Serial Line Internet Protocol - SLIP protocol

The need for a data link layer protocol to let IP operate over serial links was identified very early on in the development of TCP/IP. To solve the problem they created a very simple protocol that would frame IP data grams for transmission across the serial line. This protocol is called the Serial Line Internet Protocol, or SLIP for short.
SLIP modifies a standard TCP/IP data gram by appending a special "SLIP END" character to it, which distinguishes data gram boundaries in the byte stream. SLIP requires a serial port configuration of 8 data bits, no parity, and either EIA hardware flow control, or CLOCAL mode (3-wire null-modem) UART operation settings.

- Serial Line Interface Protocol (SLIP) is a TCP/IP protocol used for
communication between two machines that are previously configured for communication with each other.
- The dial-up connection to the server is typically on a slower serial line rather than on the parallel or multiplex lines.
- SLIP does not provide error detection, being reliant on other high-layer protocols for this.
- A SLIP connection needs to have its IP address configuration set each time before it is established.
- The Serial Line Internet Protocol (SLIP) is a mostly obsolete encapsulation of the Internet Protocol designed to work over serial ports and modem connections.
- A version of SLIP with header compression is called CSLIP (Compressed SLIP).
- The Parallel Line Internet Protocol (PLIP) is very similar to SLIP, but works at higher speeds via a parallel port.
- SLIP is a STREAMS-based computer networking facility that provides for the transmission and reception of IP packets over serial lines.
- SLIP can be used to connect one host to another via a single, physical serial line connection between serial ports or over longer distances using a modem at each end of a telephone line.

RS422 Standard

RS-422 is a telecommunications standard for binary serial communications between devices. It is the protocol or specifications that must be followed to allow two devices that implement this standard to speak to each other. RS-422 is an updated version of the original serial protocol known as RS-232.

This standard was introduced in 1975 to offer improvements over the older RS-232 standard. It provides a balanced line with optional termination. The standard uses a voltage differential of 2v min to 5v max to represent the binary 0's and 1's. The specification allows data rates up to 10M baud at 40 feet maximum cable length.
The maximum cable length that can be driven will depend on the baud rate, the driver/receiver IC's, the cable type, and the amount of electrical noise in the surrounding environment. RS-422 can be used for point-to-point communication or for multi-drop one-master/many-slave systems.

The RS-422 standard only defines the characteristic requirements for the balanced line drivers and receivers. It does not specify one specific connector, signal names or operations. RS-422 interfaces are typically used when the data rate or distance criteria cannot be met with RS-232. The RS-422 standard allows for operation of up to 10 receivers from a single transmitter. The standard does not define operations of multiple tri-stated transmitters on a link.

RS-422 is a balanced four wire system. The signal sent from the DTE device is transmitted to the DCE device through two wires and the signal sent from the DEC device to the DTE device is transmitted through the other two wires. The signals on each pair of wires are the mirror opposite of each other, i.e., a "1" datum is transmitted as a plus 2 volt reference on one wire and a minus 2 volt reference on the other wire. To send a "0" datum, a minus 2 volt reference is transmitted through one wire and a plus 2 volt reference on the other wire. That is the opposite of what was done to transmit a \'1\' datum.

The RS-422-A interfaces between the Data Terminal Equipment (DTE) and Data Communication Equipment (DCE) or in any point-to-point interconnection of signals between digital equipment. It employs the electrical characteristics of balanced-voltage digital interface circuits.

RS485 Standard

RS-485 is a telecommunications standard for binary serial communications between devices. It is the protocol or specifications that need to be followed to allow devices that implement this standard to speak to each other. A RS-485 compliant network is a multi-point communications network. The RS-485 standard specifies up to 32 drivers and 32 receivers on a single (2-wire) bus. RS-485 drivers are now even able to withstand bus contention problems and bus fault conditions. A RS-485 network can be constructed as either a balanced 2 wire system or a 4 wire system. If a RS-485 network is constructed as a 2 wire system, then all of the nodes will have equal ranking. A RS-485 network constructed as a 4 wire system, has one node designated as the master and the remaining nodes are designated as slaves. The maximum cable length can be as much as 4000 feet because of the differential voltage transmission system used. The typical use for RS485 is a single PC connected to several addressable devices that share the same cable.

RS485 meets the requirements for a truly multi-point communications network, and the standard specifies up to 32 drivers and 32 receivers on a single (2-wire) bus. With the introduction of "automatic" repeaters and high-impedance drivers / receivers this "limitation" can be extended to hundreds (or even thousands) of nodes on a network. RS485 extends the common mode range for both drivers and receivers in the "tri-state" mode and with power off. Also, RS485 drivers are able to withstand "data collisions" (bus contention) problems and bus fault conditions.

