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LEDs and Switches on a SimmStickTM Bus.


This is the first Input/Output board that everyone wants and needs. Something to flash a LED to, and read a switch from. This board can monitor 24 SimmStickTM bus signals and output the results via the LED driver chips to the 24 individual LEDs if required.

4 Momentary and/or 8 LINKED switch positions can be easily connected to any bus signal.

The signals monitored by the 8 LEDs with E1 ULN2803 chip are:
A1, A2, A3, SCL, SDA, SI, SO, and IO.
E2 monitors D0 to D7, and E3 monitors D8 to D15.

When used with the DT004 board, a stand alone Printer Port to LEDs and Switches board is produced, no Microcontroller!!!

These DT203 Two Inch boards are $11AUD each.(~$9USD)
All of the documentation is now on this page.
Download Schematic in PDF format.

SimmStick and other PCB Prices.

Free Local And International Air Mail Postage. on Printed Circuit Boards. Conditions Apply.

Board Components:

NOTE *** If your DT203 is only ever going to be used with an 18 pin PIC chip, then the components associated with signals D8 to D15 don't need to be installed. This is E3, the 8 Resistors next to it, and the 8 LEDs next to them.

1 by DT203 PCB
3 by ULN2803 Driver Chips
3 by 18 pin sockets. (Optional)
24 by 330 Ohm Resistors .25 watt
8 by 10K Resistors .25 watt
24 by 5mm LEDs.
4 by PCB mount Push button Switches. These can be the Hat-key type with a .2" by .2" mounting pattern, or a 4 legged zippy switch with a .2" by .3" pattern.
A set of 2 by 8 male header pins suitable for connecting 8 Shorting or Test Links to.

Switch signal connections can be done with permanent jumper wires, or two strips of single row male header pins and female jumper leads or clips. This is done with a strip of 24 pins and a strip of 8 pins.

If you use the header pin jumper principle, it means that wiring alterations can be done very easily.
I use wire wrap for this sort of thing myself rather than jumpers that may be hard to obtain.
Either way, it's your choice.

Not that difficult, but I would recommend using sockets for the chips if this is the first you have put this board together.

All hand assembly should be done by installing the shortest height components first.
Resistors don't matter as the legs can be bent over to hold these in place at any time.

I would suggest:
Install the 3 Sockets.
Install the 24 LEDs. The Cathode end is nearest the bus. It is shown on the overlay as the dark flat marking near the lower leg. The LED itself has one leg shorter than the other. This is the cathode. It also has a flat edge. This also marks the cathode.
Install the 2 by 8 header block for the test or shorting links.
Install the 2 single strips of bus signal and swich signals if this is the way you are going.
Install all Resistors standing upright with one leg bend over the body of the resistor so that each one will fit the .1" pattern as shown on the overlay of the board..
Install the 4 switches.

Then insert your 3 chips. You are finished.

NOTE **** A small artwork error has been found on this board. (My fault!, but keep it quiet.) The linked Switch positions SW5 to SW8 don't have the 10K pullups connected to the links. This amounts to 4 small .1 Inch long tracks missing. Small jumpers can be added to the solder side of the board if these last 4 switches are to be used with the links.

Date: Fri, 30 Jan 1998 19:21:04 +1100
From: Don McKenzie don@dontronics.com
To: Jim Moulton jim@standard.net.au

Jim Moulton wrote:
> Dear Don, Bits from you arrived today, thanks.
> I'v just had a very frustrating couple of days with problems with RA4
> using your dt001, dt101 and dt 203 and have at last found the problem
> The pull_up resistor on RA4 on dt101 is too big.
> To read in a logic 1 on RA4 the voltage must be atleast 0.85 Vdd (ie
> 4.25V)
> The input current drawn by an input pin of ULN2803 on dt 203 is 1.35ma
> max
> Thus the pull up resistor needs to be (5-4.25)/1.35 k     (ie 560 ohms
> not the 10k provided on dt101)
> I have added 560 ohm to the dt203 as this is the board with the
> problem load (ie the ULN2803)
> what do you think ?

> Plenty of code will work;
> the problem is for reading the RA4 pin not
> writing to it.
> eg    bsf    PORTA,4        pin goes high
>                             (as it can with the r pull-up and ULN load)
>       other code
>       bsf    PORTA,6        RA 6 goes high and RA 4 pin goes low !!!

> this is because bsf etc is read/modify/write and when it reads
> it sees a '0' on RA4 and this second instruction actually
> affects RA4 I'v deleted my test code but it is simple to
> demonstrate the affect.
> The figures I quoted are from the data books.

> Jim Moulton
> R.M.B 2128
> Colcott St.
> TIMBOON, 3268
> PH: 03 55983544

I can show you a set of standard boards ticking away on a walking LED
program here on an 84 without problems on RA4, but I have to believe you
of course Jim.

I feel to install it on the board that displays the load problems is the
correct solution.
So that your frustrating couple of days is put to good use, I'll add
this message to the hardware html file of the DT203 so that others will

Cheers Don...

I have found when using FED Basic that if you load SCL and SDA with a DT203
LEDs and Switches board, it may not run. Remove the first ULN2803 chip for
initial tests. Don...

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