One of the SITCOM training computer prototypes
SITCOM 85 Training Computer
 
  In order for anyone to build and program their own �Sitcom�, they will need certain tools and facilities to hand. In the case of such single-use items such as an EEPROM / EPROM programmer, it might be easier to borrow or loan one from work / College / Uni / School / a friend, in order to get the Sitcom up and running. Bear in mind that one of the great features of Sitcom, is that all programs are subsequentially written into the RAM via the PC Assembler, thus the programmer will never be needed for it again!   Sitcom discussion session
 
  1)  
An old 386 PC will be perfect for assembling programs   An MS-DOS or Windows based PC computer with ONE serial port free. As a mouse is not required by the Assembler COM1 may be most convenient. Dust off that old 386 in the attic as here is the perfect use for it!
 
  2)   A simple TEXT EDITOR such as Windows Notepad, or EDIT if using one of the last issues of DOS. Remember that a word processing program like Word or WP will be unsuitable as it will �add� characters to your program listing which will cause serious problems with your Assembler! Other shareware and freeware text editors are available from many sources but not really necessary here.
 
  3)  
An ASSEMBLER. Well, what more is there to add? The Assembler will convert your text based instructions into the final MACHINE CODE that will eventually be downloaded into the RAM and used to instruct the microprocessor. Once again, there are several freely available, but only one that is highly recommended, highly specified AND free, and that is San�s excellent SB Assembler!   A plug for San Bergman's SB-Assembler
 
  If however, you are happy with the Assembler you are currently using and do not wish to try out the one that San has written, you may have to use an additional TERMINAL PROGRAM to download the code to the Sitcom. (San�s SB-Assembler has this facility already built-in.)
 
  4)   DOCUMENTATION  In order to start programming the Sitcom�s 8085 processor, you will need some suitable documentation. See our DOWNLOADS page for all those essentials. However, for a complete understanding of the 8085 instruction set, a proper programming manual will help a great deal with those more complex programming tasks.
 
  5)  

A BOOT PROM. This is the heart of the 8085 based Sitcom which allows the computer to download your program into the right place and run it. Once again see the DOWNLOADS page for the full program listing, in both binary format and ASM that may be used directly by the SB-Assembler to program an EEPROM or EPROM. As mentioned at the top of this page, a PROGRAMMER will be needed once to write the program into the boot prom. Almost all will accept coding from an assembler such as San�s. I myself used the EEPROM programmer featured on my own site. The PROM device itself may either be a 27256, a 2764 or any of the same sized variants.

Incidentally, it goes without saying that San wrote and debugged the boot prom routine on his own SB-Assembler!

 
  6)  

Bill of materials or BOM

One of the essential features of the Sitcom Training Computer was the requirement for ALL component parts to be freely available.

 
      The bits and pieces that will make a SITCOM computer
 
 

A circuit board to build it on. Either strip board, pad board or one of the Eurocards may be used. Both San and I used a similar sized board, in my case it was the 160mm x 100mm Roadrunner card. Some may wish to use a slightly larger version to allow more room for expansion and power supply components. The basic hardware needed (which it is not necessary to rigidly adhered to ) for this board is as follows:

4x rubber feet with screws and nuts / adhesive.

Power connector / socket to suit.

Wire and 9 way �D� socket for serial connection (or 25 way if this is what you will use)

Two SPST push button switches.

Vero pins (optional) for testing / hanging scope or meter probes from.

40 way header for expansion or test modules.

One LED (unless you wish to use a second one as a power �on� indicator).

IC Sockets. The absolute minimum number is as follows: 2 x 40 pin,  2 x 28 pin,  1 x 20 pin,  1 x 16 pin and  3 x 14 pin DILs. If using one or two optional DL1414 displays you will need another 28 way socket, as well as two further 16 pin ones if the 100Hz divider chain using 4040�s is to be built onto the board rather than the project utilising the mains frequency derived 100Hz/120Hz.

Stripboard and Roadrunner Wiring Combs  

Wiring system wire and accessories. Both San and myself used the �Roadrunner� system. Other alternatives are wire-wrapping, the Vero wiring system, or simply careful point to point connections. A lot more difficult using the latter option but it WILL work (as long as you don�t make any errors that is!). Of course one could make a special PCB if that is within your field of interest. Unfortunately at the time of writing this, we have no CAD or board layout to help you with.

 
 

Integrated circuits

COMPONENT NOTES
8085 CPU 80C85 should work perfectly but not tried
8255 PPI 82C55 should work perfectly but not tried
27256 or 2764 EPROM either 32 or 8k x 8 bit Or any similar variants. San used the 27256 and I used a 27C64.
62256 / 55257  RAM  32k x 8 bit Most access time devices will work. San used a 15ns in his prototype and mine was 100nS
74LS132 We both used LS devices. HC should work too.
74LS00 ` `
74LS32 ` `
74LS138 ` `
74LS373 San used LS, I used HC, both worked fine.
74HC4040 x 2  (optional) for test 100 / 120Hz Not needed if mains derived Frequency is to be used.
DL1414 x 2 intelligent displays (optional) Optional but very useful! Even one is worth using.
 
 

Miscellaneous Components

COMPONENT NOTES
0.1uF capacitors Qty. approx 12. For decoupling purposes and RESET button.
Pull-up resistors (1K to 10K) Use on floating inputs etc.
10K resistors x 2 Pull ups for BOOT and RESET lines to push switches.
47K resistor x 2 In series with serial data in and to ground to stop noise.
270R resistor Used for LED(s) current limiting.
470uF cap 35V and 47uF 35V caps Used for optional EPROM programming module hi-volts I/F.
10uF 16V x 3 General +5V supply decoupling and for BOOT button.
1N4148 x 3 BOOT button, RS232 interface and optional 100Hz/120Hz input.
1N4004 (Qty approx. 7) Optional PSU rectification and voltage doubler circuit.
7805 +5V regulator with heat sink etc. Optional if using non-regulated power supply.
6.144MHz or 6.000MHz crystal For 8085 system clock. Use 3.075MHz or 3.000MHz for oldest slow 8085�s.
BC548 (or NPN equivalent) Transistor used for RS232 interface.
22pF capacitors x 2 For 8085 oscillator circuit.
Yet unspecified for mains I/P frequency  
 
 
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