Problems with mice!

 Not the furry four legged type, but the sort with the "ball sticking out their bottom" variety (as one learned PC treatise describes the popular pointing device).
Lately I've been plagued by mouse difficulties. A common problem is when a machine is booting up and a rectangular window appears with the dreaded information that a mouse has not been detected. This despite the fact that the computer was working fine a few minutes before it was re-booted.
Nowadays mice are fitted with the small "PS2" plug rather than the older 9-pin canon style and usually a new mouse will be supplied with an adaptor which converts the PS2 plug into a canon socket.
The thing to remember above all is that the PS2 mouse requires a power supply of nominally 5 volts and if a plug is fitted the wrong way round in a PS2 socket and any attempt is made to force it to fit then a mini-disaster may occur if the computer is running or, at that time even though shut down and plugged into the mains. Windows does warn of this in the rectangular box when it cautions you to switch off before plugging in your mouse if it's the PS2 type.
Unlike "AT" PCs, "ATX" machines normally have power applied to the motherboard even when the computer is ostensibly not running. Some motherboards have a red LED fitted to indicate this but as it's fitted directly to the board one cannot see it when the case is in place. On these ATX motherboards there is usually a tiny soldered-in fuse protecting the computer from a short circuit at a PS2 port but this fuse is not easy to replace and even when this has been done it's not guaranteed that a motherboard will work normally ever again.
Anyway it's a fact that a newer type of PS2 mouse needs to be supplied with power but a 9-pin canon connector wired as a standard COM1 or COM2 type of serial port does not carry power as such. Admittedly if one were to check the pins with a voltmeter one would observe various voltages present but this is basically a by-product of the circuitry connected to the plug. This circuitry is usually carried on the motherboard although in older computers is often carried on the hard drive interface card which is plugged into an ISA socket. This circuitry can supply a limited amount of power by virtue of the driving capability of the transistors connected to a particular pin but this may be of the order of only a few milliamps.
So..if your new PS2 mouse is connected to a computer it will need a certain amount of power in order to work. If an adaptor is used to allow a mouse to operate into the serial port, adequate power may or may not be within the capability of the circuitry wired to the 9-pin canon connector. If the current demand is too high the voltage at the connector pin will drop below the threshold required to make the mouse work properly and if it's a marginal situation then sometimes the mouse will work and sometimes it will not.
In the case of a PS2 mouse connected to a modern PS2 connector there won't be a problem as that mode of working is designed to provide power to the mouse.
So re-capping….if the power requirement of the mouse is within the capability of the canon connector all is well. If the power requirement is just a little bit too high the mouse will intermittently not be detected and if the power requirement is definitely too high the mouse will never be detected. A different computer will almost certainly have a circuit using alternative components connected to the serial port and this may or may not have the same power capability. This explains why a mouse will work on one computer but not another.
A mouse fitted solely with a canon connector should always work when plugged into a 9-pin connector as the power equation will have been worked out with that method of connection in mind. A mouse fitted with an adaptor converting its PS2 format into the canon format may or may not work. because the designers may not have done their job properly when it comes to downwards compatibility.
There are, however, potential problems other than those associated with power. Unlike DOS, if Windows can't find a PS2 mouse, it has always required a "serial" mouse to be connected to either COM1 or COM2. Ideally COM1 should be the first choice as there won't then be any DOS/Windows interoperability problems.
Unfortunately the intricate workings of modern PCs still hark back to the earliest types. Each device connected to a PC needs to be assigned an address from where data supplied by the device can be read and data intended for the device can be written. To let the PC know that the device needs attention requires a signal called an "Interrupt" or in PC parlance an "IRQ". As there were only a limited number of IRQs available on early PCs, and this part of the concept has changed little, there is sometimes a problem connecting more than a handful of devices to a modern PC. To help alleviate the situation COM1 shares its IRQ with COM4 and COM2 with COM3. If a mouse happens to be configured on a port which shares its IRQ with something else all sorts of weird things can happen (including the PC crashing when the mouse is moved). A PS2 mouse gets round this because it uses a dedicated connection to the PC thus avoiding any IRQ upsets. The key here then is to always use the PS2 connection if it's available.
Finally a word on smooth mouse operation. Treat a mouse as a mini vacuum cleaner and like a vacuum cleaner it needs emptying. In a traditionally designed mouse the ball picks up fluff and carries it to the rollers. There are two long rollers, one for forward motion and the other for sideways motion, each carrying at its end a disk with holes in it. Associated with each disk is an LED which shines light through the holes to a sensor where circuitry determines mouse movement in each of the two planes.
Over a period fluff gets wrapped around the rollers making the ball movement rough. There is also a small stabilising wheel which gets coated in hardened fluff. In really bad cases fluff gets wedged in the optical parts interfering with the way the LED and its sensor work.. Usually you can remove the fluff by detaching the small plate on the underside and after removing the ball, using a fingernail or a moistened cotton bud clean away the debris. If you are faced with a mouse full of fluff you'll have to remove the screws securing the top and bottom plastic housing, split it open, and do the job thoroughly. Note first how the parts clip together and how the cable is routed so you can get it back together. You'll be surprised how smoothly the mouse operates after a Spring clean.


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