This project is this basic board used in many of the control systems in my house. It is old technology at this point, but is still doing exactly what it was designed to do almost 25 years ago. There has been no reason to change since the required functions remain the same. It consists of the microprocessor, memory, and support components needed to construct an 8 bit microcomputer for general purposes. The plans here cover only the processor board; no I/O or other boards are included. As built, this board includes a decoder for 16 Input and 16 Output ports, All are an active low level.
The Data Bus and Address Bus are fully decoded and available as an output from this board. The board itself has locations for two 2732 (4Kx8) EPROMS and a 6116 SRAM. These are assigned memory addresses; 0XXX, 1XXX, and 2XXX by default. Other address locations may be assigned off-card using the Address and Control Busses.
The control Bus contains 4 functions, I/O Read and Write, and Memory Read and Write. These are active low and may be used in conjunction with the address and data bus by an external I/O or Memory device.
There are several input leads to this card as well. The Reset lead is intended to provide a means to reset the CPU from a remote point. This lead is normally held high, a reset is accomplished by momentarily grounding this point. There are two leads marked as "Spare". These are brought to several 8085 control input leads that are jumpered to either ground or +5 Volts on the card. Should you desire to use one of these functions, cut the appropriate trace on the card and connect a jumper to the Spare input. Consult the 8085 manual for details on how to use the function you've selected. The Hold pin will stop the CPU at the conclusion of its current instruction anytime the lead goes high. The Hold Acknowledge will go low when Hold is accomplished. This will also force the data bus and address bus into its high impedance state (tri-stated). Operation of the card requires the Hold Input be grounded or low for the card to work. Consult the 8085 manual for details.
All functions on this card are accomplished using a single 5 Volt power supply rated at 1 amp. This regulated supply is not shown and is located off-card. It is permissible to use the same supply for other external applications, however it must be well regulated and free of any noise spikes or other anomalies.
The schematic of the 8085 Microprocessor Card: