7.3 SERIAL COMMUNICATIONS

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Serial communication channels send data one bit at a time.

These techniques are often slower, but are essential when transmitting using radio waves, telephone lines, or similar media.

Serial lines have become one of the most common methods for transmitting data to instruments, and most personal computers now have two serial ports.

Serial data can be transmitted two ways,

synchronous - all of the data bits are timed between sender and receiver.

asynchronous - the data bits arrive unannounced, without any coordination between sender and receiver. (this is the most common)

RS-232/RS-232C - the most common standard using voltage levels

RS-422 - a current loop standard well suited to noisy conditions

RS-485 -

A typical data byte looks like the one below. The voltage/current on the line is turned on/off. The width of the bits determines the possible bits per second (bps). The value shown before is used to transmit a single byte.

The bits shown correspond to,

before - this is a period where no bit is being sent and the line is high

start - a single bit to help get the systems synchronized

data - this byte could be 7 or 8 bits, but is almost always 8 now. The value shown here is 01001000 binary.

parity - this lets us check to see if the byte was sent properly. The most common choices here are no parity bit (just drop it out), or even parity, or odd parity. In this case there are two bits set. If we are using even parity the bit would be set. If we are using odd parity the bit would be off

stop - the stop bits allow form a natural pause at the end of the data.

after - a period of time where the line is high before the next byte.

7.3.1 RS-232

This standard is based on a high being +3 to +15V, and an off being -3 to -15V (+/-12V is commonly used).

The typical connection scheme is shown below. notice that the lines are crossed between connectors. Also note that in some cases the dsr/dtr lines are not used.

The line names are,

TXD/RXD - (transmit data, receive data) these lines pass data one way from sender to receiver.

DSR/DTR - (data set ready, data terminal ready) these handshaking lines indicate when the remote machine is ready to receive data.

COM - a common ground to keep things from blowing up, etc.

A simpler wire is also used for some devices (in low noise situations) called a null modem cable. In this cable only the data and common lines are used.

There are typically two connectors used for these devices. (Note: the connector have very fine numbers on them)

The main pin assignments for the DB-25 are,

2 - TXD

3 - RXD

6 - DSR

7 - COM

20 - DTR

The main pin assignments for the DB-25 are,

2 - TXD

3 - RXD

- COM

- DSR

- DTR