COM20020_00 Datasheet, PDF(26/82 Page) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

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COM20020_00 Datasheet, PDF (26/82 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

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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

IMPEDANCE

ATTENUATION PER 1000 FT.

AT 5 MHz

93Î©

5.5dB

75Î©

7.0dB

75Î©

5.5dB

150Î©

7.0dB

100Î©

17.9dB

105Î©

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 â Cabling Guidelines for the COM20020 ULANC,

available from Standard Microsystems Corporation.

FUNCTIONAL DESCRIPTION

MICROSEQUENCER

is stored in the ROM and the instructions are

The COM20020 contains an internal fetched and then placed into the instruction

microsequencer which performs all of the control registers. One register holds the opcode, while

operations necessary to carry out the ARCNET the other holds the immediate data. Once the

protocol. It consists of a clock generator, a 544 x instruction is fetched, it is decoded by the internal

8 ROM, a program counter, two instruction instruction decoder, at which point the COM20020

registers, an instruction decoder, a no-op proceeds to execute the instruction. When a no-

generator, jump logic, and reconfiguration logic.

op instruction is encountered, the microsequencer

enters a timed loop and the program counter is

The COM20020 derives a 10 MHz and a 5 MHz temporarily stopped until the loop is complete.

clock from the output clock of the Clock Multiplier. When a jump instruction is encountered, the

These clocks provide the rate at which the program counter is loaded with the jump address

instructions are executed within the COM20020. from the ROM. The COM20020 contains an

The 10 MHz clock is the rate at which the program internal reconfiguration timer which interrupts the

counter operates, while the 5 MHz clock is the rate microsequencer if it has timed out. At this point

at which the instructions are executed. The the program counter is cleared and the

microprogram

MYRECON bit of the Diagnostic Status Register is

set.

DISCONTINUED DATASHEET

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