SC126, v1.0, Assembly Guide

Click here for a list of the parts required to build an SC126, v1.0, Z180 Motherboard.

Experienced builders READ THIS.

Before you start assembly, read the section below about assembly options. After that, just go ahead and populate the board. There shouldn’t be any further surprises to catch you out.

Also, you should fit jumper shunts to JP1 and JP2, write protect positions, and JP4’s DCD0 side.

Assembly options

This design allows for a choice of connectors around the edge of the circuit board.

The picture below shows the parts supplied in the kit to provide these options. Note, you have a choice; you do not use them all.

Part options

Serial ports and SPI ports (typically used for micro SD cards) are connected with adapters as shown below. The adapters could be soldered directly to the motherboard, plugged in to sockets on the motherboard, or connected via cables to header pins on the motherboard. The choice is yours.

Adpaters

To make the assembled board as flexible as possible, the serial and SPI ports each have space for two connectors. It is suggested that one be used for header pins and the other for a socket.

Suggested combinations

Fitting sockets means adapters can be plugged in directly.

Sockets for adapters

Fitting header pins allows the adapters to be remote from the motherboard. This might be desirable when mounting in a case or just for convenience on the bench.

Header pins for remote adapters

Avoid the combination of straight pins in the edge position (as illustrated above) with a socket in the inner position, as the straight pins obstruct an adapter plugged in to the socket.

Update: This is my current preference (having now built several)

While direct soldering of the adapters to the motherboard is possible, it is not generally recommended due to the difficulty changing them if it becomes necessary.

Adapters soldered directly to the motherboard

Some of the header pins provided for jumper shunts also allow options. These header pins are cut from the longer strips provided in the kit. You can use either straight or angled pins.

Options for jumpers

It is a good idea to think now about how you wish to use these options. If you don’t know which will be best for you, it is recommended that you follow the suggestions below.

The assembly instruction below will indicate when a choice is available and will show a suggested option.

Introduction

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 SC126, Z180 Motherboard should look like, depending on your choice of assembly options.

SC126 Assembled

Resistors 2k2

Fit and solder the 2k2 resistors R1 to R6 (shown below in yellow).

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

Resistors 100k

Fit and solder the 100k resistors R7 to R9.

Resistor 4k7

Fit and solder the 4k7 resistor R10.

Resistors 470R

Fit and solder the 470R resistors R11 to R19.

Resistors 10k

Fit and solder the 10k resistors R20 to R22.

Crystal (32.867 kHz)

Fit and solder the 32.768 kHz crystal X2.

This should lay flat on the board in the position shown below.

This component can be fitted either way around, as it is not polarity dependent.

IC sockets

Fit and solder the IC sockets (shown below in yellow).

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.

IC socket X1

You may wish to fit the oscillator X1 in a socket. If so, fit and solder an IC socket (shown below in yellow). If you would rather solder it directly to the board, you should do that later.


Resistor network

Fit and solder the 8x10k resistor network RP1.

This must be fitted the correct way around. The component should have pin 1 marked with a dot, as illustrated right.

Pin 1 is also marked on the picture below.

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PLCC socket

Fit and solder the 68-pin PLCC socket for U5.

This type of socket can be difficult to insert into the PCB holes as there are so many fragile pins, which must be carefully aligned.

It is vital this socket is fitted the correct way around. The socket has a small chamfer on one corner, as indicated below in red.

Note the position of the chamfered corner, illustrated to the right and also below.

Quick Test

It is now worth repeating the check made earlier for a short on the power supply tracks. Connect the meter probes to each terminal of one of the capacitors, such as C1. This should be an open circuit, not a short.

Capacitor (1 nF)

Fit and solder capacitor C16.

This capacitor can be fitted either way around, as it is not polarity dependent.

The exact value of this component is not critical. The datasheet for the DS1233 specifies a value from 0.5 to 10 nF, thus a very low cost component with a wide tolerance is acceptable.

Capacitors 100 nF

Fit and solder capacitors C1 to C14.

These capacitors 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 50 to 100 nF is acceptable.

Bus sockets

Fit and solder right angled female header, 2 row x 40 pin, S1 (shown below in red).

Fit and solder female headers, 2 row x 40 pin, S2 and S3 (shown below in yellow).

Switches

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

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

Quick Test

Again, repeat the check made earlier for a short on the power supply tracks. Connect the meter probes to IC U6 pin 20 (shown below in red) and U6 pin 10 (shown in green). 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 100 kΩ (100000 ohms) is acceptable.

Now measure the resistance between IC U6 pin 20 (red) and U6 pin 1 (blue). This is measuring the pull up resistor R10. It should read 4k7 Ω (4700 ohms) plus or minus 5%.

