Micro Projects Home Page


 Microprocessor Basics

Whoops! An alternative use for the blackboard Iím afraid!

On this page we will cover more of the operational information, plus the files you will need to download in order to make your own SITCOM.

Micro Basics

Site Projects

Site Projects

Construction Techniques


Programming concepts




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


Schematic SIT 1

Main SITCOM schematic SIT1.gif

Schematic SIT2

SITCOM Ďglue logicí etc. SIT2.gif

Optional schematic SIT3

*Optional logic derived 100Hz  SIT3.gif

SITCOM core wiring list (if needed)

Optional SITCOM core wiring list**.

Boot8085.bin     The SITCOM BOOT program

Binary Code for the SITCOM BOOT EPROM     boot8085.bin

*Note that the 100Hz signal is only needed for future experiments and NOT in making the SITCOM work initially.

**The core wiring list contains the wiring for a 2764 EPROM rather than the 27256.

SITCOM No.2 and EEPROM Programmer

The second prototype seen working alongside my EEPROM programmer. The EEPROM in the SITCOM had just been programmed from this project by using Sanís BOOT program and his excellent SBASM assembler.

San when busy programming SITCOM


Once the new SITCOM is completed, all the wiring is checked and there are no shorts on the power rails, carefully insert the ICís into their sockets.

When applying power for the first time, check the current consumption. San reckoned his prototype used about 250mA without the interface connected. Mine used just over 200mA with two displays on and a 27C64. Clearly if your SITCOM uses a great deal more than this it may be prudent to switch off and try to ascertain why.

Assuming that all is OK, a correctly booted SITCOM will flash itís SOD LED at a rate of about 0.7Hz with a 50% mark space ratio if you are using the standard 6.144MHz crystal. With an older 8085 using a 3.075MHz crystal, this will be about 0.35Hz - i.e. twice as slow :)   It is important to realise that the value of the crystal determins the baud rate for downloading the program data from the assembler, so anything other than these values is likely to cause difficulties on the comms. front.


If the SOD LED is NOT flashing, try pressing the BOOT button. This is effectively the same as pressing a normal hardware reset button. If you have the two displays connected to your SITCOM, then at BOOT a message will appear on both displays. If nothing still happens, switch off and try to locate the problem. If you are at all unsure about your BOOT prom, might I suggest that you substitute another one containing a short loop to output a value to one of the I/O ports of the LS138? Using something like 3E, 41 ,D3 ,20 ,18 ,FAh,  (from 0000h to 0005h) you can easily test for an output pulse with a logic probe on the LS138 pin 14 in this instance and NOTHING on any other I/O port.


When a program is being downloaded from the Assembler, the SITCOM will automatically see this on itís SID line (connected - hopefully via the transistor shown on SITCOM schematic Page 2 to the correct PC comm port) and the LED will freeze in the ON position. until it is all loaded. At that point the SOD LED will flash with a much shorter pulse. If you have the displays connected, a rotating bar will appear until the program is loaded, then the message OK RESET will replace it. On depressing the RESET button, the SOD LED will stay on constantly and - hopefully - the program will run. And that is it!

Concluding Ideas

Concluding ideas




Ah! That all important task! presumably anyone building a SITCOM will want to use the best assembler around? Which one is that then? Why! The SB ASSEMBLER written by San and available to download FREE from:  www.sbprojects.net.  Compare itís features against the one you have been using and I guarantee youíll never go back to it!

Enough of plugging Sanís program :) so what is necessary to download a development program to a SITCOM then?

In order to send data directly to the project, you have to tell the Assembler to send the results of itís efforts to the COM PORT you are using. If you are using COM 1, then the header at the top of the program would read as in the table to the right::









If using an older 8085 with the 3.075MHz crystal, change the 9600 to 4800. The port is set to COM1 here, but may be changed to any other you  might be using such as COM2


Hopefully we will be adding programs and projects to these pages. If YOU develop a program using SITCOM and would like others to try it, please send it to San and weíll credit you with the work on our NEW projects and programs for SITCOM page.