SC705 – RCBus Serial ACIA Module

SC705 is a serial port using the 68B50 ACIA with a fixed baud rate of 115200 baud.



Nothing known


KitsWebsiteShips from
Stephen C CousinsTindieUK
PCBsWebsiteShips from
Stephen C CousinsTindieUK
Assembled and TestedWebsiteShips from
Not available
See parts list

Parts List

PCB1SC705, v1.0, PCB
C1 to C44Capacitor, ceramic, 100 nF
C50There is no C5
C6 and C72Capacitor, ceramic, 22 pF
JP11Header, male, 2 row x 8 pin, angled
JP2+3+7+81Header, male, 2 row x 9 pin, angled
J4+5+61Header, male, 1 row x 12 pin, angled
Jumper18Jumper shunt
LED1 and 22LED, green, 3mm, angled
P11Header, male, 2 row x 40 pin, angled
P21Header, male, 1 row x 6 pin, angled
R1 to R44Resistor, 2k2, 0.25W
R5 and R62Resistor, 100k, 0.25W
R71Resistor, 1M, 0.25W
R8 to R103Resistor, 1k, 0.25W
RP11Resistor network, 8x100k, SIL, 9-pin
S11Header, female, 1 row x 6 pin, angled
Screw (for spacer)1Machine screw, 6mm, M3
Spacer1Spacer, 10mm, M3, nylon
U2168B50 ACIA
X11Crystal, 7.3728 MHz
IC socket 20-pin
1Socket, DIP, 20-pin
IC socket 24-pin
1Socket, DIP, 24-pin
IC socket 14-pin
1Socket, DIP, 14-pin

Printed Circuit Board

Note: The PCB is designed to allow a DIP switch to be fitted instead of the header pins JP1.


This module conforms to the RCBus specification v1.0 for RCBus-2014, RCBus-Z80 and RCBus-68xx.

User Guide

The serial port provides bidirectional asynchronous serial communications usually at 115200 baud, 8 data bits, 1 stop bit, no parity. RTS/CTS handshaking provides hardware flow control in both directions. The interface is at 5-volt (TTL) levels.

The RCBus I/O port address for this module is set in binary with JP1. Fit a jumper shunt to each bit position where the address bit should be a logic ‘1’ (high voltage).

The default address is 128 decimal, which is 0x80 in hexadecimal and 1000000 in binary. To set this as the address requires a jumper shunt in bit position 8, as illustrated below.

Default RCBus address selection (decimal 128, hexadecimal 0x80, binary 10000000)

The module occupies two addresses so bit zero of the address can be in either state. The address jumper bit zero is labelled ‘x’ to indicate this.

Jumper JP2 connects the module’s 5-volt supply to the serial port. A jumper should should be fitted here if the RCBus system is being powered from this serial port or if the device connected to this serial port needs to be powered from the RCBus system.

Jumper JP3 enables the ACIA’s interrupt output to be connected to the INT, INT1 or INT2 bus signals. The default position is INT.

Default interrupt selection (INT)

Jumper JP4 enables selection of the clock source for the ACIA. This can either be the on-board 7.3728 MHz oscillator or the RCBus clock signal (CLK). Selecting the on-board clock means the serial port baud rate is independent of the processor speed.

JP5 and JP6 enable selection of the ACIA’s R/W and E inputs. Normally these are set to the IORQ and WR signals that are common to all implementations of the RCBus. Alternative, if the processor supports native R/W and E signals then these can be selected instead.

Default jumper positions for JP4, JP5 and JP6

JP7 and JP8 enable the ACIA’s RXD and TXD signals to be connected to the RCBus RX and TX pins, and the ACIA’s CTS and RTS signals to be connected to the RCBus RX2 and TX2 pins. To connect all of these signals, fit jumper shunts in the positions illustrated below.

TXD, RXD, RTS and CTS connected to the RCBus signals TX1, RX1, TX2 and RX2

It is also possible to connect the ACIA’s RXD and TXD signals to the RCBus RX2 and TX2 pins. This is achieved by fitting one pair of jumper shunts in the middle positions, as illustrated below.

TXD, RXD connected to the RCBus signals TX2 and RX2
Input/output port functions
RCBus I/O Address ReadWrite
Configurable *10x800x80
  1. The RCBus I/O address should be set to match the software you are using. Typically, this is 0x80 or 0x40.
Jumper options
JP 1Set SC705’s RCBus I/O address
JP 2Connects 5-volt power to serial port
JP 3Selects which interrupt signal is generated
JP 3.1 Interrupt goes to RCBus INT signal
JP 3.2 Interrupt goes to RCBus INT1 signal
JP 3.3 Interrupt goes to RCBus INT2 signal
The default is INT
JP 4Select the clock source
JP 4.1-2 Clock source is the RCBus CLOCK signal
JP 4.3-4 On-board 7.2728 MHz oscillator
If jumper shunts are fitted in both positions the on-board oscillator supplies the RCBus CLOCK signal
The default is the on-board oscillator
JP 5Select the E signal source
JP 5.1-2 RCBus pin 38 (for use with 68xx processors)
JP 5.2-3 RCBus nIORQ (inverted)
The default is the inverted nIORQ signal
JP 6Selects to R/W signal source
JP 6.1-2 RCBus pin 39 (for use with 68xx processors)
JP 6.2-3 RCBus nWR signal
The default is the nWR signal
JP 7Connects the RXD input and the CTS input to the bus
JP 7.1-2 Connects RXD to bus RX(1) signal
JP 7.3-4 Connects CTS to bus RX2 signal
JP 8Connects the TXD output and the RTS output to the bus
JP 8.1-2 Connects TXD to bus TX(1) signal
JP 8.3-4 Connects RTS to bus TX2 signal

Assembly Guide

Below is the suggested order of assembly. A general guide to assembling circuit boards can be found here.

  1. Resistors R1 to R10
  2. Socket S1
  3. Decoupling capacitors C1 to C5
    These can be fitted either way around
  4. Resistor network RP1
  5. IC sockets for U1 to U3
    Ensure the notch in the socket matches the marking on the PCB
  6. Capacitors C6 and C7
  7. Header pins JP4, JP5 and JP6
    Fit as one strip with pins 4 and 8 removed
  8. Header pins P2
  9. Header pins JP1
    You may wish to remove the pins for address bit zero as the module occupies two I/O addresses and therefore bit zero is not used
  10. Header pins JP2, JP3, JP7 and JP8
    Fit as one strip with pins 2 removed
  11. Bus header pins P1
  12. LED1 and LED2
    The angled LEDs in the kit only fit one way around, but standard LEDs need to have the short lead in the hole marked with a flat line
  13. Crystal X1
  14. Fit the IC’s in their sockets
    Ensure the notch in the IC matches the PCB and IC socket
  15. Fit jumper shunts in the default positions (see below)
  16. Fit the nylon spacer in the mounting hole
Default jumper positions


  • 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.

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