Click here for a list of the parts required to build an SC125, v1.0, Serial and timer card.
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 SC125 Serial and timer card should look like.
Resistors
Fit and solder the eight 2k2 resistors, R1 to R8 (shown below in yellow).
Fit and solder the four 100k resistors, R9 to 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.
IC sockets
Fit and solder the IC sockets for U1 to U4 (shown below in yellow).
You may wish to fit a socket for oscillator X1 (shown below in red). 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, P3, JP1 and JP2.
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.
Bus header
Fit and solder the Z50Bus header pins, P1 (shown below in yellow).
Jumper pins
Fit and solder jumper header pins, JP3 and JP 4 (shown below in yellow).
Fit and solder jumper header pins, JP5 and JP6 (shown below in red).
These can be fitted as a single strip of 6 pins.
Capacitors
Fit and solder the six 100 nF capacitors, C1 to C6 (shown below in yellow).
Oscillator
If you decided to solder the 1.8432 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.
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.
With a suitable FTDI style TTL level serial to USB adapter connected from P2 to a powered USB socket, and a jumper shunt fitted to JP1, check the supply voltage on this circuit board between U3 pin 7 and U3 pin 14. This should be 4.5 to 5.5 volts, preferably 4.75 to 5.25 volts. Unplug the serial adapter.
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.
If you fitted a socket for the oscillator, X1, fit the oscillator now.
Test
Fit a jumper shunt to JP5 and another to JP6, in the positions shown below in red. These select the default address for the SIO and CTC chips.
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 yellow. The default port is normally port A, so JP1 is the most likely position to fit 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 JP1 or 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 red.
You are now ready to play!
Plug this card and a processor card, such as SC118, into a suitable backplane. Connect an FTDI style serial to USB adapter from serial port A (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.
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.
With a suitable FTDI style TTL level serial to USB adapter connected from P2 to a powered USB socket, check the supply voltage on this circuit board between, say, U3 pin 7 and U4 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 (P3) and jumper shunts to JP1 and JP2. Now check these voltages on port A header (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 (X1). This should be 1.8432 MHz.
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