Click here for a list of the parts required to build an SC506, v1.0, Digital I/O card.

Important note: How to modify box headers

Experienced builders can just go ahead and populate the board. There shouldn’t be any surprises to catch you out.

Getting started

This guide assumes you are familiar with assembling circuit boards, soldering, and cleaning. If not, it is recommended you read some of the guides on the internet before continuing.

First check you have all the required components, as listed in the parts list.

Before assembling it is worth visually inspecting the circuit board for anything that looks out of place, such as mechanical damage or apparent manufacturing defects.

If you have a multimeter that measures resistance or has a continuity test function, check there is not a short on the power supply tracks. Connect the probes to each terminal of one of the capacitors, such as C1. This should be an open circuit, not a short.

The picture below shows what a completed SC506 should look like.

Resistor (470R)

Resistors can be fitted either way around, as they are not polarity dependent.

IC sockets

Be sure to fit them with the notch matching the legend on the circuit board, so you do not end up fitting the IC the wrong way around too.

Resistor networks

The 100k network will be marked 104 while the 1k network will be marked 102. If you want particularly bright lights you could fit 470R resistor networks instead of 1k resistor network. Alternatively, if you would like less bright lights fit, say, 2k2 resistor networks instead of 1k resistor network.

Light Emitting Diodes (LEDs)

Capacitors (100 nF)

These can be fitted either way around as they are not polarity dependent.

The exact value of this component is not critical. The use of very cheap capacitors within the range of about 30 to 200 nF is acceptable.

Double row angled header pins

Capacitor (100uF)

The silkscreen has a plus sign for the positive terminal and a hashed area for the negative terminal.

Test button

Header pins (straight)

Input/Output connector

Bus connector

Take care to ensure the connector is fitted such that the card will stand vertically when fitted to a backplane. A good method is to just solder two pins, one at each end, and then check it looks correct. Then solder two more, one at each end but in the other row, and check again. At each stage, if necessary, adjust the position by heating the required solder joint and moving the connector slightly. Solder two pins in the middle of the connector and check again. Then solder all the others.

Inspection

Remove any solder ‘splats’ with a brush, such as an old toothbrush.

Visually inspect the soldering for dry joints and shorts.

Clean the flux off with suitable cleaning materials.

Visually inspect again. The importance of visual inspection can not be overstated.

Quick Test

Check for a short on the power supply tracks by measuring the resistance between IC U2 pin 14 and U1 pin 7 (indicated below). This should be an open circuit, not a short. If you are using a digital meter set to measure resistance it will likely take a few seconds for the reading to stabilise as there are now capacitors on the power lines. A reading of more than 100k Ω (100000 ohms) is acceptable.

Integrated circuit

Insert the integrated circuits (ICs) into their sockets, taking care to insert them the right way around, as illustrated below. Be careful not to bend any legs over.

Address select jumpers

Header JP1 is the 8 address select jumpers which are labelled with their bit numbers.

The card responds to input/output addresses matching the address set with these jumpers. When a jumper shunt is fitted, that bit must be a 1 (high voltage). When the shunt is not fitted, that bit must be a 0 (low voltage).

Set the address to 0xA0 hexadecimal (160 decimal, 10100000 binary). This is selected by fitting a jumper shunt to bit 7 and another to bit 5.

You are now ready to give it a try.

Testing reset

Connect the Digital I/O card to a Z50Bus compatible system and power up.

If your system has a working power on reset then none of the card’s LEDs should be on. If your system usually needs a manual reset then reset it now and check all the card’s LEDs are turned off.

If any of the output LEDs (LED1 to LED16) are turned on at reset, check for a problem around IC U3.

If any of the input LEDs (LED9 to LED16) are turned on at reset, check for a problem around IC U4.

Testing inputs

Input bit 0 can be tested with the test button (SW1). When this button is pressed, the input LED for bit 0 (LED16) should light.

To test the other inputs, simply connect each input pin on header P4, in turn, to 5 volts, as illustrated below. When the input is connected to 5 volts the LED should light. This test should work with or without IC U4 fitted.

If an input LED does not light, the most likely explanation is the LED has been fitted backwards.

The inputs can also be tested from BASIC or from the Small Computer Monitor (SCM). With the card’s address set to zero (no jumper shunts fitted) the input port can be displayed with the following commands.

From BASIC:
PRINT INP(160)
The result will be the input port value shown in decimal.

From SCM:
I A0
That’s the letter “I”. The result will be the input port value shown in hexadecimal.

In both cases the result should be zero if there are no external connections to header P4 or P5.

Testing outputs

The outputs can be tested from BASIC or from the Small Computer Monitor. With the card’s address set to zero (no jumper shunts fitted) the output port can be written to with the following commands.

From BASIC:
OUT 160, 1
Where ‘1’ is the required decimal value to be written to the output port.

From SCM:
O A0 1
Where ‘1’ is the required hexadecimal value to be written to the output port.
Note: That’s the letter “O”.

The output signals can also be tested with a multimeter by measuring the voltage on each of the output bit pins in header P2 or P3.