WM5LL writes "<p align="center"><big><big>The Magic of Serial Comunication</big></big></p> I've been tossing around the idea of writing an article on serial ports for quite some time now. No, this is not going to be about the orifice that you put your breakfast food into every morning. This is going to be about the little 9 pin port (sometimes 25 pin) on the back of your PC and, if it's new enough, your rig and any other piece of HAM equipment that supports serial communication. I don't have a clue how this is going to turn out at this point. One thing that I can be sure of, however, is that it will, in some places, be incredibly boring to most people. I don't believe that a good article on serial communications could exclude the boring parts and be effective, however. I am by no means an expert on the subject of serial communication though I do make use of it in my work. I have not tried to cover serial communication in the PIC (or other) microcontrollers. That is outside of my knowledgebase (though I am studying). I've borrowed heavily from "Serial Port Complete" by Jan Axelson and numerous tech articles that I use at work to be able to write this article. I hope that you like it.</p> For the purposes of this article, I am going to stick with just two of the serial protocols that are available. RS-232 and RS-485. RS-232 is for shorter, slower links and RS-485 is for longer, faster links. RS-485 also has multidrop capabilities. Ie. You can connect more than two devices (nodes) to an RS-485 network. </p> None of this information is required to be able to use a serial link but it can be useful for design work or troubleshooting. Toward the end, I will list a connection diagrams. That should be somewhat useful.</p> <h4>Formats and Protocols</h4> The devices in a serial link may be different from one another, but all of the devices must agree on conventions and rules for the data that they exchange. This section will introduce the data formats and protocols used in serial links. </p> <h5>Sending Serial Data</h5> In a serial link, the transmitter, or driver, sends bits one at a time, in sequence. Each transmitter may have it's own separate communication path or each transmitter may be required to take turns. When there are three or more devices on the network all three usually share a path and a network protocol determines when each can transmit.</p> All serial links require a clock, or timing reference, to control data flow. The transmitter and the receiver use a clock to decide when to send and when to receive data. The two types of serial-data formats that we will concern ourselves with in this article are synchronous and asynchronous. Each uses the clocks differently.</p> <h5>Synchronous Format</h5> In a synchronous transmission, all devices use a common clock generated by one of the devices or an external source. The clock may toggle at irregular intervals or have a fixed frequency. All transmitted bits are synchronized to the clock. Each transmitted bit is valid at a defined time after a clock transition, rising or falling edge, cycle. The receiver uses the clock cycle transitions to decide when to read each incoming bit. The ecaxt details of the protocol can vary. Synchronous formats use a variety of ways to signal the start and end of a transmission, including Start and Stop bits and dedicated chip-select signals. Because of the need to transmit a clock signal, synchronous formats are sueful for short links, cables 15 feet or less or even circuit board component to circuit board component.</p> <h5>Asynchronous Format</h5> Synchronous transmisssions don't use a clock line. Each end of the link has it's own clock. Each end must agree on the clock's frequency and all of the clocks must match within a few percent of one another. Each transmitted byte includes a Start bit to synch the clocks, and one or more Stop bits to signal the end of a transmitted word. Typical devices that use asynchronous formats to communicate are modems and PC's. </p> One of the more common formats for asynchronous transmission is 8-N-1, where the transmitter sends each data byte as 1 Start bit, then 8 Data bits, beginning with bit 0 (the LSB, or least significant bit), and ending with 1 Stop bit.