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+ +
+ +
+
+
+ Source: http://www.Computer-Engineering.org
+ Author: Adam Chapweske
+ + Last Updated: 05/09/03
+
+
+ Legal Information:
+
+ All information within this article is provided "as is" and without +any express or implied warranties, including, without limitation, the implied + warranties of merchantibility and fitness for a particular purpose.  
+
+ + This article is protected under copyright law.  This document may + be copied only if the source, author, date, and legal information is included.
+
+ Abstract: +

This document descibes the interface used by the PS/2 mouse, PS/2 keyboard, + and AT keyboard.  I'll cover the physical and electrical interface, + as well as the protocol.  If you need higher-level information, such + as commands, data packet formats, or other information specific to the keyboard + or mouse, I have written separate documents for the two devices: +

+ +
The PS/2 (AT) Keyboard Interface + +
+ The PS/2 Mouse Interface
+ The Physical Interface:
+ +

The physical PS/2 port is one of two styles of connectors:  The + 5-pin DIN or the 6-pin mini-DIN.  Both connectors are completely +(electrically) similar; the only practical difference between the two is + the arrangement of pins.  This means the two types of connectors can + easily be changed with simple hard-wired adaptors.  These cost about + $6 each or you can make your own by matching the pins on any two connectors.  + The DIN standard was created by the German Standardization Organization + (Deutsches Institut fuer Norm) .  Their website is at http://www.din.de (this site is +in German, but most of their pages are also available in English.) + +

+ +

PC keyboards use either a 6-pin mini-DIN or a 5-pin DIN connector.  + If your keyboard has a 6-pin mini-DIN and your computer has a 5-pin DIN + (or visa versa), the two can be made compatible with the adaptors described + above.  Keyboards with the 6-pin mini-DIN are often referred to as + "PS/2" keyboards, while those with the 5-pin DIN are called "AT" devices + ("XT" keyboards also used the 5-pin DIN, but they are quite old and haven't + been made for many years.)  All modern keyboards built for the PC +are either PS/2, AT, or USB.  This document does not apply +to USB devices, which use a completely different interface.

+ +

Mice come in a number of shapes and sizes (and interfaces.)  The + most popular type is probably the PS/2 mouse, with USB mice gaining popularity.  + Just a few years ago, serial mice were also quite popular, but the computer + industry is abandoning them in support of USB and PS/2 devices.  This + document applies only to PS/2 mice.  If you want to interface a serial + or USB mouse, there's plenty of information available elsewhere on + the web.
+ +
+ The cable connecting the keyboard/mouse to the computer is usually +about six feet long and consists of four to six 26 AWG wires surrounded +by a thin layer of mylar foil sheilding.  If you need a longer cable, +you can buy PS/2 extenstion cables from most consumer electronics stores. + You should not connect multiple extension cables together.  If +you need a 30-foot keyboard cable, buy a 30-foot keyboard cable.  Do +not simply connect five 6-foot cables together.  Doing so could result +in poor communication between the keyboard/mouse and the host.
+

+ +

As a side note, there is one other type of connector you may run into + on keyboards. While most keyboard cables are hard-wired to the keyboard, + there are some whose cable is not permanently attached and come as a separate + component.  These cables have a DIN connector on one end (the end + that connects to the computer) and a SDL (Sheilded Data Link) connector +on the keyboard end.  SDL was created by a company called "AMP."  + + This connector is somewhat similar to a telephone connector in that it +has wires and springs rather than pins, and a clip holds it in place.  +If you need more information on this connector, you might be able to find +it on AMP's website at http://www.connect.amp.com.  Don't confuse the SDL +connector with the USB connector--they probably both look similar in my +diagram below, but they are actually very different.  Keep in mind +that the SDL connector has springs and moving parts, while the USB connector +does not.

+ +

The pinouts for each connector are shown below:
+   + + + + + + + + + + + + + +
+ +
Male
+ +
+ (Plug)
+ 		+ +
Female 
+ +
+ (Socket)
+ 		5-pin DIN (AT/XT): 
+ + 1 - Clock
+ 2 - Data
+ 3 - Not Implemented
+ 4 - Ground
+ 5 - Vcc (+5V)
+
+   + + + + + + + + + + +
+ +
Male
+ + +
+ (Plug)
+ 		+ +
Female
+ +
+ (Socket)
+ + 		6-pin Mini-DIN (PS/2):
+ 1 - Data
+ 2 - Not Implemented
+ 3 - Ground
+ 4 - Vcc (+5V)
+ + 5 - Clock
+ 6 - Not Implemented
+
+   + + + + + + + + + + + + +
+ +
+
+ 		+ +
+
+ 		6-pin SDL:
+ + A - Not Implemented
+ B - Data
+ C - Ground
+ D - Clock
+ E - Vcc (+5V)
+ F - Not Implemented
+

+ +

+ +


+ The Electrical Interface:
+

+ + +

Note:  Throughout this document, I will use the more general term + "host" to refer to the computer--or whatever the keyboard/mouse is connected + to-- and the term "device" will refer to the keyboard/mouse.

