SC136 is a simple digital input module designed for the RC2014 bus.

• SC136 – Forum (rc2014)
• SC136 – Forum (retro-comp)
• SC136 – Kit contents sheet (PDF)
• SC136 – Schematic v1.0 (PDF)

Suppliers

Kits	Website	From	Currency
Small Computers Direct	SCDirect	UK	GBP
Stephen C Cousins	Tindie	UK	USD
PCBs	Website	From	Currency
Small Computers Direct	SCDirect	UK	GBP
Stephen C Cousins	Tindie	UK	USD
Assembled and Tested	Website	From	Currency
Not available
Components
See parts list

Description

SC136 is a low profile digital input module designed for the RC2014 bus.

The main features of this design are:

• 8 digital inputs accessible via header pins
• 1 digital ‘read’ pulse accessible via header pins
• Input port address configurable with jumpers
• Simple operation with BASIC’s INP() statement
• Low profile RC2014 style design

User Guide

SC136 is a simple digital input module designed for the RC2014 bus.

The module reads the state of all 8 bits of the selected input port from a pin header. This makes the module useful for interfacing to external electronics.

Port address selection

The port address is set with jumper shunts which are fitted to the header pins indicated below.

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

The module responds to input 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).

The default address for an RC2014 is zero (hexadecimal 00, binary 00000000). This is selected by having no jumper shunts fitted.

If, for example, you wish to set the module to address 32 decimal, which is hexadecimal 20 or binary 00100000, then fit a single jumper shunt to bit 5, as illustrated below.

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.

As a result of address bit zero being ignored, this module responds to two addresses. The addresses are determined by the 7 upper bits plus bit zero being either high or low. Thus, if the jumper is set to address 32, the module responds to address 32 and 33.

Reading from the digital outputs

The inputs can be read from BASIC or from the Small Computer Monitor. With the module’s address set to zero (no jumper shunts fitted) the input port can be read with the following commands.

From BASIC:

INP(0)

From SCM:

I 0

Note: That’s the letter “I” and the number zero.

Parts List

Reference	Qty	Component
PCB	1	SC136, v1.0, PCB
C1 and C2	2	Capacitor, ceramic, 100 nF
Jumper shunts	7	Jumper shunt
P2 and P3	1	Header, male, 1 x 13 pin, angled
JP1	1	Header, male, 2 x 8 pin, angled
RP1 and RP2	2	Resistor pack, 8 x 10k, SIL, 9-pin
U1	1	74HCT688
U2	1	74HCT245
IC socket 20-pin U1 and U2	2	20-pin PDIP socket

Printed Circuit Board

Links

SC136, v1.0, PCB only (Tindie)

SC136, v1.0, Complete kit (Tindie)

SC136, v1.0, PCB design files (OSHWLab)

SC136, v1.0, Gerber files (ZIP)

Assembly Guide

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.

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 SC136, digital input module should look like.

Bus connector

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.

Capacitors 100 nF (0.1 µF)

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.

Resistor network

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 double row angled header pins or double row straight pins in position JP1.

Header (angled, single row)

You can either fit P2 and P3 as two separate strips, a 10-pin strip and a 2-pin strip, or you can use a 13-pin strip by removing the 11th pin. Unwanted pins can easily be pulled out of the plastic with a small pair of pliers. Cutting a long strip into shorter pieces can be done with a pair of wire cutters.

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 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.

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

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 JP1 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 output 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 32 decimal, which is hexadecimal 20 or binary 00100000, then fit a single jumper shunt to bit 5.

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.

Testing inputs

Connect the digital input module (SC136) to an RC2014 or compatible system and power up.

The inputs can be tested from BASIC or from the Small Computer Monitor. With the module’s address set to zero (no jumper shunts fitted) the output port can be written to with the following commands.

From BASIC:
PRINT INP( 0)

From SCM:
I 0
Note: That’s the letter “I” and the number zero.

Notes

• This design is made in accordance with the “designed for RC2014” labelling scheme.
• RC2014 is a trademark of RFC2795 Ltd.
• This product is designed for hobby use and is not suitable for industrial, commercial or safety-critical applications.
• The product contains small parts and is not suitable for young children.