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4.1.12 - Asynchronous Serial Communication [U3 Datasheet]

The U3 (hardware version 1.21+ only) has a UART available that supports asynchronous serial communication. On hardware version 1.30 the TX (transmit) and RX (receive) lines appear on FIO/EIO after any timers and counters, so with no timers/counters enabled, and pin offset set to 4, TX=FIO4 and RX=FIO5. On hardware version 1.21, the UART uses SDA for TX and SCL for RX.

Communication is in the common 8/n/1 format. Similar to RS-232, except that the logic is normal CMOS/TTL. Connection to an RS-232 device will require a converter chip such as the MAX233, which inverts the logic and shifts the voltage levels.

This serial link is not an alternative to the USB connection. Rather, the host application will write/read data to/from the U3 over USB, and the U3 communicates with some other device using the serial protocol. Using this serial protocol is considered an advanced topic. A good knowledge of the protocol is recommended, and a logic analyzer or oscilloscope might be needed for troubleshooting. Also consider that a better way to do RS-232 (or RS-485 or RS-422) communication is with a standard USB<=>RS-232 adapter/converter/dongle, so the user should have a particular reason to not use that and use a U3 instead.

There is one IOType used to write/read asynchronous data:


The following are special channels used with the asynch IOType above:

LJ_chASYNCH_ENABLE // Enables UART to begin buffering RX data.
LJ_chASYNCH_RX // Value= returns pre-read buffer size. x1= array.
LJ_chASYNCH_TX // Value= number to send (0-56), number in RX buffer. x1= array.
LJ_chASYNCH_FLUSH // Flushes the RX buffer. All data discarded. Value ignored.

When using LJ_chASYNCH_RX, the Value parameter returns the size of the Asynch buffer before the read. If the size is 32 bytes or less, that is how many bytes were read. If the size is more than 32 bytes, then the call read 32 this time and there are still bytes left in the buffer.

When using LJ_chASYNCH_TX, specify the number of bytes to send in the Value parameter. The Value parameter returns the size of the Asynch read buffer.

The following is a special channel, used with the get/put config IOTypes, to specify the baud rate for the asynchronous communication:

LJ_chASYNCH_BAUDFACTOR // 16-bit value for hardware V1.30. 8-bit for V1.21.

With hardware revision 1.30 this is a 16-bit value that sets the baud rate according the following formula: BaudFactor16 = 2^16 – 48000000/(2 * Desired Baud). For example, a BaudFactor16 = 63036 provides a baud rate of 9600 bps. With hardware revision 1.21, the value is only 8-bit and the formula is BaudFactor8 = 2^8 – TimerClockBase/(Desired Baud).

Following is example pseudocode for asynchronous communication:

//Set data rate for 9600 bps communication.
ePut(lngHandle, LJ_ioPUT_CONFIG, LJ_chASYNCH_BAUDFACTOR, 63036, 0);

//Enable UART.

//Write data.
eGetPtr(lngHandle, LJ_ioASYNCH_COMMUNICATION, LJ_chASYNCH_TX, &numBytes, array);

//Read data. Always initialize array to 32 bytes.
eGetPtr(lngHandle, LJ_ioASYNCH_COMMUNICATION, LJ_chASYNCH_RX, &numBytes, array);

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