+ + +

Vcc/Ground provide power to the keyboard/mouse.  The keyboard or + mouse should not draw more than 275 mA from the host and care must be taken + to avoid transient surges.  Such surges can be caused by "hot-plugging" + a keyboard/mouse (ie, connect/disconnect the device while the computer's + power is on.)  Older motherboards had a surface-mounted fuse protecting + the keyboard and mouse ports.  When this fuse blew, the motherboard + was useless to the consumer, and non-fixable to the average technician. + Most newer motherboards use auto-reset "Poly" fuses that go a long +way to remedy this problem.  However, this is not a standard and +there's still plenty of older motherboards in use.  Therefore, I +recommend against hot-plugging a PS/2 mouse or keyboard.
+

+ + +
+

Summary: Power Specifications
+ Vcc = +4.5V to +5.5V.  
+ Max Current = 275 mA.
+

+
+ +

The Data and Clock lines are both open-collector with pullup resistors + to Vcc.  An "open-collector" interface has two possible state: low, + or high impedance.  In the "low" state, a transistor pulls the line + to ground level.  In the "high impedance" state, the interface acts + as an open circuit and doesn't drive the line low or high. Furthermore, +a "pullup" resistor is connected between the bus and Vcc so the bus is pulled + high if none of the devices on the bus are actively pulling it low.  The + exact value of this resistor isn't too important (1~10 kOhms); larger resistances + result in less power consumption and smaller resistances result in a faster + rise time.  A general open-collector interface is shown below:
+ +

+ +
+

Figure 1: General open-collector interface.  Data + and Clock are read on the microcontroller's pins A and B, respectively. + Both lines are normally held at +5V, but can be pulled to ground by + asserting logic "1" on C and D.  As a result, Data equals D, inverted, + and Clock equals C, inverted.
+

+
+ +
+

+
+ +

+
+ +


+ Note: When looking through examples on this website, you'll notice +I use a few tricks when implementing an open-collector interface with +PIC microcontrollers.  I use the same pin for both input and output, +and I enable the PIC's internal pullup resistors rather than using external + resistors.  A line is pulled to ground by setting the corresponding + pin to output, and writing a "zero" to that port.  The line is set +to the "high impedance" state by setting the pin to input.  Taking +into account the PIC's built-in protection diodes and sufficient current +sinking, I think this is a valid configuration.  Let me know if your +experiences have proved otherwise.
+
+ Communication: General Description
+ +

+ +

The PS/2 mouse and keyboard implement a bidirectional synchronous serial + protocol.  The bus is "idle" when both lines are high (open-collector). +  This is the only state where the keyboard/mouse is allowed begin + transmitting data.  The host has ultimate control over the bus and + may inhibit communication at any time by pulling the Clock line low.  
+

+ +

The device always generates the clock signal.  If the host wants +to send data, it must first inhibit communication from the device by pulling + Clock low.  The host then pulls Data low and releases Clock.  This + is the "Request-to-Send" state and signals the device to start generating + clock pulses.
+ +

+ +
+

Summary: Bus States
+ Data = high, Clock = high:  Idle state.
+ Data = high, Clock = low:  Communication Inhibited.
+ Data = low, Clock = high:  Host Request-to-Send

+ +
+   All data is transmitted one byte at a time and each byte is +sent in a frame consisting of 11-12 bits.  These bits are: + + + + +

The parity bit is set if there is an even number of 1's in the data bits + and reset (0) if there is an odd number of 1's in the data bits.  + The number of 1's in the data bits plus the parity bit always add up to +an odd number (odd parity.)  This is used for error detection.  The + keyboard/mouse must check this bit and if incorrect it should respond + as if it had received an invalid command.
+

+ +

Data sent from the device to the host is read on the falling edge +of the clock signal; data sent from the host to the device is read on the + rising edge.  The clock frequency must be in the +range 10 - 16.7 kHz.  This means clock must be high for 30 - 50 microseconds +and low for 30 - 50 microseconds..  If you're designing a keyboard, +mouse, or host emulator, you should modify/sample the Data line in the +middle of each cell.  I.e.  15 - 25 microseconds after the appropriate +clock transition.  Again, the keyboard/mouse always generates the clock +signal, but the host always has ultimate control over communication. +

+ + +

+ Timing is absolutely crucial.  Every time quantity I give +in this article must be followed exactly.
+
+ Communication: Device-to-Host
+ +

The Data and Clock lines are both open collector.  A resistor is +connected between each line and +5V, so the idle state of the bus is high. + When the keyboard or mouse wants to send information, it first checks +the Clock line to make sure it's at a high logic level.  If it's not, + the host is inhibiting communication and the device must buffer any to-be-sent + data until the host releases Clock.  The Clock line must be continuously + high for at least 50 microseconds before the device can begin to transmit + its data. 

