 COM20019

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 COM20019 contains an

internal reconfiguration timer which interrupts the

microsequencer if it has timed out. At this point

the

program

counter

cleared

and

the

MYRECON bit of the Diagnostic Status Register is

set.

COM20019IP датащи(PDF) 23 Page - SMSC Corporation

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