SC521, v1.0, Assembly Guide

Click here for a list of the parts required to build an SC521, v1.0, serial card for Z50Bus.

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 circuit.

The picture below shows what a completed SC521 serial card should look like.


Fit and solder the eight 2k2 resistors, R1 to R4 and R7 to R10 (shown below in green).

Fit and solder the four 100k resistors, R5, R6, R11, and R12 (shown below in red).

Fit and solder the 10k resistor, R13 (shown below in blue).

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


Fit and solder the 1k (1000 ohm) resistor R14 (shown below in green).

Fit and solder the 1M resistors R15 (shown below in red).

Sockets (6-pin)

Fit and solder the 6-pin angled sockets, S1 and S2.

Ensure the socket lays flat against the circuit board.

IC sockets

Fit and solder the IC sockets for U1 to U5 (shown below in red).

You may wish to fit a socket for oscillator X1 (shown below in blue). My preference is to solder this component as I like low-profile boards.

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.

Serial header

Fit and solder the serial port header pins, P2 and P3.

You can either fit these as a single continuous strip, by pulling out the unwanted pins with pliers, or you can cut the strip into the correct side for each header.

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.


Fit and solder the 7.3728 MHz crystal X2.

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

Capacitors 22 pF

Fit and solder 22pF capacitors C7 and C8.

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

Resistor network

Fit and solder the 8x100k resistor network RN1 (shown below in red).

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.

Jumper pins

Fit and solder jumper header pins, JP3, JP4, and JP7 (shown below in blue).

Fit and solder jumper header pins, JP1 and JP2 (shown below in red).

Fit and solder header pins, JP5 and JP6 (shown below in green).

These may need to be cut from longer strips. The plastic is easy to cut with long-nosed wire cutters.

Header JP1 is the 6 address select jumpers that are labelled with their bit numbers. The card occupies 4 addresses, thus address bits 0 and 1 are not included in the address decoding. The board includes placeholders for bits 0 and 1 for consistency with other cards in the SC500 series. It is suggested that you remove or cut the pins for bits 0 and 1 as a reminder that they are not used.


This part is optional and is not included in the kit.

If you decided to solder the 7.3728 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.

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.

If you fitted a socket for the oscillator, X1, fit the oscillator now.


Fit a jumper shunts in the positions shown below in red. These select the default address and clock source.

If the system is to be powered from one of the serial ports, fit a jumper shunt in one of the positions indicated below in green. The default port is normally port A, so port A is the most likely to require a jumper shunt.

Do not attempt to power the system from two different sources. If the system is not being powered from a serial port, then only fit a jumper shunt to JP2 if the serial device is being powered from the system.

The jumpers shown in blue are optional. They are only needed if you wish to connect one of the serial ports to the Z50Bus RX and TX lines. To connect serial port A to the bus lines, fit the shunts in the positions shown below in blue.

Default jumper shunt positions

You are now ready to play!

Z80 systems with the Small Computer Monitor in ROM:

Plug this card and a Z80 processor card, such as SC516, into a suitable backplane. Connect an FTDI style serial to USB adapter from serial port A (S1 or P2) to a computer running a terminal emulation program. The terminal should be configured for 115200 baud, 8 data, 1 stop, no parity. Flow control can be either Off or hardware RTS/CTS.

Turn the power on to the system. The terminal should show something similar to the illustration below. Check the SC521 user guide for more information.

Z180 systems with RomWBW:

This card is not supported in the 3.0.1 release version of RomWBW, but you can download the latest pre-release version from the development branch and configure it for this card. Later release versions of RomWBW should include support for this card. Check the SC521 user guide for more information.

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.

Power the system from the backplane or serial port and check the supply voltage on this circuit board between, say, U3 pin 7 and U3 pin 14. This should be 4.5 to 5.5 volts, preferably 4.75 to 5.25 volts.

If your system is running the Small Computer Monitor and you have a means of displaying the self-test results, typically an LED output, check if the serial port has been detected. See the processor card and SCM documentation for details.

If the serial port has not been detected, the problem is probably one of the signals from the Z50Bus to the SIO.

If the serial port has been detected, the main bus connections and SIO chip are probably good. Remove the card from the backplane. Connect a powered FTDI style cable to port B header (S2 or P3) and fit jumper shunts to JP2. Now check these voltages on port A header (S1 or P2):

  • Pin 1, GND, less than 0.1 volts
  • Pin 2, /RTS, more than 4.5 volts
  • Pin 3, 5V, more than 4.5 volts
  • Pin 4, RXD, more than 4.5 volts
  • Pin 5, TXD, more than 4.5 volts
  • Pin 6, /CTS, less than 0.4 volts

If you have a logic probe or oscilloscope, check the output of the oscillator (X2). This should be 7.3728 MHz.

Homebrew 8-bit retro computing