Also, measure the resistance between IC U6 pin 10 (green) and U6 pin 1 (blue). This is measuring the resistance of the reset switch (SW2). It should currently be open circuit. Again the reading may take a few seconds to stabilise. A reading of more than 100 kΩ (100000 ohms) is acceptable. Whilst still measuring the resistance, press the reset button. The reading should now be a short circuit. A reading of less than 1 Ω is ideal, but less than 10 Ω is acceptable. The 10 ohm limit has been suggested mainly to allow for measurement accuracy with cheap meters.

Check the ON/OFF switch is operating by measuring the resistance between J2 pin 1 (yellow) and U6 pin 20 (red). This should be a short circuit when the switch if ON and an open circuit when the switch is ON. Acceptable resistance readings are less than 1 Ω and greater then 1 MΩ, respectively (ignore inaccuracies of the meter).

Headers (straight)

Fit and solder the pin headers JP3 and JP4 ( 1 row of 3 pins).

These may need to be cut from longer strips using wire cutters to cut the plastic.

Fit and solder the pin header JP5 ( 2 rows of 2 pins).

Headers (angled)

Assembly option: Fit either angled or straight header pins. If in doubt it is suggested that angled pins should be used.

If P4 and P7 headers are the same type of connector as used for P5 and P6, then it may be easier to fit them now as a single strip. This will require removing some pins as illustrated to the right.

Fit and solder angled header pins (1 row of 2 pins) P4 and P7 (shown below in yellow) or the strip P4, P5, P6 and P7 (with the unused pins removed).

Fit and solder angled header pins (1 row of 2 pins) P8, P9, and P10 (shown below in yellow).

Fit and solder angled header pins (1 row of 3 pins) JP1 and JP2 (shown below in red).

Peripheral ports

Assembly options: The peripheral ports, shown below in yellow, should be fitted with your choice of connector (as described in the “Assembly options” section, above. Each peripheral port has two connectors with identical connections. In most cases, it is best to fit a male connector in one and a female connector in the other (as illustrated below). This provides the most flexibility but it may not suit everybody.

My current preference

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).

Capacitor (100 µF)

Fit and solder capacitor C15 (shown below in yellow).

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.

To make the board as low profile as possible, this capacitor can be fitted laying flat, in the position shown below.

LEDs

The LEDs can either be fitted vertically, with straight leads, or horizontally, by bending the leads. Take your time aligning them carefully as they look awful if the are not aligned.

Fit and solder yellow LED (LED1) in the position shown below.

Fit and solder green LEDs (LED2 to LED 9) in the positions shown below.

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).

Battery holder

Fit and solder the battery holder in the position shown below.

Quick Tests

Repeat the check made earlier for a short on the power supply tracks. Connect the meter probes to IC U6 pin 20 (shown below in red) and U6 pin 10 (shown in green). 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.

Turn the power switch (SW1) OFF (toggle towards the LEDs). Connect a 5 volt power source to J1, J2 or P10, and perform the following tests.

If you do not have a suitable 5 volt power source, connect an FTDI style serial adapter to serial port A and fit a jumper shunt to P4. The power LED should light. In this configuration the power is being supplied ‘after’ the ON/OF switch, so the motherboard will work regardless of the O/OFF switch poition. However, set the ON/OFF switch to ON (toggle away from the LEDs) so that power is allowed through to J1, J2 and P10.

  • The voltage on the two screw heads of the terminal block J2, should be between 4.75 and 5.25 volts.
  • Turn the power switch to ON (toggle away from the LEDs). The power LED (LED1) should now light.
  • Test the other 8 LEDs. Touch one end of a piece of wire to U7 pin 20 (5 volts). Touch the other end to each of the following pins: U7 pins 19, 16, 15, 12, 9, 6, 5, and 2.
  • Check the voltage at the middle pin of U12 (RESET) is above 4.5 volts and drops to below 0.5 volts when the reset button is pressed. This should be pulled up by R10.
  • Check U4 pin 3 is above 4.5 volts and drops to below 0.5 volts when a jumper shunt is fitted to P9. This should be pulled up by R22.
  • Check U1 pin 31 (WR to Flash 1) is below 0.5 volts but rises to above 4.5 volts when a jumper shunt is fitted to the write protect position of JP1. Without the shunt fitted the signal is floating, so with a digital meter it may not be stable but should be below 0.5 volts.
  • As above, check U2 pin 31 (WR to Flash 2) with a jumper shunt on JP2.
  • Check JP4 pin 1 (DCD0) is above 4.5 volts. This should be pulled up by resistor network RP1.
  • Check JP4 pin 3 (DREQ1) is above 4.5 volts. This should be pulled up by resistor network RP1.
Bus connector viewed from component side of PCB (note the unusual pin numbering)
  • Check bus connector pin 66 (NMI) is above 4.5 volts. This should be pulled up by resistor network RP1. Note, the pin number for the bus connector runs from pin 1 to pin 40 on one row and 41 to 80 on the other. Numbering is not the typical odd numbers on one row and even on the other.
  • Check bus connector pin 65 (WAIT) is above 4.5 volts. This should be pulled up by resistor network RP1.
  • Check bus connector pin 64 (BUSRQ) is above 4.5 volts. This should be pulled up by resistor network RP1.
  • Check bus connector pin 22 (INT) is above 4.5 volts. This should be pulled up by resistor network RP1.
  • Check bus connector pin 36 (RX) is above 4.5 volts. This should be pulled up by R7.
  • Check bus connector pin 76 (RX2) is above 4.5 volts. This should be pulled up by R8.
  • Check P1 pin 4 (SDA) is above 4.5 volts. This should be pulled up by R20.
  • Check P2 pin 4 (MISO) is above 4.5 volts. This should be pulled up by R21.
  • Check U5 pin 11 (INT1) is above 4.5 volts. This should be pulled up by resistor network RP1.
  • Check U5 pin 12 (INT2) is above 4.5 volts. This should be pulled up by resistor network RP1.

