SMSC COM20020 manual Functional Description, Attenuation, Nominal, Cable Type Impedance AT 5MHZ

Table 1 - Typical Media

		ATTENUATION
	NOMINAL	PER 1000 FT.
CABLE TYPE	IMPEDANCE	AT 5MHZ
RG-62 Belden #86262	93Ω	5.5dB
RG-59/U Belden #89108	75Ω	7.0dB
RG-11/U Belden #89108	75Ω	5.5dB
IBM Type 1* Belden #89688	150Ω	7.0dB
IBM Type 3* Telephone Twisted
Pair Belden #1155A	100Ω	17.9dB
COMCODE 26 AWG Twisted
Pair Part #105-064-703	105Ω	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 5MHz and a 2.5MHz clock from the external crystal. These clocks provide the rate at which the instructions are executed within the COM20020. The 5MHz clock is the rate at which the program counter operates, while the 2.5MHz 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.

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