4.3 Digital Communication

Transcription

1 Data Acquisition & Computer Control Digital Communication Why digital communication (often termed digital input/output, or DIO)? Because the language of computers (and many peripheral devices) is digital and digital communication is extremely robust (even if there is an error, error-correction algorithms can be implemented)! Consider the original digital signal below Even with considerable signal attenuation, distortion, or transmission in a noisy environment a digital signal can be recovered. There are two fundamental types of data transmission, parallel and serial. In parallel data communications each bit in a signal, where a bit is a single binary digit (e.g., a 1 or a 0 ), is carried on its own conductor (or pair of conductors) while in serial data communications a single conductor (or pair) is used for all of thedata Logic Before considering types of digital communication we need to discusshowcomputers write a 1 or a 0. Logic refers to a set of digital inputs and/or outputs that follow asetofrules. Therearetwofundamentaltypesoflogic asynchronous logic where a system responds immediately to a change in logic state and synchronous logic where a system waits for some system-clock pulse to check the logic state. An important fundamental question when dealing with digital communicationisdohigh

2 12 Data Acquisition & Computer Control voltages correspond to 1 or do low voltages correspond to 1. When the high voltage range corresponds to 1 and the low voltage range corresponds to 0 we term this positive logic. Alternatively, when the low voltage range corresponds to 1 and the high voltage range corresponds to 0 we term this negative logic. Both are inwideuse. The next question is exactly what voltage ranges correspond to HIGH and LOW. There are many standards defining these ranges. TTL One of the most common digital IO standard is known as TTL or Transistor-Transistor Logic. TTL is a single-ended logic protocol meaning all signals are referenced to a common ground. In TTL signals a voltage between 2.4 and 5.0 volts are considered HIGH and 0.0 to 0.8 volts are considered LOW. TTL can be either positive or negative logic. RS-422/RS-485 RS-422 is one of the more common differential digital IO standards and is sometimes referred to as differential TTL. It is a rigorous standard thatdefinesahisignalasbetween 0.2 and 6.0 volts and a low signal as between -0.2 and -6.0 volts. As it is a differential standard each signal is carried on a line pair and it is important to know which line is which. The pair members may be referred to as the TRUE and the COMPLIMENT or as A and B. In any case the signal is the voltage difference across the pair. When the TRUE or A signal is between 0.2 and 6.0 volts greater than the COM- PLIMENT or B signal the line is in a 0 state which is known as the SPACE or ON state. When the TRUE or A signal is between 0.2 and 6.0 volts less than the COMPLI- MENT or B signal the line is in a 1 state which is known as the MARK or OFF state. RS-485 is very similar to RS-422 in its logic definitions with the exception that it is a

3 Data Acquisition & Computer Control 13 bit more tolerant of a wider range of voltages appearing across any line pair. RS-422 can withstand either A or B being ±7 voltsrelativetoacommonground(e.g.,theground reference for the entire cable - frequently the connector shield or D -ring housing). This level is known as the common mode voltage. RS-485 can withstand a common mode voltage across A or B in the range -7 to +12 volts. RS-232 RS-232 is rigorously defined serial digital protocol (often written with a - letter at the end, e.g., RS-232-E which essentially is the version number Thecurrentversionis EandwillsometimesbereferencedasEIA232EastheEIA,theElectronic Industries Association, maintains the specification). The logic states aredefinedacrossasingleline pair A 0,knownasaSPACE,willbetransmittedasavoltagedifference between +3 and +25 volts. A 1,knownasaMARK,willbetransmittedasavoltagedifference between -3 and -25 volts. The region between +3 and -3 volts is undefined. More on RS-232 shortly. IEEE-1394 IEEE-1394, known as firewire, isahigh-speedserialstandard. Itnowhasthreeflavors: the original 1995 standard, 1394a (2000 standard), and 1394b (2002standard). Thebus can address up to 63 separate devices per controller. As experimentalists we will rarely have to interface IEEE-1394 decives direclty, as these are generally plug-n-play devices. One needs to keep in mind that the IEEE-1394 design specification allows for a maximum cable length of only 4.5m but this can be extended to up to 72m using inexpensive hubs as receivers. The standard 6-pin IEEE-1394 connection supplies power over the cable (up to 1.5A) but laptops, even if they have a 6-pin connection, generallydonotprovide

