SC139, v1.0, Assembly Guide

Click here for a list of the parts required to build an SC139, v1.0, Serial 68B50 module.

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 SC139, serial 68B50 module should look like.


Fit and solder the 2k2 resistors R1, R2, R3, and R4 (shown below in blue).

Fit and solder the 1k (1000 ohm) resistor R5 (shown below in green).

Fit and solder the 1M resistors R6 (shown below in red).

Fit and solder the 100k resistors R7 and R8 (shown below in yellow).

Bus connector

Fit and solder the bus header pins P1.

Serial port socket

Fit and solder the 6-pin female angled header socket P2.

Serial port pins

Fit and solder the 6-pin male angled header pins P3.

Make sure the pins are parallel to the top of the socket P2. If they point down towards the socket you may have trouble fitting a Dupont cable. A good tip is to fit a Dupont cable to the pins when soldering them. Press the Dupont cable housing agains the socket, P2, to ensure the pins are parallel.

IC sockets

Fit and solder the IC sockets U1 to U3.

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 pack

Fit and solder the 8x10k resistor network RP1 (shown below in red).

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.

Capacitors 100 nF (0.1 µF)

Fit and solder 100nF capacitors C1 to C4.

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.

Capacitors 22 pF

Fit and solder 22pF capacitors C4 and C5.

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


Fit and solder the 7.3728 MHz crystal X1.

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

Header (double row)

Address selection is via jumper shunts fitted to a double row of header pins. These can either be straight pins or angled pins. Angled pins stick out above the module allowing easy access to the jumper shunts when the module is fitted to a backplane. Straight pins look neater but do not allow the address to be changed when the module is fitted in a backplane with another module immediately in front of it.

Fit and solder the double row straight or angled header pins, JPA.

As only bits 7 to 1, not bit 0, are used to set the address, you may wish to remove the pins for bit zero. See the user guide for an explanation of this.

Headers (straight, single row)

Fit and solder the pin headers JP1 to JP5.

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

Quick Test

Repeat the check made earlier for a short on the power supply tracks. Connect the meter probes to IC U1 pin 20 (shown below in red) and U1 pin 10 (shown in green). This should not be a short circuit. A reading of more than 1 kΩ (1000 ohms) is acceptable.


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

If all the above tests check out okay, insert the integrated circuits into their sockets. Take care to insert them the right way around, as illustrated below. Be careful not to bend any legs over.

Address select jumpers

Header JPA is a block of header pins with 2 rows of 8 pins. These are the 8 address select jumpers and are labelled with their bit numbers.

The module responds to addresses matching the address set with these jumpers. When a jumper shunt is fitted, that bit must be a 1 (high voltage). When the shunt is not fitted, that bit must be a 0 (low voltage).

If, for example, you wish to set the module to address 128 decimal, which is hexadecimal 80 or binary 10000000, then fit a single jumper shunt to bit 7 (as illustrated below). This is the default address for a 68B50 ACIA.

The least significant address bit (bit 0) does not do anything with this design, but has been included as a place holder for consistency with other modules. You may wish to cut off the pins for bit 0 as a reminder that bit 0 is not used.

You are now ready to give it a try.

Set the module’s address by fitting a jumper shunt in the position illustrated above.


Connect the module (SC139) to an RC2014 or compatible system that does not have any other serial modules.

Connect a suitable serial cable to the serial port on this module. Pin 1 of the serial port is indicated below with a red arrow. This is the ground or zero volt pin.

Connect the other end to a computer running suitable terminal software, such as Tera Term. The terminal program should be set for 115200 baud, 8 data bits, 1 stop bit. Flow control is optional but initially it is recommended that flow control is turned off.

If the system is to be powered from the serial port, fit a jumper shunt to JP5 (indicated below in green).

Power up and reset the system. You should get a sign-on message on the terminal.

More information

The SC139 User Guide can be found here.

Other information about SC139 can be found here.

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