So far we have introduced RS-232 Communications in relation to the PC. RS-232 communication is asynchronous. That is a clock signal is not sent with the data. Each word is synchronized using it's start bit, and an internal clock on each side, keeps tabs on the timing.

TTL/CMOS Serial Logic Waveform

The diagram above, shows the expected waveform from the UART when using the common 8N1 format. 8N1 signifies 8 Data bits, No Parity and 1 Stop Bit. The RS-232 line, when idle is in the Mark State (Logic 1). A transmission starts with a start bit which is (Logic 0). Then each bit is sent down the line, one at a time. The LSB (Least Significant Bit) is sent first. A Stop Bit (Logic 1) is then appended to the signal to make up the transmission.

The diagram, shows the next bit after the Stop Bit to be Logic 0. This must mean another word is following, and this is it's Start Bit. If there is no more data coming then the receive line will stay in it's idle state(logic 1). We have encountered something called a "Break" Signal. This is when the data line is held in a Logic 0 state for a time long enough to send an entire word. Therefore if you don't put the line back into an idle state, then the receiving end will interpret this as a break signal.

The data sent using this method, is said to be framed. That is the data is framed between a Start and Stop Bit. Should the Stop Bit be received as a Logic 0, then a framing error will occur. This is common, when both sides are communicating at different speeds.

The above diagram is only relevant for the signal immediately at the UART. RS-232 logic levels uses +3 to +25 volts to signify a "Space" (Logic 0) and -3 to -25 volts for a "Mark" (logic 1). Any voltage in between these regions (ie between +3 and -3 Volts) is undefined. Therefore this signal is put through a "RS-232 Level Converter". This is the signal present on the RS-232 Port of your computer, shown below.

 
RS-232 Logic Waveform

The above waveform applies to the Transmit and Receive lines on the RS-232 port. These lines carry serial data, hence the name Serial Port. There are other lines on the RS-232 port which, in essence are Parallel lines. These lines (RTS, CTS, DCD, DSR, DTR, RTS and RI) are also at RS-232 Logic Levels.

Almost all digital devices which we use require either TTL or CMOS logic levels. Therefore the first step to connecting a device to the RS-232 port is to transform the RS-232 levels back into 0 and 5 Volts. As we have already covered, this is done by RS-232 Level Converters.

Two common RS-232 Level Converters are the 1488 RS-232 Driver and the 1489 RS-232 Receiver. Each package contains 4 inverters of the one type, either Drivers or Receivers. The driver requires two supply rails, +7.5 to +15v and -7.5 to -15v. As you could imagine this may pose a problem in many instances where only a single supply of +5V is present. However the advantages of these I.C's are they are cheap.

Another device is the MAX-232. It includes a Charge Pump, which generates +10V and -10V from a single 5v supply. This I.C. also includes two receivers and two transmitters in the same package. This is handy in many cases when you only want to use the Transmit and Receive data Lines. You don't need to use two chips, one for the receive line and one for the transmit. However all this convenience comes at a price, but compared with the price of designing a new power supply it is very cheap.

There are also many variations of these devices. The large value of capacitors are not only bulky, but also expensive. Therefore other devices are available which use smaller capacitors and even some with inbuilt capacitors. (Note : Some MAX-232's can use 1 micro farad Capacitors). However the MAX-232 is the most common, and thus we will use this RS-232 Level Converter in our examples.

In order to do anything useful with our Serially transmitted data, we must convert it back to Parallel. (You could connect an LED to the serial port and watch it flash if you really want too, but it's not extremely useful). This in the past has been done with the use of UART's. However with the popularity of cheap Microcontroller's, these can be more suited to many applications. We will look into the advantages and disadvantages of each method.

 

8250 and Compatible UARTs

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We have already looked at one type of UART, the 8250 and compatibles found in your PC. These devices have configuration registers accessible via the data and address buses which have to be initialized before use. This is not a problem if your device which you are building uses a Microprocessor. However if you are making a stand alone device, how are you going to initialize it?

Most Microprocessors / Microcontrollers these days can be brought with build-in Serial Communication Interfaces (SCI). Therefore there is little need to connect a 40 pin 16550 to, for example a 68HC11 when you can buy one built in. If you are still in love with the Z-80 or 8086 then an 16550 may be option! (or if you are like myself, the higher chip count the better. After all it looks more complicated and impressive! - But a headache to debug!)

Pin Diagrams for 16550, 16450 & 8250 UARTs

Pin Assignments for 16550A UART CDP6402, AY-5-1015 / D36402R-9 etc UARTs

Pin Description for CDP6402, AY-5-1015 / D36402R-9 and compatible UART's

However one disadvantage of these chips over the 8250's is that these UART's have no inbuilt Programmable Baud Rate Generator, and no facility to connect a crystal directly to it. While there are Baud Rate Generator Chips such as the AY-5-8116, a more cheaper (and common) alternative is the 74HC4060 14-bit Binary Counter and Oscillator.

The 74HC4060, being a 14 bit binary counter/divider only has outputs for some of it's stages. Only Q4 to Q14 is available for use as they have external connections. This means higher Baud Rates are not obtainable from common crystals, such as the 1.8432 Mhz and 2.4576 Mhz. The UART requires a clock rate 16 times higher than the Baud Rate you will be using. eg A baud rate of 9600 BPS requires a input clock frequency of 153.6 Khz.

The 1.8432 Mhz crystal gives some unfamiliar Baud Rates. While many of these won't be accepted by terminal programs or some hardware, they are still acceptable if you write your own serial programs. For example the PC's baud rate divisor for 7200 BPS is 16, 3600 BPS is 32, 1800 BPS is 64 etc. If you require higher speeds, then it is possible to connect the UART to the OUT2 pin. This connection utilizes the oscillator, but has no frequency division applied. Using OUT2 with a 1.8432 Mhz crystal connected gives a baud rate of 115,200 BPS. The CMOS CDP6402 UART can handle up to 200 KBPS at 5 volts, however your MAX-232 may be limited to 120 KBPS, but is still within range.