</p> The N in 8-N-1 indicates that the transmission does not use a parity bit. Parity bits are used, in other formats, as a simple method of error checking. Parity can be Odd, Even, Mark, or Space. RTTY operators may recognize the terms MArk and Space.With Even parity, the parity bit is set or cleared so that the data bits plus the parity bit contain an even number of 1's. With odd Parity so that the opposite is true (data plus parity equal an odd number of 1's). It's important to know that data is transmitted on serial line by alternating voltage from high or 1 to Low or 0. The voltages are low and noise can be a problem. Lets say that you send a data packet that looks like this 00011001. With Even parity, the Parity bit is 1 since there are 3 1's in the data packet, which is an odd number. When you add the 1 from the Parity bit you get Even parity. The receiver, if set up correctly, will have the same Parity setting and it will expect data + Parity to equal Even. Any added signal, noise, should change the bit pattern therefore triggering error correction algorithms in the hardware or software where applicable.</p> Mark and Space are forms of Stick parity: with Mark parity, the parity bit is always 1, and with Space parity it is always 0. These are less useful as error indicators, but one use for them is in 9-bit networks. These networks use a parity bit to indicate whether a byte contains an address or data.</p> A link's bit rate is the number of bits per second transmitted or received per unit of time, usually expressed as bits per second (bps). Baud rate is the nuber of possible events or data transitions, per second. The two values are often identical because in many links each transmission period represent a new bit. High-speed modems, over phone lines, use phase shifts and other tricks ot encode multiple bits in each data period, so the baud rate is actually much lower than the bit rate.</p> All of the bits required to transmit a value, from Stop to Start bit, form a word. The data bits in a word form a character. In some links the characters are actually text characters (letters and numbers). In other links the character values are binary and have nothing to do with text. For instance, the CAT protocol used by my Yaesu FT-847 uses 1 Start bit, 8 Data bits, No Parity bit, and two Stop bits. All of the commands sent from the computer to the receiver consist of five-byte blocks, with 200ms (milliseconds) between each byte. The last byte in each block is the instruction opcode, while the first four bytes of each block are arguements (either parameters for that instruction, or dummy values required to pad the block out to five bytes). As an example, let's say that we want to set the VFO frequency to 439.70 Mhz. Set Frequency is Opcode 0A (set by the P1 command byte). Placing the opcode in the 5th data bit position, we would then enter the frequency into the first four data bit positions. It looks like this (hexadecimal format):</p> <div align="center"><center> <table border="1" width="100%"> <tr> <td width="20%" align="center">Data 1</td> <td width="20%" align="center">Data 2</td> <td width="20%" align="center">Data 3</td> <td width="20%" align="center">Data 4</td> <td width="20%" align="center">Data 5</td> </tr> <tr> <td width="20%" align="center">00</td> <td width="20%" align="center">00</td> <td width="20%" align="center">97</td> <td width="20%" align="center">43</td> <td width="20%" align="center">0A</td> </tr> </table> </center></div><div align="center"><center> <table border="1" width="100%"> <tr> <td width="80%" align="center">Parameters</td> <td width="20%" align="center">OpCode</td> </tr> </table> </center></div> <p align="left">In this example the Start bit is sent, then Data 1, then Data 2, etc, etc. until the OpCode is sent. The receiver sees the OpCode and follows the instruction.</p> <p align="left">The CAT DATA BYTE FORMAT looks like this:</p> <div align="center"><center> <table border="1" width="100%"> <tr> <td width="9%" align="center">Start Bit</td> <td width="9%" align="center">Bit 0</td> <td width="9%" align="center">Bit 1</td> <td width="9%" align="center">Bit 2</td> <td width="9%" align="center">Bit 3</td> <td width="9%" align="center">Bit 4</td> <td width="9%" align="center">Bit 5</td> <td width="9%" align="center">Bit 6</td> <td width="9%" align="center">Bit 7</td> <td width="9%" align="center">Stop Bit</td> <td width="10%" align="center">Stop Bit</td> </tr> </table> </center></div> The CAT 5-BYTE COMMAND STRUCTURE looks like this:</p> <div align="center"><center> <table border="1" width="100%"> <tr> <td width="20%" align="center">L.S.D. Parameter 4</td> <td width="20%" align="center">Parameter 3</td> <td width="20%" align="center">Parameter 2</td> <td width="20%" align="center">Parameter 1</td> <td width="20%" align="center">M.S.D. Command</td> </tr> </table> </center></div> Let me save you some time, 97h and 43h when converted to decimal, do not resemble 439.7 Mhz.</p> The number of characters transmitted per second equals the bit rate times the number of bits in a word. Adding one Startbit and one Stop bit to a byte increases the transmission time of each byte by 25 percent (because there are 10 bits per byte instead of 8). With 8-N-1 format, a byte transmits at 1/10th the bit rate: a 9600 bits-per-second link transmits 960 bytes per second.