Eight Bit Standard Connector Bus for Sixteen Bit Microcomputer Using Mirrored Memory Boards

This invention relates to microcomputer systems, and more particularly to the use of a sixteen bit microcomputer with standard eight bit bus arrangement and memory boards.

One of the most favorable factors in the development of the home or personal computer industry is de facto standardization of the bus arrangement used to connect one printed circuit board to another. This standard, which is fairly widely accepted, is the "Altair" or "S-100" bus used in the original home computer kit, the Altair 8800. The S-100 bus contains one hundred conductors, many of which are not used or not assigned at present. Certain pins or conductors are designated for power supplies, clocks and ground, others for various interrupts and controls such as WAIT, INTERRUPT ACKNOWLEDGE, MEMORY WRITE, HOLD, etc. Sixteen pins are designated for the memory address, allowing 64K bytes of memory to be directly accessed. Separate unidirectional eight bit data pins are provided, eight Data In pins and eight Data Out pins. This bus system was built for the 8080 eight bit microprocessor, which is the most widely used at present, but it can be used with other eight bit microprocessor parts. Several manufacturers of microcomputers use the S-100 bus, and dozens of manufacturers of microcomputer boards use the S- 100 to make hundreds of standard parts such as various types of memory boards as well as processor boards.

The Proceedings of the IEEE, February 1978, p. 117, in an article entitled "Low-Cost Microcomputing: The Personal Computer and Single-Board Revolutions", states:

"The Altair 8800 and S100 Bus: The original MITS Altair 8800 kit . . . featured . . . 8080 central processing unit (CPU) . . . and a 100 line bus (known as the Altair or `S100" bus). The bus allows expansion memory and peripherals to be connected. Expansion kits included additional memory, real-time clock, and vectored interrupt cards 11.

"The 100-line Altair bus has been widely copied. The bus data width is 8 bits, with separate lines for input and output. 65,536 bytes of memory may be addressed. Fourteen lines remain unused, and extensions have been proposed . . . Dozens of manufacturers offer Altair plug-compatible modules including READ-WRITE RAM, READ-ONLY memory (ROM) and combination RAM/ROM cards. Compatible EPROM programmers are available for the ultraviolet-Erasable, reProgrammable Read Only Memories (EPROM's) used for nonvolatile storage. S100 interface cards will drive black-and-white video displays, color video displays, magnetic cassette tape controllers, real-time clocks, and parallel and serial digital input/output (I/O). There are Altair-compatible controllers for flexible disks such as the mini-floppy (approximately 64K bytes), standard floppy (256 K bytes), and even for Calcomp Trident hard disks (80 000 K bytes). Besides digital-to-analog and analog-to-digital converters, there are S100-compatible speech and music synthesizers, modems for communication via telephones, video camera controllers, and kits for experimenting with speech recognition."

Sixteen bit microprocessors employ a sixteen bit bidirectional data bus which obviously is not directly compatible with the two eight bit unidirectional data bus sets in the S-100. A sixteen bit processor of course uses a sixteen bit instruction word which is of many times more capability than an eight bit instruction word. Arithmetic operations can be performed in sixteen bit machines in from one third to one tenth the instruction words needed in eight bit machines. The cost of generating software is directly related to the number of statements or lines of code needed, so if the number of statements needed is halved, the cost is likewise halved. Further, sixteen bit processors often contain additional features such as more flexible addressing modes and more interrupt capability. The TMS9900 sixteen bit processor manufactured by the assignee hereof, Texas Instruments, has the added features of hardware multiply and divide instructions, multiple sets of sixteen general purpose registers and context switching which allows rapid interrupt response and user extension of the hardware instruction set. And so, many designers seek to employ sixteen bit processors in place of the older but widely accepted eight bit processors.

Various interconnect arrangements have been proposed for interfacing a sixteen bit processor with the eight bit S-100 bus. Certainly, additional ones of the unused pins could be employed, producing two sixteen bit unidirectional buses. This would not be desirable because the system would not be compatible with the many available memory boards now on the market, and further a user could not use any of his existing boards if he wished to upgrade to a sixteen bit system. Compatibility can be achieved by forcing the sixteen bit processor to handle data input/output to the memory boards in eight bit bytes instead of sixteen bit words, but this sacrifices much of the advantage of using a sixteen bit processor in the first place, since each memory access is twice as long. See Journal of West Coast Computer Faire, March 1978, p. 394-401, and BYTE Magazine, March 1978, p. 148.

It is the principal object of this invention to provide a microcomputer system wherein the board-to-board connectors employ a standardized bus which has a word length of fewer bits than that of the data bus of the processor chip.

Another object is to provide an interconnection arrangement wherein a sixteen bit microprocessor can be used with existing eight bit memory boards having a standardized bus, particularly without sacrificing the speed inherent in a more powerful processor.

A microcomputer system uses a standardized S-100 bus with eight-bit Data In and eight-bit Data Out lines, but has a sixteen bit microprocessor with sixteen bit bidirectional data input terminals. An arrangement is provided to cross-connect the Data In lines and Data Out lines for one memory board compared to another, these two memory boards being accessed by the same address. A processor board containing the microprocessor connects the Data In and Data Out lines to the data input/output terminals by controllable unidirectional buffers which criss-cross the bytes on write compared to read operations.

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to the detailed description which follows, read in conjunction with the accompanying drawing, wherein:

The single FIGURE is an electrical diagram represented in block form showing a microcomputer system according to the invention.

