SC129 is a simple digital I//O module with 8 inputs and 8 outputs. Each input and each output has an LED to indicate its state.

This module should be compatible with all RC2014 systems and any system that claims to support the standard or enhanced RC2014-Z80 bus.

Address

Headers JP1 has 2 rows of 8 pins. These are the 8 address select jumpers and are labelled with their bit numbers.

The module responds to input/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). The module has tight address decoding and only occupies a single input/output port address.

The default address for an RC2014 is zero (0x00). 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.

Many of these modules can be used at the same time providing the addresses are all different.

Outputs

The outputs can be written from lots of software, including BASIC and SCM. 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:
OUT 0, 1
Where ‘0’ is the port address and ‘1’ is the required decimal value to be written to the output port.

From SCM:
O 0 1
Where ‘0’ is the port address and ‘1’ is the required hexadecimal value to be written to the output port.
Note: That’s the letter “O” and the numbers zero and one.

Outputs are driven from a 74HCT series logic chip running at 5 volts. Each output can sink (output low) up to 4 mA and source (output high) up to 2 mA (after allowing 2 mA for the output LED).

Inputs

The inputs can be read from lots of software, including BASIC and SCM. With the module’s address set to zero (no jumper shunts fitted), the input port can be read with the following commands.

From BASIC:
PRINT INP(0)
The result will be the input port value shown in decimal.

From SCM:
I 0
That’s the letter “I” and the number zero. The result will be the input port value shown in hexadecimal.

In both cases, the result should be zero if there are no external connections to header P2.

Inputs are connected to a 74HCT series logic chip running at 5 volts and also an LED with a current limit resistor. A high-level input should be between 2.0 volts and the supply voltage (nominally 5.0 volts). A low-level input should be between GND and 0.8 volts. A high-level input signal needs to be able to source at least 2 mA to drive the input LED.