2nd serial port =============== Frequently there is the need to connect a BBC micro or Master to the serial port on a PC for example, to transfer files or other data back and forth. This ties up the serial port leaving you with no spare port to connect other peripherals or communications equipment. To solve this a second serial port card is needed, which simply connects to the 1MHz bus under the beeb. PARTS... The circuit diagram shows electrically how to assemble the unit, requiring the following parts: 1x MAX232 DIL serial line driver 1x 6850 ACIA 1x 2.4576MHz TTL oscillator module, 14 pin DIL outline 1x 220uF capacitor 5x 22uF 5mm diameter electrolytic capacitor 1x 5 pin domino DIN socket, panel mounting 1x 4075BE 1x 74LS02 1x 74LS04 1x 74LS32 2x 4512B mux 1x 4040B divider 1x 40 pin 0.1" pitch header 1x 40 way PCB mount IDC cable terminator 1x 40 way length of IDC cable 1x 40 way IDC socket 4x 9 pin SIL 2.2k resistor packs (plus optional sockets) In addition, you'll need a low current power supply. It must be capable of supplying +5v. BENEFITS... *A "pass through" connector so that you can continue to use any other 1MHz bus devices such as a Music 5000 synthesiser or Teletext unit *Uses the same serial chip as the main motherboard serial port which means excellent compatibility with exisiting software *Receive/transmit rates independantly adjustable between 75 and 19200 baud BUILD... 1.Place all but the 6850 and MAX232 (and associated passive components) onto the board and power it up 2.Use the supplied software to set the serial baud rate to some value between 75 and 19200 baud 3.Verify that this is the case by measuring the frequency of the signal from pin 2 of the 4512 mux. It will be 16 times the programmed rate. 4.Repeat this for the transmit clock as well as the receive clock 5.Lastly, add the 6850 and MAX232 line driver and remaining passives. DRIVER... The software is simplicity itself - install the "2ndserial" ROM either temporarily in sideways RAM or by blowing the image into EPROM. From then on, it behaves exactly as per the internal port provided you prefix with the command *COM2. For example *COM2 *FX7,6 *FX8,4 Further detail is given in the "2ndserial" ROM user guide for programmers. HINTS... *You could use the +5v power from the main BBC micro and fit the board internally to the case - using the often unsed Econet interface socket hole at the rear for the second serial port *Socket the MAX232 line driver, as this is potentially the most likely part to be damaged by plugging in wrongly wired serial cables - this allows it to be easily replaced. *Only add the 2.2k line termination to the address and data lines if the second serial port is the last item in the chain of devices on the 1MHz bus. In this case, ensure the termination has been removed from the other devices in the chain. *A newer MAX232A allows smaller capacitors for the charge pump, so the 22uF components could be downsized to 1uF for example. FROM... Robert Sprowson. Happy soldering!