The Cerebellum has a built-in serial port for communications with external devices or computers. A header withh TTL voltage levels and a DB9 connector with RS-232 voltage levels are available. It is necessary to jumper select between the TTL or DB9 connectors.

TTL vs RS-232 voltage levels

RS-232 signals are signals that range between -10 and +10 volts, and therefore require voltage conversion circuitry to interface to a microcontroller - on the Cerebellum, this is accomplished using an RS-232 level converter. Since these signals swing over a wider voltage range, they are less affected by signal noise, allowing RS-232 serial to operate over cable lengths of 30 meters.

TTL serial refers to serial communication using signals that range between 0 and 5 volts. This is sometimes used on sensors because no voltage conversion circuitry is necessary. TTL serial only operates over short distances.

Connecting via the DB9 serial port

Pin-out of the Cerebellum DB9 connector
In order to connect to the Cerebellum over the DB9 port, simply connect the provided serial cable between a computer and the Cerebellum. If you wish to connect to a device, such as the CMUCam, or another Cerebellum, you may need to place a null modem between the Cerebellum and the other device. A null modem effectively swaps Rx(receive) and Tx(transmit), such that one device's receive line is hooked up to another device's transmit line and vice versa. You may also create a null modem cable by swapping pins two and three between the two ends of the cable. A null modem is not necessary when communicating with a standard PC.


A null-modem connection

Connecting via the TTL serial header

	It is sometimes convenient to connect to other devices using TTL serial instead of RS-232. In order to do this, you just connect Tx of the Cerebellum to Rx of the device, and vice versa, as well as connecting ground, unless both devices already share a ground connection.
		Do not connect RS-232 level signals to the TTL serial header. Doing so will harm the Cerebellum serial port and may prevent future programming of and communications with the Cerebellum

Listening in on the serial port

It is possible to connect both TTL serial and RS-232 to listen in to what the Cerebellum is sending out. This may be useful when debugging a serial interface between the Cerebellum and a sophisticated sensor. If the sensor is connected over TTL serial, with the serial jumper set to TTL, you can connect the Cerebellum to a PC and monitor the Cerebellum's output. You will not be able to read the information coming in to the Cerebellum, only the information your program is sending out.