4 14 Data Acquisition & Computer Control this power and an external powered hub is needed of your device requirespoweroverthe firewire cable. Most recently a 1394c (2006 standard) was published which shifts the port to an RJ45 (ethernet port) but as of yet there is no news of any hardware manufacturer using this standard. USB USB stands for Universal Serial Bus and comes in three flavors: USB1.0, USB1.1 and USB2.0 (USB3.0 now a standard but it won t appear on hardware until 2010). As the name implies USB is a serial bus that allows up to 127 devices per USB host controller. Like firewire, USB decices are plug-n-play and we will rarely need to interface USB devices directly. One thing to keep in mind for high-bandwidth devices is that connection quality and length are important. At high data rates these devices can become negatively impacted by poor data cables and electronically noisy enfironments (large transformers can give off signifant RF signals). In reality this is true of firewire as well. The maximum cable length allowed for in the USB standard is 5.0 m which has to do with the round-trip travel time of the signal. There are some USB repeaters out there to extend this limit but for high-speed (2.0) devices they can by finicky Parallel Data Transmission We used to encounter parallel data transmission regularly inpersonalcomputersasthey all had a printer port, also known as the parallel port, one standard form of parallel transmission. However, with the rise of USB and firewire we have seen the demise of the printer port and standard parallel data transmission in personal computing. These ports were governed by the IEEE 1284 standard that was set in From a computer control point of view an important parallel data transmission protocol is the IEEE-488 standard commonly known as GPIB for General Purpose Interface Bus and sometimes referred to as HPIB for Hewlett Packard Interface Bus (HP was the developer of the original IEEE-488 protocol).

5 Data Acquisition & Computer Control 15 Agenericcharacteristicofallparallelprotocolsistheytransmit at least 8 bits (1 byte) of data simultaneously. Eachbitistransmittedoveradatalinewhichmaybereferenced to acommonground(single-ended) ortoitsownuniqueground(differential). In addition, there are generally control lines and handshaking lines. Control lines are used to enable or disable certain functionality for example to disable any buttonsorswitchesonaunit when the unit is under computer (remote) control. The handshaking lines are used such that the device and the computer know when signals on the data line are valid, when they ve been read, etc. Many instruments work on non-standard parallel digital protocols. For example digital CCD/CMOS cameras used to all have generally the same parallel digitalprotocol(afew still use parallel but many have moved to serial) - data lines for each bit of pixel depth (e.g., an 8-bit camera will have 8 data lines if it is single-ended or 8 pairs if differential) and then three control lines: Pixel Clock, Line Clock, and Frame Enable. The three control lines tell a frame-grabber (a digital data acquisition card) at what rate each pixel value is coming across the data lines (pixel clock), when an individual line of data is coming (line clock), and when an individual frame is coming (frame enable). The exact form of these signals will vary from manufacturer to manufacturer, and even the names, but in general three signals will be provided. While most digital cameras have moved to transmitting the data from camera to host computer via serial interfacestherealityis the camera is still internally supplying the three control signals (Pixel Clock, Line Clock, and Frame Enable) to drive the camera and as experimentalists itisoftenimportantto understand these control lines and their exact timing so that wemaysynchronizelight sources to the camera or potentially other aspects of experimental design Serial Data Transmission RS-232 Of course when dealing with digital data it is most natural to work in parallel data transmission as entire bytes can be sent at a single time (clock cycle). For example the ASCII character set is a set of characters, numbers and symbols as well as control

