SC503, v1.0, Assembly Guide

Click here for a list of the parts required to build an SC503, v1.0, Z180 Processor card for Z50Bus.

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 SC503, Z180 Processor card should look like.

SC503 Assembled

Resistors 2k2

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

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 10k

Fit and solder the 10k resistor R10.

Resistor 1k

Fit and solder the 1k resistor, R11.

Header sockets

Fit and solder the 6-pin angled header sockets, P2, P6, and P10.

Ensure they are fitted tight to the board.

Header pins (2-pin angled)

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

Fit and solder the 2-pin angled headers, P4 and P8.

Header pins (3-pin angled)

This may need to be cut from a longer strip using wire cutters to cut the plastic.

Fit and solder the 3-pin angled header, JP1.

Header pins (6-pin angled)

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

Fit and solder the 6-pin angled headers, P1, 56, and P9.

Ensure they are fitted tight to the board and the pins are parallel to the surface of the 6-pin angled sockets.

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 8x10k resistor network, RP1.

The 10k network will be marked 103.

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.

Oscillator 18.432 MHz

Fit and solder the 18.432 MHz oscillator, X1. 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.

PLCC socket

Fit and solder the 68-pin PLCC socket for U4.

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 green

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.

Capacitors 100 nF

Fit and solder capacitors C1 to C7.

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 30 to 200 nF is acceptable.

Header pins (straight)

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

Fit and solder the pin header JP2 plus PJ3 (1 row of 4 pins), shown below in red.

Fit and solder the pin headers P3 and P7 (1 row of 2 pins), shown below in green.

Light Emitting Diode (LED)

Fit and solder green LED (LED1) 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).

Bus connector

Fit and solder the bus connector, P11. 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.

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 (supply lines)

Repeat the check made earlier for a short on the power supply tracks. Connect the meter probes to IC U5 pin 20 (shown below in red) and U5 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.

Quick Tests (voltages)

Fit SC503 to a backplane, such as SC501. If the backplane does not have a built in power supply and reset circuit, then you will also need to fit a power/reset card, such as SC502.

  • The voltage measured from U5 pin 20 (red) and U5 pin 10 (green) should be between 4.75 and 5.25 volts.
  • Test the status LED by touching one end of a piece of wire to U6 pin 5 (medium blue) and the other end to U6 pin 7 (dark blue).
  • The voltage measured from U6 pin 7 (dark blue) to U6 pin 1 (light blue) should be between 4.00 and 5.25 volts.
  • The voltage measured from U6 pin 7 (dark blue) to U6 pin 1 (light blue) should be between 0.00 and 0.20 volts when the reset button is pressed.
  • With a jumper shunt fitted to JP1 in the “write protect flash” position, check the voltage form U1 pin 31 (light orange) to U1 pin 16 (dark orange) is between 4.50 and 5.25 volts.

Integrated Circuits

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

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.

Insert the other 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.

Quick Test (reset)

Fit a jumper shunt in the positions shown below in red. This will write protect the Flash memory.

Fit SC503 to a backplane, such as SC501. If the backplane does not have a built in power supply and reset circuit, then you will also need to fit a power/reset card, such as SC502.

If your system includes a voltage supervisor and reset device, such as the one included in the power and reset card SC502, then when turned on the status LED should light for a fraction of a second, blink off for about half a second, then light again. This indicates the self test has passed.

If your system does not include a voltage supervisor and reset device, you may need to press the reset button after turning the system on.

If the status LED does not perform as indicated above, check the power supply voltage is between 4.75 and 5.25 volts, as detailed above.

Brief Test (serial port)

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 startup message.

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 all is well set the terminal software to use hardware flow control and check you still see the startup message. It is recommended that hardware flow control be used where possible.

Getting Started

The SC503 User Guide can be found here.

Other information about SC503 can be found here.

Homebrew 8-bit retro computing