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Table 1 - Typical Media
CABLE TYPE
NOMINAL
IMPEDANCE
ATTENUATION
PER 1000 FT.
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.