SC505, v1.0, Assembly Guide

Click here for a list of the parts required to build an SC505, v1.0, RTC and I2C 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.


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 SC505, RTC & I2C Card should look like.


Fit and solder the 10k resistors, R1 to R4 (shown below in green).

Fit and solder the 1k resistors, R5 and R6 (shown below in red).

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

Sockets (6-pin)

Fit and solder the 6-pin angled sockets, P3, P5, and P7.

Ensure the socket lays flat against the circuit board.

Single row angled header pins

Fit and solder the single row angled header pins, P2, P4, P6, P8, P9, JP2, and JP3.

Ensure the pins are parallel to the circuit board, especially P2, P4, and P8.

IC sockets

Fit and solder the 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 network

Fit and solder the 8x100k resistor network, RN1.

The 100k network will be marked 104.

Take care to fit the resistor network the correct way around. Pin 1 is usually marked with a dot. This end is indicated on the PCB and on the illustration below.

Double row angled header pins

Fit and solder the double row angled header pins, JP1.

Capacitor 100 nF (0.1 µF)

Fit and solder capacitors, C1 to C6.

This capacitor can be fitted either way around, as it is 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.

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.

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

Light Emitting Diodes (LEDs)

Fit and solder green LEDs (LED1 and LED2) in the position shown below.

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

If using standard LEDs care must be taken to ensure they are 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).


Fit and solder the 32.768 kHz crystal, X1.

The crystal can be fitted either way around, as it is not polarity dependent.

The crystal can be secured in place with a small piece of wire fitted in the position indicated below in green. An off-cut resistor leg is suitable for this.

Battery holder

Fit and solder the CR2032 battery holder, B1.

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.

Quick Tests

Repeat the check made earlier for a short on the power supply tracks. Connect the meter probes to the pins 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.


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.

Integrated Circuits

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

Getting Started

The card’s I/O address needs to be set with jumper shunts on JP1.

The card address should be set to match the software you are using. Most software currently assumes the I/O address is 0x0C. The other likely address is 0xA1.

Jumper positions for I/O address 0x0C
Jumper positions for I/O address 0xA1.

Use with SC503 Z180 processor card: RomWBW firmware should now identify the real-time clock at start up. RomWBW does not currently offer specific support for I2C so this is left to the user and the application software.

Use with Z80 based systems: There is not currently any firmware support for the real-time clock or the I2C bus master. This is left to the user and the application software.

Both processor types allow use of the I2C bus master with existing example software for SC400 series I2C modules.

The SC505 User Guide can be found here.

Other information about SC505 can be found here.

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