Modern PCs can support between one and (at least) four RS232 serial communication ports, either built in to the computer or by means of USB-to-serial converters. On PCs that have these ports built in, they are usually used to connect to printers, modems, or sometimes a mouse, and are commonly called COM1 to COM4 depending on how many there are on the PC. On PCs with USB ports, if you plug in a USB-to-serial converter, the RS232 port usually appears as one of COM1 to COM8 (you can click the Discover Ports button when configuring external bells or switches, to find out which port it is).

Each serial port has a number of pins to which you can connect external sensors - when the sensor changes the voltage on the pin, this causes a control signal in the PC. Abel can detect these control signals and use them to decide when a bell should ring. Up to four of these signals on each serial port are available, and Abel deliberately sets one additional signal high so that the pin can act as a voltage source, thus potentially removing the need for an external power supply to drive the sensors. Note however that this mechanism does not work on all computers, and even if it does work, an external power supply may still be needed if there is a long cable run between the computer and the sensors, of if the sensors use a lot of power.

The signal connections and their associated pin numbers for both 9 and 25 pin connectors are shown below:

Signal name	9 pin connector	25 pin connector
RTS (set high by Abel for use as a voltage source)	pin 7	pin 4
GND	pin 5	pin 7
These signals are used as input to Abel:
RLSD or DCD	pin 1	pin 8
DSR	pin 6	pin 6
CTS	pin 8	pin 5
RI	pin 9	pin 22

Sample circuit diagrams given in Tower Bells and Handbells show how these signals are used in practice.

See also:

  Connecting bells using the Multi-Bell Interface
  Tower bells as external bells
  Handbells as external bells
  Using external switches
  Configuring Abel to use bell sensors
  Configuring Abel to use control switches