In a normal S-100 bus system operating with an 8080 processor, the data bus is two 8-bit unidirectional buses, Data In and Data Out. During a Write cycle eight bits of data are sent out from the processor on the Data Out lines, and the data is written into the appropriate eight memory cells at the byte selected by the address bus. When a Read cycle is performed the data is sent out from the memory board to the processor on the Data In bus. So, during any given Read or Write cycle only one bus is used, either Data In or Data Out, with eight lines being wasted. A single eight-bit bidirectional bus could have been used, since only one set of tristate devices is ever active on the bus at any one time. Data is written to the memory on one set of lines, and read back from memory on a different set of lines.

Referring now to the FIGURE, a modification of an S-100 bus system is illustrated to make use of the fact that the Data In or Data Out buses are not ever in use at the same time. The system includes a processor board 10, an S-100 motherboard or connector board 11, and standard memory boards 12 and 13. The motherboard 11 is a printed circuit board with one hundred conductor lines or traces running parallel along its length, with a number of slots or connectors 14 into which are plugged the pins of the various boards such as the processor and memory boards, using conventional edge connectors. A typical size for small microcomputers is twelve of the slots 14. The traces include an eight bit Data Out bus 15 and an eight bit Data In bus 16, along with an address bus 17, power supply and clock lines 18, and a large number of control lines 19 and other lines not pertinent to this invention. To provide sixteen bit operation, a standard memory card 12 is plugged into a slot 14 on the left, and another "mirror image" memory card 13 is plugged into a slot 14 on the right. The cards 12 and 13 are set to select for the same address space. This modification of one of the memory boards by transposing the unidirectional buffers is all that is required to use S-100 boards in a sixteen bit system.

If the CPU chip 25 puts a write command on the bus, and the sixteen bits of data on the bidirectional data output pins 26 on the chip and appropriate pins on the board 10, then each of the memory cards 12 and 13 will store one of the bytes of data. Unidirectional buffers 27 in each of the eight Data In lines on the memory card 12 will allow the data on the lines 15 to be applied to the "D" inputs 28 of the memory chips in an array 29 of memory devices on the board 12. Likewise, unidirectional buffers 30 on the mirror image memory board 13 allow the data from lines 16 to be applied to the data input lines 31 in the array 32 of memory chips. The unidirectional buffers 27 and 30 would be actuated to allow data on the bus 15 and bus 16 to be applied to the inputs 28 and 31 only when a WRITE command is applied to the control lines 19 of the bus, along with board select or chip select when a large number of pairs of memory boards or chips is used. For a Read operation, the output 33 of the array 29 on the board 12 is connected by eight unidirectional buffers 34 to the Data In lines 16, while the output 35 from the mirror memory array 32 is connected by eight unidirectional buffers 36 to the Data Out lines 15. The sets of unidirectional buffers 34 and 36 are controlled by a READ command on control lines 19, as well as board or chip select. Thus, if the CPU 25 puts a WRITE command on the control lines 19 and drives sixteen bits of data onto the bus lines 15 and 16, each of the memories 29 and 32 will store one of the bytes of data. But when a READ cycle is performed by putting a READ command on the lines 19 and activating unidirectional buffers 34 and 36, the data will return on the opposite bus lines. This byte swap could be reconciled by the CPU, by appropriate programming, but it can be eliminated so that the operation of the system will be faster. The unidirectional buffers ordinarily used on the processor card are wired slightly differently to eliminate the byte swap. For WRITE, high order byte is connected by eight lines 37 and eight controllable unidirectional buffers 38 to the pins going to the lines 15, and likewise the low order byte is connected by eight lines 39 and eight controllable unidirectional buffers 40 to pins going to the bus lines 16. For READ, the pins for the bus 15 are connected by eight unidirectional buffers 41 to the lines 39, rather than the lines 37 as in conventional sixteen bit processor boards. Likewise, the pins for the bus 16 are connected by eight unidirectional buffers 42 to the lines 37. This transpostion or criss-cross eliminates the byte swap. The unidirectional buffers 38, 40, 41 and 42 are controlled by WRITE and READ commands from the CPU 25 in conventional manner.

The system thus far described provides operation with a sixteen bit CPU in an eight bit S-100 bus, but a more practical product would have both standard and mirrored buffers on the mirrored memory card 13, allowing its use on either a sixteen bit system or an eight bit S-100 bus. Thus, eight unidirectional buffers 45 transpose the output 35 to the pins going to the lines 16, and eight unidirectional buffers 46 transpose the lines from the pins connected to the bus 15 to the input 31 to the memory array 32. A switch or jumper on the memory board 13 would select the normal or mirror mode by connecting the READ or WRITE commands and board select controls to the appropriate ones of the controllable unidirectional buffers. That is, for normal operation, the buffers 30 and 36 would never be operative, and for mirror operation, the buffers 45 and 46 would never be operative.

The decision of whether to use the mirror memory boards of this invention or the cutting and criss-cross of the motherboard as in my copending application Ser. No. 898,736, now U.S. Pat. No. 4,214,302, depends upon whether the user is upgrading an existing system, or creating a new one. The user with a large investment in existing S-100 memory and peripheral cards would be inclined to modify (or purchase pre-modified) an S-100 bus motherboard. The designer of a new system would tend to purchase mirror cards and use an unmodified bus. It should be noted that modification of the bus motherboard does not preclude the use of the older eight bit processor card. If the user wants to revert to the eight bit bus and CPU card, he merely changes the address select switches on the LOW Byte cards, and moves them to a HIGH Byte slot.

While this invention has been described with reference to an illustrative embodiment, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiment, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.