|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.|
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.
For whatever method of construction is going to be used, it is best to break this down into two different stages.
Gather together the pieces
Stage one complete
|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?|
|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.|
|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.|
|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.|
TESTING YOUR NEW SITCOM
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.
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.
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.
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,
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:
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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