AT&T 3B2/300 Technical Reference Manual page 424

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Local RAM
The NI card contains 32K bytes of Static Random Access Memory (SRAM) configured as 16K by 16
bits. The DRAM is accessed via the Low Memory Chip Select (LCSO) from 80186 Microprocessor and
address bits 12-00. Note that 256K bytes of RAM address space is reserved. The 32K bytes of RAM
is used for intermediate data storage of the data being transmitted/received to/from the network.
Local ROM
Firmware for the 80186 Microprocessor is stored in the ROM. The NI card ROM contains
16K bytes configured as 8K by 16 bits. The ROM is accessed via the Upper Memory Chip Select
(UCS[O]) and address bits 14-00.
Network Interface
The AT&T 3BNET is an Ethernet compatible, local area network. Data is transferred over the
network by attaching a destination identification to the data to be transferred. The nisend command is
used to attach a destination code and file name to the data to be transferred. The NI firmware handles
the transfer of the data between the 3B2 computer main memory and the Network Interface Local
RAM. The data to be transferred is divided into packets of 1024 bytes for transfer over the network.
The Ethernet Controller autonomously reads the data from local RAM, converts the data to serial
stream, and transmits the stream over the network. If collisions are detected, the information packet is
retransmitted automatically. The receiving system acknowledges the receipt of the data.
Ethernet Controller Circuit
The Network Interface is an 82586 Ethernet Controller that manages the process of transmitting and
receiving data over the network. The primary functions of the controller are as follows:
• Decode the serial data
• Check for data integrity
• Convert serial data into parallel data format
• Store parallel data in the local RAM
• Save and report networking errors for the node.
The Network Interface connects to the network via a coaxial drop cable and a transceiver circuit.
The interface between the Network Interface and the transceiver circuit consists of the following signals.
Receive Pair
A differential signal that is active when any data is received.
Transmit Pair
A differential signal that is active when any data is transmitted.
Collision Presence Pair
A differential signal that is active while a collision is in progress.
Power Pair
The transceiver is powered by +12 volt DC supplied by the 3B2 computer.
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