Computer acquisition of data from nonaddressable switches

In a computer system in which address lines and data lines are utilized to interface bus devices to the computer, data is provided to the computer from nonaddressable switches by connecting at least a portion of the data lines to ground through a pull-down resistor and to a positive voltage through a switch and a pull-up resistor. Depending upon the position of the switches associated with each data line, each data line will be pulled to a high logic level or a low logic level when the data lines are in the tristate high impedance mode. The computer can read the contents of the switches even though the switches have no associated address decoding or bus driving circuitry.

This invention relates to method and apparatus for transferring information 
to a computer. In a specific aspect this invention relates to method and 
apparatus for transferring information to a computer from switches which 
have no associated address decoding circuitry or bus driving circuitry. 
In a typical computer system, an address bus and a data bus is utilized to 
interface a computer to bus devices such as memory or input/output 
devices. In such computer systems, it is also typical to use various types 
of switches such as thumb wheel, rotary or push button to transfer 
information to the computer. These switches are typically utilized to 
allow operator interface with the computer. Generally, the use of switches 
to transfer information to the computer requires that the particular 
switch must be addressed and this address must be decoded to enable a 
particular switch. Once the particular switch is enabled the data 
programmed on the switch is transferred through bus driving circuitry to 
the data bus and thus to the computer. Thus, in addition to the switches, 
address decoding circuitry and bus driving circuitry are generally 
required to enable the transfer of data from a switch to the computer via 
the data bus. 
In any system, cost considerations are important. Computer based systems 
are no exception. While it is generally necessary to use switches to allow 
operator intervention in a computer system, it would be desirable to be 
able to transfer information from switches without the need for address 
decoding circuitry or bus driving circuitry. It is thus an object of this 
invention to provide method and apparatus for transferring information to 
a computer from switches which have no associated address decoding 
circuitry or bus driving circuitry. 
In accordance with the present invention, method and apparatus is provided 
whereby at least a portion of the data lines in a computer system are 
electrically connected to ground through pull-down resistors and are 
electrically connected to a positive voltage supply through pull-up 
resistors. A switch is electrically connected between each of the pull-up 
resistors and the respective data lines electrically connected to the 
pull-up resistors. When it is desired to transfer data from the switches 
to the computer, the computer provides an address to the bus devices which 
is not decoded by any of the bus devices. When this condition occurs, the 
data bus lines will be in a tristate or high impedance mode. This will 
allow the data bus lines to be pulled to a high logic level or a low logic 
level depending upon the position of the switches associated with each of 
the pull-up resistors. Thus, the computer can read the contents of each 
switch (the position of each switch) even though the switch has no address 
decoding or bus driving circuitry. This results in a considerable cost 
savings for computer based systems which utilize switches to allow 
operator intervention in the computer system.

The invention is described in terms of a particular computer system which 
has eight data lines with each of the eight data lines being associated 
with a switch. However, the invention is applicable to other computer 
systems having more or less data lines and is also applicable to the use 
of only one data line to transfer information from a switch to the 
computer. 
Referring now to the drawing, there is illustrated a very basic 
conventional computer system. A computer 11 is electrically connected to a 
plurality of bus devices which include the random access memory 12, the 
read only memory 13 and the input/output devices 14 through the A0-A15 
address lines and the DO-D7 data lines. Preferably, the computer is a 6800 
microprocessor manufactured by Motorola Semiconductor. The bus devices are 
devices which may be interfaced to the 6800 microprocessor. Typically, the 
computer 11 addresses a particular bus device to enable that particular 
bus device. The information contained in that particular device is then 
transferred to the computer via the D0-D7 data lines. In a conventional 
computer system, this addressing would also be required for the switches 
which are utilized to allow an operator to transfer information to the 
computer. As has been previously stated, this address would require 
decoding and the response of the switches would require bus driving 
circuitry in order to transfer information to the computer. 
In the present invention, the plurality of switches 21-28 are utilized to 
allow an operator to transfer information to the computer. Since there are 
eight data lines, eight bits of information can be transferred to the 
computer using the switches 21-28. If it was desired to transfer only one 
bit of information to the computer, only one switch, electrically 
connected to one data line, would be required. The D7 data line is 
electrically connected to ground through the pull-down resistor 31 and is 
also electrically connected to the plus five volt power supply 32 through 
the switch 21 and the pull-up resistor 33. The D6 data line is 
electrically connected to ground through the pull-down resistor 35 and is 
electrically connected to the plus five volt power supply 32 through 
switch 22 and the pull-up resistor 36. The D5 data line is electrically 
connected to ground through the pull-down resistor 38 and is also 
electrically connected to the plus five volt power supply 32 through the 
switch 23 and the pull-up resistor 39. The D4 data line is electrically 
connected to ground through the pull-down resistor 41 and is also 
electrically connected to the plus five volt power supply 32 through the 
switch 24 and the pull-up resistor 42. The D3 data line is electrically 
connected to ground through the resistor 44 and is also electrically 
connected to the plus five volt power supply 32 through the switch 25 and 
the pull-up resistor 45. The D2 data line is electrically connected to 
ground through the pull-down resistor 48 and is also electrically 
connected to the plus five volt power supply 32 through the switch 26 and 
the pull-up resistor 49. The D1 data line is electrically connected to 
ground through the pull-down resistor 51 and is also electrically 
connected to the plus five volt power supply 32 through switch 27 and the 
pull-up resistor 52. The D0 data line is electrically connected to ground 
through the resistor 54 and is also electrically connected to the plus 
five volt power supply 32 through switch 28 and the resistor 55. 
When it is desired to transfer information from the switches 21-28 to the 
computer 11, the computer 11 provides an address which is not decoded by 
any of the bus devices. When this condition occurs, the D0-D7 data lines 
will be in a tristate or high impedance mode. Depending upon the position 
of the switches 21-28, the D0-D7 data lines will be pulled to a low logic 
level or a high logic level. For the 6800 microprocessor, if the switches 
21-28 are open then the corresponding data lines will be pulled to a 
voltage level which has a maximum of 0.8 volts by the pull-down resistors 
which are electrically connected to ground. If the switches 21-28 are 
closed then the respective data lines will be pulled to a voltage level 
which has a minimum of 2 volts by the pull-up resistors which are 
electrically connected to the plus five volt power supply 32. In this 
manner, eight bits of information may be transferred from the switches 
21-28 to the computer 11 without the need for any address decoding or bus 
driving circuitry. 
The presence of the pull-up resistors, pull-down resistors and the switches 
in the data lines do not effect the transfer of data from the bus devices 
to the computer 11 because the bus devices each contain drivers which are 
active devices. The level of the voltage on the data lines may be effected 
but the effect will not be sufficient to change the data transferred from 
the bus devices to the computer. 
In the present invention, the preferred value of the pull-up resistors is 8 
K ohms. The preferred value of the pull-down resistors is 100 K ohms. 
Obviously, other resistance values could be utilized so long as the high 
logic level exceeds the threshold level for the particular computer system 
and the low logic level does not exceed the low logic threshold for the 
particular computer system. The resistance value of the pull-up resistors 
and pull-down resistors will typically be a function of the number of bus 
devices in the computer system which are electrically connected to the 
data lines. 
While the invention has been described in terms of the presently preferred 
embodiment, reasonable variations and modifications are possible by those 
skilled in the art, within the scope of the described invention and the 
appended claims.