</p> If the receiver requires a little extra time to accept received data, the transmitter may stretch the Stop bit width to 1.5 or 2 bits. The original purpose of using a longer Stop bit was to allow time for mechanical teletype machines to settle to an idle state.</p> <h4>Transmitting a Byte</h4> <h5>The Bit Format</h5> A transmitters output is at logic state 1 when it is idle. The output sends a logic 0 for the length of one bit to signal the beginning of a transmission. This is the Start bit. At 300 bps (bits-per-second), a bit is 3.3 milliseconds, while at 9600 bps, it is 0.1 millisecond. The Start bit synchronizes the transmit and receive clocks at the beginning of each new word that is transmitted.</p> At the receiving end, the transition from logic 1 to the Start bits logic 0 signals that a byte is arriving and determines the timing for detecting the following bits. The receiver measures the logic state of each bit near the middle of the the bit. This is done so that the receiver reads the bits correctly even if the respective clocks do not exactly match. More on clocks in a minute.</p> After the Start bit, the transmitter sends the 8 data bits in sequence with bit 0, the LSB (least-significant bit). Then the transmitter sends a logic 1, which functions as the Stop bit. The output reains at logic 1 for at least the width of one bit. Immediatly following this, or at any time after, the transmitter may send a new Start bit 0 to announce the beginning of a new byte.</p> Some interfaces, RS232 for instance, use inverted voltages from those listed above. In that case, the Stop bit is logic state 0 (negative voltage) and the Start bit is positive.</p> The serial port in a PC is an asynchronous port controlled by a UART. The UART (universal asynchronous receiver / transmitter) is responsible for performing the main task in serial communications with computers. The device changes incomming parallel information to serial data which can be sent on a communication line. A second UART can be used to receive the information. The UART performs all the tasks, timing, parity checking, etc. needed for the communication. The only extra devices attached are line driver chips capable of transforming the TTL level signals to line voltages and vice versa. An entire article on the UART could be written. The UART typically uses a receive clock frequency of 16 times the bit frequency. If the data rate is 300 bps then the clock must be 4800 bps. After detecting the transition that signals a Start bit, the UART waits 16 clock cycles (1 bit) for the Start bit to end, then waits 8 more cycles to read bit 0 in the middle of the bit. It then reads each of the following bits 16 clock cycles after the previous one.</p> If the transmit and receive clocks do not exactly match, the receiver will read each successive bit closer and closer to an edge of the bit. To read all of the bits in a 10-bit word correctly, the transmit and receive clocks should vary no more than 3 percent. Any more difference than this, and the timing may be off by so much that it will read bits either before they have begun or after they have finished. The clocks only need to stay in synch for the length of one word since each word begins a new Start bit the resynchronizes the clocks. I've said this twice in this section. It's an important concept.</p> Because accurate timing is very important, asynchronous interfaces require a stable timing reference. Most are controlled by a crystal or ceramic resonator. The frequency of the reference should allow even division by the frequencies the receive clocks use for standard bit rates. In PC's, the standard UART clock frequency is 1843.2 kHz. Division by 16 gives 115,200, which is the top bit rate that the UART supports.</p> <h4>Data Formats</h4> Data bits in a serial transmission may represent anything at all. Sensor readings, error codes, status information, text messages or device commands and responses. The information may be encoded as binary or text data.</p> <h4>Binary Data</h4> With binary data, the receiver interprets a received byte as a binary number with a value from 0 to 255. The bits are conventionally numbered 0 through 7, with each bit representing the bit's value (o or 1) multiplied by a power of 2. For example: <ul> [*]Bit 0 = BitValue * (2^0) [*]Bit 1 = BitValue * (2^1) [*]Bit 2 = BitValue * (2^2) [/list] A byte of 1111 1111 translates to 255, or FFh (the small h indicates that the number is hexadecimal format), and 0001 0001 translates to 17 or 11h. In asynchronous links, bit 0, the least-significant-bit (LSB) arrives first. If you are viewing the data on an oscilloscope or logic analyzer, remember to reverse the order when translating to conventional notation of most-significant-bit (MSB) first.</p> <h4>Text Data</h4> Binary data works fine most of the time. Sometimes you need to send messages or files containg text. For that reason it is also possible to send binary data encoded as text.</p> To send text, the program uses a code that assigns a numeric value to each text. There are several coding conventions. The most common coding convention is ASCII (American Standard Code for Information Exchange), which consists of 128 codes and requires only 7 data bits. An 8th bit, if used, may be 0 or a parity bit. ANSI (American National Standards Institute) uses 256 codes, with the higher codes representing special chaacters. Many of the higher codes of the ASCII text used in the original IBM PC were used by DOS programs for line and box drawing characters. These were used to add simple graphics to text screens and printouts.</p> The Unicode standard uses 16 bits per character and allows 65,536 different characters. This standard supports hundreds of additional alphabets. DBCS (Double-Byte Character Set is an earlier standard tha supports many Asian languages.</p> <h5>ASCII Hex</h5> You can use text to transfer binary data by expressing the data in ASCII Hex format. Each byte is represented by a pair of ASCII codes that represents the bytes two hexadecimal characters.This format can represent any value using only ASCII codes 30h to 39h (for 0 thru 9) and 41h to 46h (for A thru F).</p> Instead of sending a byte to represent a value from 0 - 255, the sending device sends two, one for each character in the hex number that represents the byte. The receiving computer treats the values like ordinary text. After the computer receives the values, it can process the data any way that is necessary, including converting it back to binary data.</p> For example, consider the decimal number</p> 163</p> Expressed as a binary number, it is</p> 1010 0011</p> In hexadecimal it is</p> A3h</p> The ASCII codes for A and 3 are</p> 41h and 33h</p> So the binary representation of this value in ASCII hex consists of these two bytes</p> 01000001 00110011</p> A serial link that uses ASCII hex format would send the value A3h by transmitting these two bytes. An obvious downside to using ASCII hex is that each data byte requires two characters, so data takes twice as long to transfer. ASCII hex has it's uses though. For one, it frees all of the other codes for other uses, like handshaling or an end-of-file indicator. It also allows protocols that only support 7 data bits to transmit a numeric value. I see this quite a bit in my work.</p> I'm going to end this article with two RS232 Pin function charts. The first is a 25-pin configuration and the second is the 9-pin configuration. I will continue with preventing missed data (Handshaking, buffers, polling an interrupts, acknowledgements, and error checking) and more info specific to RS2323 and RS485 at a later time if this article generates sufficient interest. Pins marked with a * represent signals that are included in a PC interface. DTE stands for Data Terminal Equipment and DCE stands for Data Circuit-terminating equipment. It doesn't matter which device in a link is the DTE or DCE, but each link must have one of each. In a PC / Modem application the DTE is the PC and the modem is the DCE. All of the signal names are from the perspective of the DTE. For example, TD is an output on a DTE and an input on an DCE. </p> <div align="center"><center> <table border="1" width="73%" height="623"> <tr> <td width="9%" align="center" height="17">Pin Number</td> <td width="9%" align="center" height="17">Circuit Name</td> <td width="12%" align="center" height="17">Popular Name</td> <td width="8%" align="center" height="17">Source</td> <td width="8%" align="center" height="17">Type</td> <td width="54%" align="center" height="17">Description</td> </tr> <tr> <td width="9%" align="center" height="18">1</td> <td width="9%" align="center" height="18">Shield</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">-</td> <td width="8%" align="center" height="18">-</td> <td width="54%" align="left" height="18">-</td> </tr> <tr> <td width="9%" align="center" height="18">2*</td> <td width="9%" align="center" height="18">BA</td> <td width="12%" align="center" height="18">TD</td> <td width="8%" align="center" height="18">DTE</td> <td width="8%" align="center" height="18">data</td> <td width="54%" align="left" height="18">Transmitted data</td> </tr> <tr> <td width="9%" align="center" height="18">3*</td> <td width="9%" align="center" height="18">BB</td> <td width="12%" align="center" height="18">RD</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">data</td> <td width="54%" align="left" height="18">Received data</td> </tr> <tr> <td width="9%" align="center" height="18">4*</td> <td width="9%" align="center" height="18">CA/CJ</td> <td