+ + +

As I mentioned in the previous section, the keyboard and mouse use a +serial protocol with 11-bit frames.  These bits are:

+ + + The keyboard/mouse writes a bit on the Data line when Clock + is high, and it is read by the host when Clock is low.  Figures 2 +and 3 illustrate this.
+ + +

Figure 2:  Device-to-host communication.  + The Data line changes state when Clock is high and that data is valid + when Clock is low.
+

+ +
+
+ +

+ + +

Figure 3:  Scan code for the "Q" key (15h) being +sent from a keyboard to the computer.  Channel A is the Clock signal; +channel B is the Data signal.

+ +
--- +
+
+ +

The clock frequency is 10-16.7 kHz.  The time from the rising + edge of a clock pulse to a Data transition must be at least 5 microseconds.  + The time from a data transition to the falling edge of a clock pulse +must be at least 5 microseconds and no greater than 25 microseconds.  + +
+

+ +

The host may inhibit communication at any time by pulling the Clock + line low for at least 100 microseconds.  If a transmission is inhibited + before the 11th clock pulse, the device must abort the current transmission + and prepare to retransmit the current "chunk" of data when host releases + Clock.  A "chunk" of data could be a make code, break code, device + ID, mouse movement packet, etc.  For example, if a keyboard is interrupted + while sending the second byte of a two-byte break code, it will need to + retransmit both bytes of that break code, not just the one that was interrupted.
+

+ +

If the host pulls clock low before the first high-to-low clock transition, + or after the falling edge of the last clock pulse, the keyboard/mouse +does not need to retransmit any data.  However, if new data is created +that needs to be transmitted, it will have to be buffered until the host +releases Clock.  Keyboards have a 16-byte buffer for this purpose. + If more than 16 bytes worth of keystrokes occur, further keystrokes +will be ignored until there's room in the buffer.  Mice only store +the most current movement packet for transmission.

+ + + +

Host-to-Device Communication:
+

+ +

The packet is sent a little differently in host-to-device communication... +

+ +

First of all, the PS/2 device always generates the clock signal.  + If the host wants to send data, it must first put the Clock and Data +lines in a "Request-to-send" state as follows:

+ + + The device should check for this state at intervals not to exceed + 10 milliseconds.  When the device detects this state, it will begin + generating Clock signals and clock in eight data bits and one stop bit.  + The host changes the Data line only when the Clock line is low, +and data is read by the device when Clock is high.  This is opposite +of what occours in device-to-host communication. + +

After the stop bit is received, the device will acknowledge the received + byte by bringing the Data line low and generating one last clock pulse.  + If the host does not release the Data line after the 11th clock pulse, the + device will continue to generate clock pulses until the the Data line is +released (the device will then generate an error.)

+ + +

The host may abort transmission at time before the 11th clock pulse + (acknowledge bit) by holding Clock low for at least 100 microseconds. +

+ +

To make this process a little easier to understand, here's the steps + the host must follow to send data to a PS/2 device:

+ +
1)   Bring the Clock line low for at least 100 microseconds. +
+ 2)   Bring the Data line low.
+ 3)   Release the Clock line.
+ + 4)   Wait for the device to bring the Clock line low. +
+ 5)   Set/reset the Data line to send the first data bit +
+ 6)   Wait for the device to bring Clock high.
+ 7)   Wait for the device to bring Clock low.
+ + 8)   Repeat steps 5-7 for the other seven data bits and + the parity bit
+ 9)   Release the Data line.
+ 10) Wait for the device to bring Data low.
+ 11) Wait for the device to bring Clock  low.
+ + 12) Wait for the device to release Data and Clock
+ +


+ Figure 3 shows this graphically and +Figure 4 separates the timing to show which signals are generated by the +host, and which are generated by the PS/2 device.  Notice the change +in timing for the "ack" bit--the data transition occours when the Clock + line is high (rather than when it is low as is the case for the other +11 bits.)

+ +

Figure 3:  Host-to-Device Communication. +
+ + +

+ +

Figure 4:  Detailed host-to-device communication. +
+ +
+  

+ + +

Referring to Figure 4, there's two time quantities the host looks for. +  (a) is the time it takes the device to begin generating clock pulses + after the host initially takes the Clock line low, which must be no greater + than 15 ms. (b) is the time it takes for the  packet to be sent, which + must be no greater than 2ms.  If either of these time limits is not +met, the host should generate an error.  Immediately after the "ack" +is received, the host may bring the Clock line low to inhibit communication + while it processes data.  If the command sent by the host requires +a response, that response must be received no later than 20 ms after the +host releases the Clock line.  If this does not happen, the host generates + an error.

+ +
+
+
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