SC134 is a simple LED output module designed for the RC2014 bus.

The LEDs show the state of all 8 bits of the selected output port. This makes the module a useful debugging tool as well as providing some LED eye candy.

If the port address is set to the system’s status display port, the LEDs will display status information, such as self-test and disk activity. For a typical RC2014 system this address is zero (hexadecimal 0x00). To set the address to zero, do not fit any address jumper shunts.

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 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 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.

Writing to the output LEDs

The outputs can be written 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:

OUT 0,1

Where ‘1’ is the required decimal value to be written to the output port.

From SCM:

O 0 1

Where ‘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.