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 Microprocessor Basics

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8085 page

8085 page

Fault Finding

Fault Finding

Data Sheets

Data Sheets

EEprom Programmer


Video Information

VIDEO info

Peripheral Circuitry

Peripheral circuitry

Central Heating and Z280's

Z280 and  Central Heating  Controllers

Concluding Ideas

Concluding ideas




The ‘SITCOM’ project was designed to offer an easy to build training computer, for anybody interesting in constructing microprocessor based projects. Designed around the 8085 microprocessor (in my experience the micro. most site enquiries are asking after) all of the other components should be easy to obtain, the largest chip being the ever-popular, easy to program 8255 parallel I/O device.

San and Izabella

What makes the ‘SITCOM’ so different from any of the ‘competition’, is that once built and tested, any development programs are downloaded via a simple serial link into the 32K of RAM. By using a straight-forward but ingenious bit of logic, the RAM is then switched into location 0000h, thus allowing the micro to run as if it were loaded into the EPROM.  OK enough talk and onto the details!

Introducing San

SAN designed SITCOM, wrote the software for the BOOT prom AND built the first prototype! I built the second prototype and tried a number of small alterations to circuit component values in order to check that the project had no excuse for not working when someone else copies it.

As to how the next person might like to build it, this is entirely up to them. The first two prototypes were constructed in a similar manner, but there is no reason why other techniques might not be successfully tried.

Although this is meant to be a ‘preliminary’ project page, rest assured that any subsequent re-vamps will (hopefully) not alter any details shown here. One of the first ‘improvements’ will probably be to substitute proper CAD type schematics.

First and foremost we will cover the building of  SITCOM prototype number 2.     I will not dwell too long on this side of things as if you wish to study the techniques any closer, might I suggest that you take a look at the fully documented Site Project 1 (The Digital clock).

As I am often asked just how long a project should take to build, for your information, I timed myself for both preparing the board on Day 1, then the wiring up on Day 2.

Nothing but a collection of parts and a wiring list at 11:49 Day 1.

SM Decoupling cap positioning

A good way to save a little space is to use surface mount decoupling caps. Quite simple for little fingers... (Day 1)

The time is now nearly 5:00 PM and it has therfore taken just over 5 hours to sort out and mount the hardware, the decoupling caps and the power wiring. (including a break or two for refreshment of course!)

First step - fit the rubber feet, the two switches and the power bits.

Wiring combs and power to the IC’s shown sorted end of Day 1


It took me about 4 hours to complete the wiring.

If using one of the ‘eurocard’ type of boards, this is how I arranged my sockets to maximise the space useage. Big down point is identifying Pin 1’s. Thus I ‘blacked’ out every pad immediately to the right of each Pin 1.  (Day 1)

Wiring complete, less that needed for the 100Hz source and the 40 way header. (Day 2)

Close-up of wiring around the RAM

A close-up of prototype number 2 with most IC’s in their sockets. Please note that the two DL1414 displays are NOT needed to successfully complete the SITCOM project. They simply give a few more experimentation options IF you have one or two handy in the spares box.

Izabella's project works!

As mentioned above, whilst those two DL1414 displays are very useful, they are NOT essential, as all the SITCOM’s ‘programming’ states may be identified by the single SOD LED, shown on my version immediately to the right of  the blue BOOT button. After switch on, this LED will flash slowly, with other faster flashing rates enabled for detecting downloading program data and error conditions.