6 16 Data Acquisition & Computer Control characters (e.g., carriage return, tab) that have been assigned an 8-bit value. Thus our computer keyboards and many other digital devices work with abyteormoreofdataat any instant in time and many of these devices then send and receive their data in parallel. However, it is not always practical to send the data in parallel. Clearly more complex cabling is required as more individual lines must be present in a connector cable. Hence over longer distances serial data transmission may be a better alternative. Have you ever looked at the connector for an older (pre-usb) keyboard? There are only 4 connector pins (and in fact if you look at USB there are 4 connector pins typically as well)! Clearly the keyboard is communicating with the computer in serial as 4pinsonlyprovide2 4 = 16 potential different characters (assuming the cable shield isground) notenoughfor even the lower case characters! It turns out the keyboard cables are a CLOCK, DATA, GND (ground), and +5 Volts. Thus the first process that most be handled in serial communication is the method by which bytes will be broken down into individual bits and shipped out. This is generally handled on each end by a device known as a UART (Universal Asynchronous Receiver/Transmitter). A very common standard UART are the 8250 series of UARTs. With the data broken down to its bits the next issue is whether to send the data synchronously or asynchronously. To send the data synchronously a clock signal must be sent along with the data signals so that the two devices communicating are synchronized. This is the way a keyboard works. Alternatively, the receiving device can always be listening for the next bit which will come at some essentially random time, this is asynchronous communication and is the method used in the de facto standard in serial communication RS-232. Before we take a more detailed look at RS-232 let s define some important terms. frame sequence of bits defining a byte, including start, stop, and error information baud rate is the number of bits per second (including framing and other information) bit time is the reciprocal of the baud rate data rate is the effective number of bits per second

7 Data Acquisition & Computer Control 17 character or frame rate is (baud rate)/(number of bits per frame) We have already described the logic state definitions in RS-232. Let s know take a look at the standard. There are several standard baud rates defined. These include: 300, 600, 1200, 2400, 4800, 9600, 19200, 38400, and Note that only more recent devices may communicate at baud rates above In asynchronous communication an initial timing bit, known as a START bit, is sent to wake-up the listener. In RS-232 the START bit is a single bit andisaspace(logical 0 ). The start bit is followed by either 7 or 8 data bits the number must be established prior to the serial communication and set in each device. The data bits, which may be either MARKs or SPACEs may be followed by an optional PARITY bit. This bit is used to make the total number of MARKs in the data and the PARITY biteitherodd or even depending on how the PARITY was established. The optional PARITY bit is then followed by either one or two MARKs, known as STOP bits. This entire sequence is known as a frame. The line is then held in the MARK state until thenextframeis sent (which will begin with a START bit and hence the line will transition from MARK to SPACE). Some example transmissions are shown below:

8 18 Data Acquisition & Computer Control The most common RS-232 configuration is 8 data bits, no PARITY bits, and 1 STOP bit (10 total bits per byte of data - 1 START bit, a byte of data, and 1 STOP bit). This is often abbreviated 8N1. If no configuration is given this is agoodstartingpointwhen trying to establish communication. In general the most standard baud rate is 9600 baud. For older devices 1200 or even 300 may be the default setting. Each system knows what the baud rate is so the receiving system knowsatwhatrateto expect data but it does not know exactly when the data will come. The START bit tells the receiving system that data is coming. The STOP bit(s) have twopurposes. Oneisto give the UART enough time to convert the serial bits back to a parallel byte. The second is more subtle. The possibility that the sender and the receiver drift a bit in timing with respect to each other exists - this in known as skew. TheSTOPbits,coupledwith the START bit, give the receiver a measure of the apparent baud rate. Ifthesignalis too skewed with respect to the anticipated baud rate a framing errorwillbedeclared. Depending on the RS-232 implementation the systems may be able to re-send the byte. The next bit of complication arrises in defining senders and receivers. Serial devices are split into two categories - DCE (data communications equipment) and DTE (data terminal equipment). We typically think of a computer or terminal as a DTE device and

