Jameco Electronics 2000, 3000 manual Transmitting and Detecting a Break

2.Clear bit 4 of the Parallel Port C function register so that the output no longer comes from the serial port. Of course, this should not be done until the transmitter is idle.

A similar procedure can be used if the serial port is set up to use alternate output pins on port D. Only Serial Ports A and B can use alternate outputs on Parallel Port D.

If an RS-485 driver is being used, dummy characters can be transmitted by disabling the driver after the stop bit has been sent. This is an alternative to the above procedure.

12.9.3 Transmitting and Detecting a Break

A break is created when the output of the transmitter is driven low for an extended period. If a break is received, it will appear as a series of characters filled with zeros and with the 9th bit detected low. This could only be confused with a legitimate message if a protocol using the 9th bit was in effect. Break is not usually used as a message in such protocols.

A break can be transmitted by transmitting a byte of zeros at a very slow baud rate. Another and probably better method is to disconnect the transmitter from the output pin, and use the parallel port bit to set the line low while sending dummy characters to time out the break.

The use of break as a signaling device should be avoided because it is slow, erratically sup- ported by different types of hardware, and usually creates more problems than it solves.

12.9.4 Using A Serial Port to Generate a Periodic Interrupt

A serial port may be used to generate a periodic interrupt by continuously transmitting characters. Since the Tx output via Parallel Port C or D can be disabled, the transmitted characters are transmitted to nowhere. Because the character output path is double-buff- ered, there will be no gaps in the character transmission, and the interrupts will be exactly periodic. The interrupts can happen every 9, 10 or 11 baud times, depending on whether 7 or 8 bits are transmitted and on whether the 9th (8th) bit is sent.

12.9.5 Extra Stop Bits, Sending Parity, 9th Bit Communication Schemes

Some systems may require two stop bits. In some cases, it may be necessary to send a par- ity bit. Certain systems, such as some 8051-based multidrop communications systems, use a 9th data bit to mark the start of a message frame. The Rabbit 3000 can receive parity or message formats that contain a 9th bit without problem. Transmitting messages with parity or messages that always contain a 9th bit is also possible. It is quite easy to do so for byte formats that use only 7 data bits, in which case the 9th bit or parity bit is actually an 8th bit. Sending a 9th low bit is supported by hardware. Sending a 9th bit as a high value requires a write to the Serial Port A-F Long Stop Register (SxLR) which is the same as two stop bits.

194 Rabbit 3000 Microprocessor