SC523, v1.0, Assembly Guide

Click here for a list of the parts required to build an SC523, v1.0, Z80 Programmable ‘ROM’ Card.

Important note: How to modify box headers

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

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 circuit.

The picture below shows what a completed SC523, Z80 Programmable ‘ROM’ Card should look like.


Fit and solder the 1k resistor, R1 (shown below in red).

Fit and solder the two 100k resistors, R2 and R3 (shown below in green).

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

IC sockets

Fit and solder IC sockets for U1 to U5.

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.


Fit and solder capacitors C1 to C5.

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

Light Emitting Diode (LED)

Fit and solder green LED, LED1.

If using an angled LED the polarity is predefined and matches the PCB.

If using a standard LED care must be taken to ensure it is fitted the correct way around.

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).

Jumper Header Pins

Fit and solder header pins JP1, JP2, and JP3.

This header may need to be cut down from a longer strip.

Bus connector

Fit and solder the bus connector, P1. This can either be a right-angled box header or right-angled header pins. The box header is recommended.

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.


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.

Integrated Circuits

Insert the ICs into their sockets, taking care to insert them the right way around, as illustrated below. Be careful not to bend any legs over. ICs are usually supplied with the legs slightly spread out making them difficult to insert. It is best to bend the two rows of legs so they are parallel before inserting them. Remember, these components can be damaged by static electricity so if possible earth yourself and the components while handling them.

SC523’s Flash memory socket (U1) should contain suitable firmware. Typically this is SCM S2 or SCM S3. SCM S2 supports SC516’s serial port but does not support SC523’s software-controlled memory bank selection. SCM S3 does not support SC516’s serial port but does support SC523’s software-controlled memory bank selection.


Fit jumper shunts in the positions shown below.

Getting Started

The SC523 User Guide includes a brief guide to getting started.

Fault Finding

Check there are no chips with bent legs and thus not making contact with their socket, carefully inspect all soldering, check all the chips are inserted the right way around, check all the components are in the right place.

Fit this card into a backplane together with a suitable set of cards to make a working system and then power up. A minimal system using this card also requires:

  • Backplane (eg. SC501, SC508, SC515, or SC522)
  • Power and reset card (eg. SC502)
  • Z80 CPU and memory:
    • Z80 processor card (eg. SC516 with ROM chip removed), or
    • Z80 CPU + Z80 memory (eg. SC518+SC519 with ROM chip removed)
  • Serial card (eg. SC511, SC520, or SC521)

Check the LED is on. If it is not, then check the LED is the correct way around. Also check the RESET signal is high but goes low when the reset button is pressed. The LED should light when U4 is powered and working, and the RESET signal on U4 pin 15 is low.

If the LED is working, check it flashes off and then on again following a reset. If it flashes then the board is running code successfully. This would indicate that all the main components are generally working. If it keeps flashing the self-test has failed, most likely indicating the RAM is not working.

Homebrew 8-bit retro computing