Logisim UART

General purpose asynchronous buffered receiver/transmitter. Transfers data between Logisim and host system through a socket.

Applications

• Remote terminal using telnet
• Networking

Installing

• Download jar file from latest release
• In Logisim, Project > Load Library > JAR Library
• Then you get new library Network I/O with UART component

Properties

• Port - listen port

Pins

• clk - clock signal, input
• rd/wr' - read/write, selects operation mode, input
• en - enable, when low disables read/write operation, input
• dt - data available, high if there is some data in RX buffer, output
• con - connection, high if there is active connection, output
• RX/TX - data pins, output/input

Operations

en	clk	rd/wr'	Operation
0	↓	x	No-op
x	↑	x	Fetch
1	↓	1	Read
1	↓	0	Write

• ↑ - rising edge
• ↓ - falling edge
• x - either 1 or 0

No-op

When en is low, falling edge of clk has no effect

Fetch

On rising edge of clk, contents of RX buffer are fetched from network connection and con pin status is updated. RX pin outputs a value of the first byte of buffer.

Read

When en is high, rd/wr' is high, falling edge of clk drops the first byte of buffer and shifts all other bytes. Updates RX pin.

Write

When en is high, rd/wr' is low, falling edge of clk reads current value of TX and sends it.

Connection

Each instance of terminal opens a new socket on a port specified by Port property. If two instances have the same port, the behaviour is undefined.

As a Telnet terminal

To use this library as Telnet terminal you need some Telnet client.

For example, you can use:

• Windwos: PuTTY or Microsoft telnet
• Linux: telnet command
• MacOS: telnet from brew package manager

In order connect to an instance of UART, set IP to 127.0.0.1 and port to one that you specified in properties of Logisim component.

Example: telnet 127.0.0.1 7241

Note that you need to handle Telnet control messages yourself.

As a plain socket

This library doesn't really enforce any protocol, so you can connect to instance just like to regular socket and transfer binary data.

For example, you can use netcat to send some data from terminal. Another way is to write a program in your favorite language that connects to socket.

Connecting multiple instances together

Socket opened by UART instance acts as a server, so it can't directly connect to other sockets. If you want to transfer data between instances, you need some program on your host system that will connect to those instances and do all necessary routing.

Errors and troubleshooting

Port is in use status message

If you get this message as status, that means that specified Port is occupied, so it can't be bound. To resolve this, you can either choose another port or free up current port, for example by finding and closing program that uses it.

Other errors

If you get undefined behaviour, hanged up sockets or error in status, you can try to restart Logisim, it will close unused sockets and try to reopen used ones. On some systems you might need to wait up to minute for hanged up sockets to close. That is sometimes the case for port in use status.