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.

Resistors

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

Resistors

Sockets (6-pin)

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.

Serial header

Capacitor 100 nF (0.1 µF)

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.

Crystal

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

Capacitors 22 pF

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

Resistor network

Jumper pins

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.

Oscillator

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

Bus connector

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.

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.

Test

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.

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.