SC128, v1.0, Assembly Guide

Click here for a list of the parts required to build an SC128, v1.0, Modular backplane.

Experienced builders READ THIS.

There shouldn’t be any further surprises to catch you out so just go ahead and assemble it.

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 SC128 should look like.

Horizontal header socket

Fit and solder the header socket, SK7, in the position indicated below in yellow.

Press the connector firmly against the edge of the PCB and ensure the pins are in the centre of the pads. Solder one of the middle pins on one side of the board and then check the connector is still in the correct place and the socket is parallel to the board’s surface.

Solder one of the middle pins on the other side of the board and repeat the checks. It is important the socket is parallel to the PCB so any backplane extension section or Z50Bus card is neatly aligned with the SC128 PCB.

Solder one pin at each end of the connector and on each side of the board. Repeat the alignment checks. Now solder all the remaining pins.

Resistor 470R

Fit and solder the 470R resistor, R1.

Resistor 4k7

Fit and solder the 4k7 resistor, R2.

Header pins

Fit and solder header pins, P1 and P2.

Capacitors (100 nF)

Fit and solder the 100 nF cacacitors, C1 to C6.

Header sockets

Fit and solder the header sockets, SK1 to SK6.

Capacitor (100 µF)

Fit and solder capacitor, C7.

It is important to fit this capacitor the right way around. The negative terminal is indicated with a ‘minus’ sign, as illustrated to the right. The negative terminal also has a shorter lead.


Fit and solder toggle switch, SW1 (shown below in red).

Fit and solder push button switch, SW2 (shown below in yellow).

Power connectors

Fit and solder 2.1 mm barrel power socket, J1 (shown below in red).

Fit and solder screw terminal, J2 (shown below in yellow).

Jumper header pins

Fit and solder the jumper header pins, JP1.

Light Emitting Diode (LED)

The LED can either be fitted vertically, with straight leads, or horizontally, by bending the leads.

Fit and solder green LED, LED1.

It is important to fit the LED the correct way around. LEDs usually have a small flat side to indicate the cathode (the negative end). This should be positioned to match the flat side shown on the circuit board (illustrated to the right). Also, the cathode pin on the LED is usually shorter than the other pin (the Anode).


Visually inspect all soldering for bad joints and for shorts. Clean the flux from the board and visually inspect again.

Check there is not a short on the circuit board’s 5 volt supply by connecting the multimeter’s probes to each terminal of the screw terminal block (J2). This should read an open circuit, not a short. Test with the toggle switch in both positions.

If you really want to play safe you can use the multimeter to check for a short between all adjacent pins on the backplane. You could also check for continuity between all the pins on the same backplane signal line. Note that signals IEI and IEO form a daisy chain so do not simply connected straight through to each socket, as all the other signals do. This level of checking is very tedious and unlikely to find a problem if you have thoroughly inspected the soldering with a magnifying glass.

Connect a 5 volt supply to the screw terminals (J2), the barrel socket (J1), or the header pins (P2). The power LED should light when the toggle switch is in the ON position – the toggle switch pointing away from the LED.

Disconnect the power supply and connect a minimal set of Z50Bus cards to the backplane. This could be as little as just SC118 (a single board computer). Power up and check the system is running. If all is well, fit any other Z50Bus cards and turn off your soldering iron!

Homebrew 8-bit retro computing