SPECIFICATIONS RS485
- Mode of Operation DIFFERENTIAL
- Total Number of Drivers and Receivers on One Line 1 DRIVER & 32 RECEIVER
- Maximum Cable Length 4000 FT.
- Maximum Data Rate 10Mb/s
- Maximum Driver Output Voltage -7V to +12V
- Driver Output Signal Level (Loaded Min.) Loaded +/-1.5V
- Driver Output Signal Level (Unloaded Max) Unloaded +/-6V
- Driver Load Impedance (Ohms) 54
- Max. Driver Current in High Z State Power On +/-100uA
- Max. Driver Current in High Z State Power Off +/-100uA
- Slew Rate (Max.) N/A
- Receiver Input Voltage Range -7V to +12V
- Receiver Input Sensitivity +/-200mV
- Receiver Input Resistance (Ohms) >=12k

RS232 Standard

RS232 is a asynchronous serial communication protocol widely used in computers and digital systems. It is called asynchronous because there is no separate synchronizing clock signal as there are in other serial protocols like SPI and I2C.
In RS232 there are two data lines RX and TX. TX is the wire in which data is sent out to other device. RX is the line in which other device put the data it need to sent to the device.
Voltage levels in RS232 are HIGH=-12V and LOW=+12V.

RS-232 Specifications :
- Cabling : Single-ended
- Number of Devices : 1 transmit, 1 receive
- Communication Mode : Full duplex
- Distance(max) : 50 feet at 19.2kbps
- Data Rate(max) : 1Mbps
- Signaling : Unbalanced
- Mark(data 1) : -5V (min) -15V (max)
- Space(data 0) : 5V (min) 15V (max)
- Input Level(min) : ±3V
- Output Current : 500mA
- Impedance : 5kW (Internal)
- Bus Architecture : Point-to-Point

RS232 Data Transmission :
Transmission
1. When there is no transmission the TX line sits HIGH (STOP CONDITION).
2. When the device needs to send data it pulls the TX line low for 104uS (This is the start bit which is always 0).
3. Then it sends each bit with duration of = 104uS.
4. Finally it sets TX lines to HIGH for at least 104uS (This is stop bit and is always 1).
Reception :
1. The receiving device is waiting for the start bit i.e. the RX line to go LOW.
2. When it gets start bit it waits for half bit time i.e. 104/2 = 51uS, i.e.it is in middle of start bit, it reads it again to make sure it is a valid start bit and not a spike.
3. Then it waits for 104uS and now it is in middle of first bit. It then reads the value of RX line.
4. In the same way it reads all the 8 bits.
5. Now the receiver has the data.

Limitations of RS232 Standard :
* The large voltage swings and requirement for positive and negative supplies increases power consumption of the interface and complicates power supply design. The voltage swing requirement also limits the upper speed of a compatible interface.
* Single-ended signaling referred to a common signal ground limits the noise immunity and transmission distance.
* Multi-drop connection among more than two devices is not defined. While multi-drop "work-arounds" have been devised, they have limitations in speed and compatibility.
* Asymmetrical definitions of the two ends of the link make the assignment of the role of a newly developed device problematic; the designer must decide on either a DTE-like or DCE-like interface and which connector pin assignments to use.
* The handshaking and control lines of the interface are intended for the setup and takedown of a dial-up communication circuit; in particular, the use of handshake lines for flow control is not reliably implemented in many devices.
* No method is specified for sending power to a device. While a small amount of current can be extracted from the DTR and RTS lines, this is only suitable for low power devices such as mice.
* The 25-way connector recommended in the standard is large compared to current practice.

9 Pin Serial Port Connector

Connector may be reversed depending on which side is viewed. All pins are numbered.

Pin No. Function
1 DCD (Data Carrier Detect)
2 RX (Receive Data)
3 TX (Transmit Data)
4 DTR (Data Terminal Ready)
5 GND (Signal Ground)
6 DSR (Data Set Ready)
7 RTS (Request To Send)
8 CTS (Clear To Send)
9 RI (Ring Indicator)
Voltage sent over the pins can be in one of two states, On or Off. On (binary value "1") means that the pin is transmitting a signal between -3 and -25 volts, while Off (binary value "0") means that it is transmitting a signal between +3 and +25 volts.
An important aspect of serial communications is the concept of flow control. This is the ability of one device to tell another device to stop sending data for a while. The commands Request to Send (RTS), Clear To Send (CTS), Data Terminal Ready (DTR) and Data Set Ready (DSR) are used to enable flow control.