width="12%" align="center" height="18">RTS</td> <td width="8%" align="center" height="18">DTE</td> <td width="8%" align="center" height="18">control</td> <td width="54%" align="left" height="18">Request to Send / Ready to Send</td> </tr> <tr> <td width="9%" align="center" height="18">5*</td> <td width="9%" align="center" height="18">CB</td> <td width="12%" align="center" height="18">CTS</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">control</td> <td width="54%" align="left" height="18">Clear to Send</td> </tr> <tr> <td width="9%" align="center" height="18">6*</td> <td width="9%" align="center" height="18">CC</td> <td width="12%" align="center" height="18">DSR</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">control</td> <td width="54%" align="left" height="18">DCE (Data Set) Ready</td> </tr> <tr> <td width="9%" align="center" height="18">7*</td> <td width="9%" align="center" height="18">AB</td> <td width="12%" align="center" height="18">SG</td> <td width="8%" align="center" height="18">-</td> <td width="8%" align="center" height="18">common</td> <td width="54%" align="left" height="18">Signal Common</td> </tr> <tr> <td width="9%" align="center" height="18">8*</td> <td width="9%" align="center" height="18">CF</td> <td width="12%" align="center" height="18">CD</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">control</td> <td width="54%" align="left" height="18">Received Line Signal Indicator (Carrier Detect)</td> </tr> <tr> <td width="9%" align="center" height="18">9</td> <td width="9%" align="center" height="18">-</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">-</td> <td width="8%" align="center" height="18">-</td> <td width="54%" align="left" height="18">Reserved for Testing</td> </tr> <tr> <td width="9%" align="center" height="18">10</td> <td width="9%" align="center" height="18">-</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">-</td> <td width="8%" align="center" height="18">-</td> <td width="54%" align="left" height="18">Reserved for Testing</td> </tr> <tr> <td width="9%" align="center" height="18">11</td> <td width="9%" align="center" height="18">-</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">-</td> <td width="8%" align="center" height="18">-</td> <td width="54%" align="left" height="18">Unassigned</td> </tr> <tr> <td width="9%" align="center" height="18">12</td> <td width="9%" align="center" height="18">SCF/CI</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">control</td> <td width="54%" align="left" height="18">Secondary Received Line, Signal Detector/data signal rate selector</td> </tr> <tr> <td width="9%" align="center" height="18">13</td> <td width="9%" align="center" height="18">SCB</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">DTE</td> <td width="8%" align="center" height="18">control</td> <td width="54%" align="left" height="18">Secondary Clear to Send</td> </tr> <tr> <td width="9%" align="center" height="18">14</td> <td width="9%" align="center" height="18">SBA</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">data</td> <td width="54%" align="left" height="18">Secondary Transmitted Data</td> </tr> <tr> <td width="9%" align="center" height="18">15</td> <td width="9%" align="center" height="18">DB</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">timing</td> <td width="54%" align="left" height="18">Transmitter Signal Element Timing</td> </tr> <tr> <td width="9%" align="center" height="18">16</td> <td width="9%" align="center" height="18">SBB</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">data</td> <td width="54%" align="left" height="18">Secondary Received Data</td> </tr> <tr> <td width="9%" align="center" height="18">17</td> <td width="9%" align="center" height="18">DD</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">timing</td> <td width="54%" align="left" height="18">Receiver Signal Element Timing</td> </tr> <tr> <td width="9%" align="center" height="18">18</td> <td width="9%" align="center" height="18">LL</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">DTE</td> <td width="8%" align="center" height="18">control</td> <td width="54%" align="left" height="18">Local Loopback</td> </tr> <tr> <td width="9%" align="center" height="18">19</td> <td width="9%" align="center" height="18">SCA</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">DTE</td> <td width="8%" align="center" height="18">control</td> <td width="54%" align="left" height="18">Secondary Request to Send</td> </tr> <tr> <td width="9%" align="center" height="18">20*</td> <td width="9%" align="center" height="18">CD</td> <td width="12%" align="center" height="18">DTR</td> <td width="8%" align="center" height="18">DTE</td> <td width="8%" align="center" height="18">control</td> <td width="54%" align="left" height="18">DTE Ready</td> </tr> <tr> <td