9 Data Acquisition & Computer Control 19 amodemasadcedevice. However,bothDTEandDCEdevicesarebi-directional. The only difference is across which pins they communicate. Let s consider the most basic RS-232 communication set - a 3 line system: Signal Name Abbreviation 25-Pin D 25-Pin D 9-Pin D 9-Pin D DTE or DTC DTE DCE DTE DCE Transmit Data TxD (TD) Pin 2 Pin 3 Pin 2 Pin 3 Receive Data RxD (RD) Pin 3 Pin 2 Pin 3 Pin 2 Common (Signal Gnd) Com (SG) Pin 7 Pin 7 Pin 5 Pin 5 By convention the names of a lines are based on the DTE side of the line, hence the DTE line emanating from PIN 2 is known as the transmit line. Thus when connecting two DCE or two DTE devices you must use a cable with Pin 2 at one end wired to Pin 3 at the other end and visa-versa. This type of cable is known as a null modem cable. How do you know which pin is which? When looking at the male end connector, the top row is the longer row on the DB connecter (e.g., the top of a DB-9 is the 5-pin row) and pin 1 is the upper left (left-most pin on the 5 pin row). The pins then progress left to right as if reading a book (e.g., pin 6 is the first pin on the second row of pins). On the female connector end it is the mirror image, e.g., you start on the upper right pin (right-most pin on the 5 pin row) and the pins progress right to left. There are other signals defined in the RS-232 protocol and some systemswillrequirethat these signal lines take on particular values. The remaining signal lines are (note these are the standard definitions and are the most common, there are somevariantsoutthere on these lines):

10 20 Data Acquisition & Computer Control Signal Name Abbrev. 25-Pin D 9-Pin D Function Request to send RTS Pin 4 Pin 7 DTE is ready to exchange data Clear to send CTS Pin 5 Pin 8 DCE is ready to exchange data Data set ready DSR Pin 6 Pin 6 DCE is ready to establish link Data terminal ready DTR Pin 20 Pin 4 DTE is ready to establish link Carrier detect CD Pin 8 Pin 1 Active when devices detected Ring indicator RI Pin 20 Pin 9 Active on a ringing signal Flow Control Serial device buffers (location where data is stored before it hasbeenreadormoved elsewhere) tend to be small. Depending on what the devices aredoingthebuffersmay fill up. If this happens the receiving device needs to tell the sending device to stop sending data. Then when the receiving device is ready again it will tell the sending device that it is O.K. to start sending data again. This type of communication is known as flow control. There are two types of flow control software and hardware (actually a third is no flow control which assumes that the communication protocol issorobustthatitisnot necessary, this is frequently the case at low baud rates). Hardware flow control, known as RTS/CTS flow control, takes advantage of two additional control lines beyond the minimum 3 to communicate. The logicalstateofthe RTS and CTS lines are toggled back and forth according to when each device is ready to send and receive data (depending on the status of the buffers). Note that this does not effect the actual data rate and allows all 256 values of a byte to remain available for data transfer. Software flow control, known as Xon/Xoff, uses two characters which are known as Xon and Xoff. Xon is indicated by ASCII character code 17 and Xoff is ASCII character code 19. Thus when a receivers buffer is full it sends an Xoff to the sender and when it is ready it sends an Xon. The advantage of this type of flow control isthatnoadditional lines are needed the disadvantage is that the overall data rate may drop as each Xon and

11 Data Acquisition & Computer Control 21 Xoff character requires additional non-data bits to be sent. Also, this will only work if the communicating devices do not need all of the 256 binary values available. This is the case for most ASCII type communication. However, if binary data is being moved then the software flow control can not be used as the full 256 available values will be needed to transmit the data. You can google the ASCII character set. Codes 031 and 127 are non-printing control characters (such as 10 which is linefeed and 13 which is carriage return or 7 which is bell (make a tone). Note that in base 10 values map to 0-9. You can use the Matlab char command to find mappings. Note thefirstdisplayable charceter starts at 32 and is the space character. You can use: ascii = char(reshape(30:129,10,10) ) To print a table of 10 rows and 10 characters per row of the ASCIIcharactersthatare printable.