One of the SITCOM training computer prototypes
 SITCOM 85 Training Computer
 
  Unless you are a master at producing PCB layouts on a CAD package and etching boards using all those smelly and messy chemicals, there are several alternatives you can use to build your `Sitcom’. The most simple of these being the point to point wiring method. Whilst this is most definitely the most time consuming and awkward, in the absence of any unintentional errors, the project will almost certainly work in the end. If you are going to take this approach, I would recommend using a good quality easily-strippable wire - maybe that normally is reserved for wire-wrapping. Wrong sort of building materials Izabella!
 
  Wire wrapping turned pin IC socket
Typical wire wrap socket
WIRE WRAPPING is of course a viable alternative, though the cost will be a lot higher as special wire-wrap sockets will have to be purchased and used.
 
 

Another popular method of prototyping used for the construction of both the first Sitcoms, is by using the wiring-pencil method. Two sorts seem to be available, though may be difficult to source, a) the Roadrunner system, and b) the Vero system. For both of these, a wire dispenser is fitted to the top of a hollow ‘pencil’ which dispenses the wire from the hole at the bottom. From here it is wrapped around the first pin then taken to subsequent pins on the same node. The wire makes its progress between each pin via castellated wiring ‘combs’ glued to the board. For a full description please see the Z80 based ‘site project’ on my site http://www.izabella.freeuk.com.

If you like this approach but find it difficult to obtain the required tools I may have some tips for you.
Simple pad board, a PCB full of holes with a small island of copper around each hole can be used as the base material. This kind of board is easily obtainable, and comes in a large variety of sizes and quality.
As wire you can use the wire found on transformers and other coils. Ideally a thickness of 0.15mm2 can be used for the data lines, and 0.3mm2 is suitable for most power lines.
A wire pencil like the road-runner system pen is not absolutely necessary. I have created several projects without one myself.
Instead of the nice wiring combs that Izabella uses I always use steel pins to "guide" the wires during the building process. Place these pins at strategic points in the holes of the PCB to create nice bundles of wires. I always wire the data lines first, and use the thicker power line wires to fix the data wire bundles.

 
 

CONSTRUCTION

For whatever method of construction is going to be used, it is best to break this down into two different stages.

1) The assembling of the board with its rubber feet (if not using wire wrap), the push switches and power connectors.
2) Mounting the sockets, combs and doing the wiring.
 
  Parts

Gather together the pieces
Board with feet, buttons and power connectors

Stage one complete
 
  Roadrunner wire example Here is an example of a simple connection using the Roadrunner System. Note that one end of the wire is soldered to the + Volts plane, with the other routed a short way down the wiring comb and onto the pin of the IC socket.
 
  With some layouts, it may be an idea to consider fitting surface mount decoupling capacitors rather than through hole components - or maybe a combination of both for convenience? SM decoupling cap positioning
 
  close-up In the photo to the left we can see the fitting of the 8255 and the 62256 RAM to the second Sitcom. As it can be a bit confusing trying to identify each IC’s pins, what I sometimes do is ‘black out’ the pad immediately to the right of any ‘pin 1’ with a permanent marker. Note that in this image, the power supply wires are in position, as well as the s/m decoupling capacitors.
 
  Semi-complete Sitcom underside Underside of the partly completed Sitcom to the left, and to the right we see a close-up to show the wiring to the 62256 RAM. wiring to the RAM
 
  Applying power to the Sitcom for the first time with success! The two displays have been fitted as an optional extra and are NOT required unless you choose to fit them - or perhaps just one? All of Sitcom’s states can be identified from the flashing rate of the single LED connected to the 8085’s SOD pin. Using displays simply allows for more experimentation opportunities. The PCB to the left is my own EEPROM programmer sharing the +5 Volts. Working semi-complete Sitcom
 
 

TESTING YOUR NEW SITCOM

San hard at work

As it can be very difficult ascertaining whether or not a micro based project is working correctly, one should not simply power the thing up and hope that it doesn’t go straight into a mega melt-down. For this reason I suggest that it is well worth taking a few minutes to double-check the wiring, especially that for the power.

Once you are certain that all seems well, carefully insert the IC’s. When applying power for the first time, check the current consumption. San reckoned his prototype used about 250mA without the interface connected and 300mA with a program running. 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 its 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 determines 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.

THE BOOT

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 3Eh, 41h ,D3h ,20h ,18h ,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.

DOWNLOADING

When a program is being downloaded from the Assembler, the SITCOM will automatically see this on its SID line (connected - hopefully via the transistor shown on SITCOM schematic to the correct PC comm. port level) 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!

Should an error occur during the download then the SOD LED will flash rapidly. You must now press the Boot button, and try uploading your program again,

 
 

DEBUGGING PROBLEMS?

But if it doesn’t seem to work....... where do I start looking for the problem?

There cannot be a developer out there who has not constructed a micro based project with a fault on it at some time, however small. The trouble is that we ALL make mistakes, but our problem will be identifying its location.

I think it is fair to say that I have only ever come across a dead IC in one of my circuits once or twice at the very most. Even the most dead looking IC in my circuits has turned out to be either a design or wiring error - the latter of course, could well have been found with a little bit more patience in the pre-turn-on stage.

Testing for that error does not necessarily need expensive equipment or even a ‘scope if you are following a tried and tested design; that is not to say that one will make life a great deal easier if you have access to one. It should therefore be possible to identify almost all switch-on problems with a Sitcom with nothing more than a multimeter and a logic probe.

Ancient dual trace oscilloscope

Logic probe, pulser and tools

I may well be preaching to the converted here, but may I state the obvious in saying, understand what your logic probe tells you. I use an old HP model that offers all states on a single flashing bulb, whereas others usually have two LEDS. It is most important to appreciate how the different conditions are shown on YOUR particular model.

TABLE OF IDEAS FOR DEBUGGING

The following table shows the conditions expected on certain Sitcom pins to aid debugging:

8085 Pin 5
SID (INPUT)
HIGH when no interface connected If the SID pin that receives the serial data from the Assembler is held low, an error condition will occur.
8085 Pin 4
SOD (to LED)
0.7Hz approx when Sitcom in standby The flash rate from the SOD pin is determined by the current state of Sitcom.
IO/M pin 6 on 74LS138 Alternates approx 10 secs low then 20 clocks The alternating low then clock on this pin is when the optional displays scroll the text during standby.
74LS138 outputs 7 & 9 Clock is as above to show scrolling message Whilst Sitcom is in standby mode (reset after power up) these will be the ONLY outputs active on the decoder.
REMOVE both RAM and 8255 For test purposes these may be removed. As neither are used during standby operation, the SOD LED and the optional displays should still operate.
All static conditions OK Output from SOD LED pin at fast flashing rate. If displays fitted ‘FR ERROR’ should be displayed. This suggests that SID data pin received a spurious glitch.
74LS132 Pin 11 BOOT HIGH when Sitcom in standby mode. The BOOT line will latch low when the RESET button is pressed and only go high again when BOOT pressed.
74LS00 Pin 8 /RESET Normally HIGH The /RESET line will be taken low momentarily when either BOOT or RESET buttons are pressed.

This may come as a surprise to you, but all ICs should get a nice, clean, ripple free, +5V supply, as well as a firm ground level. Furthermore you must assure yourself that the crystal oscillator is running!

 
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