width="9%" align="center" height="18">21</td> <td width="9%" align="center" height="18">RL/CG</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">DTE/DCE</td> <td width="8%" align="center" height="18">control</td> <td width="54%" align="left" height="18">Remote Loopback / Signal Quality Detector</td> </tr> <tr> <td width="9%" align="center" height="18">22*</td> <td width="9%" align="center" height="18">CE/CK</td> <td width="12%" align="center" height="18">RI</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">control</td> <td width="54%" align="left" height="18">Ring Indicator / Received Energy Present</td> </tr> <tr> <td width="9%" align="center" height="18">23</td> <td width="9%" align="center" height="18">CH/CI</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">DTE/DCE</td> <td width="8%" align="center" height="18">control</td> <td width="54%" align="left" height="18">Data Signal Rate Selector</td> </tr> <tr> <td width="9%" align="center" height="18">24</td> <td width="9%" align="center" height="18">DA</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">DTE</td> <td width="8%" align="center" height="18">timing</td> <td width="54%" align="left" height="18">Transmit Signal Element Timing</td> </tr> <tr> <td width="9%" align="center" height="18">25</td> <td width="9%" align="center" height="18">TM</td> <td width="12%" align="center" height="18">-</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">control</td> <td width="54%" align="left" height="18">Test Mode</td> </tr> </table> </center></div> 9 pin functions-</p> <div align="center"><center> <table border="1" width="73%"> <tr> <td width="9%" align="center" height="17">9 Pin Number</td> <td width="9%" align="center" height="17">25 Pin Number</td> <td width="12%" align="center" height="17">Signal</td> <td width="8%" align="center" height="17">Source</td> <td width="8%" align="center" height="17">Type</td> <td width="54%" align="center" height="17">Description</td> </tr> <tr> <td width="9%" align="center" height="18">1</td> <td width="9%" align="center" height="18">8</td> <td width="12%" align="center" height="18">CD</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">Control</td> <td width="54%" align="left" height="18">Carrier Detect</td> </tr> <tr> <td width="9%" align="center" height="18">2*</td> <td width="9%" align="center" height="18">3</td> <td width="12%" align="center" height="18">RD</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">Data</td> <td width="54%" align="left" height="18">Received data</td> </tr> <tr> <td width="9%" align="center" height="18">3*</td> <td width="9%" align="center" height="18">2</td> <td width="12%" align="center" height="18">TD</td> <td width="8%" align="center" height="18">DTE</td> <td width="8%" align="center" height="18">Data</td> <td width="54%" align="left" height="18">Transmitted data</td> </tr> <tr> <td width="9%" align="center" height="18">4*</td> <td width="9%" align="center" height="18">20</td> <td width="12%" align="center" height="18">DTR</td> <td width="8%" align="center" height="18">DTE</td> <td width="8%" align="center" height="18">Control</td> <td width="54%" align="left" height="18">Data Terminal Ready</td> </tr> <tr> <td width="9%" align="center" height="18">5*</td> <td width="9%" align="center" height="18">7</td> <td width="12%" align="center" height="18">GND</td> <td width="8%" align="center" height="18">-</td> <td width="8%" align="center" height="18">-</td> <td width="54%" align="left" height="18">Signal Ground</td> </tr> <tr> <td width="9%" align="center" height="18">6*</td> <td width="9%" align="center" height="18">6</td> <td width="12%" align="center" height="18">DSR</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">Control</td> <td width="54%" align="left" height="18">Data set ready</td> </tr> <tr> <td width="9%" align="center" height="18">7*</td> <td width="9%" align="center" height="18">4</td> <td width="12%" align="center" height="18">RTS</td> <td width="8%" align="center" height="18">DTE</td> <td width="8%" align="center" height="18">Control</td> <td width="54%" align="left" height="18">Request to send</td> </tr> <tr> <td width="9%" align="center" height="18">8*</td> <td width="9%" align="center" height="18">5</td> <td width="12%" align="center" height="18">CTS</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">Control</td> <td width="54%" align="left" height="18">Clear to send</td> </tr> <tr> <td width="9%" align="center" height="18">9</td> <td width="9%" align="center" height="18">22</td> <td width="12%" align="center" height="18">RI</td> <td width="8%" align="center" height="18">DCE</td> <td width="8%" align="center" height="18">Control</td> <td width="54%" align="left" height="18">Ring Indicator</td> </tr> </table> </center></div> 73</p> </p> Mike, WM5LL</p> Email: WM5LL@ARRL.net</p>"