Oscillator (18.432 MHz)

If you decided to solder the 18.432 MHz oscillator, be sure to fit it the correct way around.

Pin 1 of the oscillator is normally indicated by a ‘sharp’ corner, while the other three corners are rounded.

Pin 1 on the circuit board is also indicated by a ‘sharp’ corner, while the other three corners are rounded.

Voltage supervisor and reset

Fit and solder the DS1233 voltage supervisor and reset device U12.

This device must be fitted the correct way around. Match the shape of the device to the legend on the circuit board.

It is necessary to spread the legs of the device to match the hole spacing on the circuit board. Do this gently so as too avoid straining the legs where they enter the plastic casing. DO not press the component hard into the board as this will also strain the legs.

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.

Quick Test

Connect a 5 volt supply or the FTDI cable (as described earlier). Check the voltage at U6 pin 1 (RESET). It should be above 4.5 volts. Check it drops below 0.5 volts when the reset button is pressed and returns to above 4.5 volts within one second of the reset button being released.

If you have a variable voltage supply you can check the voltage supervisor (U12) pulls the reset line low when the voltage drops below about 4.6 volts. This is not an exact voltage level, there is a significant tolerance, but it should trip between 4.5 and 4.75 volts.

Integrated Circuits

If all the above tests check out okay, disconnect the power and insert the integrated circuits into their sockets.

Fit the Flash ROM containing the Small Computer Monitor in socket U2.

Fit the Z180 CPU into its socket as illustrated to the right. This must be fitted the correct way around. The socket and IC both have a small chamfer in the position indicated.

Take care to insert the Z180 CPU level. Avoid pressing one side in while the other side is still raised. If you need to remove the IC it is worth getting a special tool which hooks under opposite corners of component so that it can be lifted out evenly.

If the oscillator has not been soldered directly to the board, insert it into socket X1.

Pin 1 of the oscillator is normally indicated by a ‘sharp’ corner, while the other three corners are rounded.

Fit jumper shunts in the positions shown below.

Jumper settings are described in the SC126 User Guide.

Initial Testing

Connect the 5 volt supply and turn on. The firmware in socket U2 will now run. This will usually be either RomWBW or the Small Computer Monitor (SCM).

  • RomWBW: As RomWBW initialises it indicates its progress by turning on the 8 status LEDs. When initialisation is complete all the LEDs should be on. Once a menu selection is made they are turned off and then used to indicate storage device activity.
  • SCM: The first thing that should happen is a self test. The 8 status LEDs should each light in turn for a fraction of a second, and should then all turn off. If the LEDs keep cycling then the simple memory (RAM) check has failed.

If the self test completes correctly, power down and connect serial port A to a computer running terminal software.

An FTDI style serial adapter and cable is typically used to connect to the computer, as illustrated to the right.

The terminal software should be set for 115200 baud, 8 data bits, no parity, 1 stop bit, and no flow control. Power up and you should see the start up message “Small Computer Monitor – SC126”.
Note: With RomWBW v2.x the baud rate is 38400 baud.

Given that the self test passed, a failure to display the start up message is most likely to be directly related to the serial port electronics or the serial adapter cable. Check for activity at the serial port connector and around the 2k2 resistors.

If you have further problem the SC126 troubleshooting guide may help.

Getting Started

The SC126 User Guide can be found here.

If you want to run CP/M, it is highly recommended you use RomWBW. Click here for help getting started with RomWBW, including setting the clock and preparing an SD card. Fit a jumper shunt at JP3 or P9 to select RomWBW rather than SCM.

A guide to SCM can be found here.

Other information about SC126 can be found here.

Homebrew 8-bit retro computing