Matrix Orbital VK204-25 Legacy User Manual

Page 19

NAK to indicate that the transmission is over.

The VK204-25-USB has some speed limitations, especially when run in I

C mode. Here are some

considerations when writing I

C code:

* to be able to read the replies of query commands (eg. cmds 54, 55) the following command must be

sent (only needs to be sent once, so this can be done somewhere in init): 254 / 160 / 0 this command puts
the reply data in the I

C output buffer instead of the RS232 output buffer. Please note that due to a 16 byte

output buffer, query commands that reply with more than 16 bytes cannot be read (eg cmd Get FileSystem
Directory)

* 3ms delay between the read commands
* 625us delay in between data bytes within a transaction is necessary
* 375us between transactions is necessary

NOTE

These delays are consrevative, and may be decreased based on performance

4.1.2

C Transaction Example

The typical I

C transaction contains four parts: the start sequence, addressing, information, and stop

sequence. To begin a transaction the data line, SDA, must toggle from high to low while the clock line, SCL,
is high. Next, the display must be addressed using a one byte hexadecimal value, the default to write to the
unit is 0x50, while read is 0x51. Then information can be sent to the unit; even when reading, a command
must first be sent to let the unit know what type of information it is required to return. After each bit is sent,
the display will issue an ACK or NACK as described above. Finally, when communication is complete, the
transaction is ended by toggling the data line from low to high while the clock line is high. An example of
the use of this algorithm to write a simple “HELLO” message can be seen in 3.

Table 3: I

C Transaction Algorithm

START

Toggle SDA high to low

Address

0x50

Information

0x48 0x45 0x4C 0x4C 0x4F

STOP

Toggle SDA low to high

4.1.3

Serial Communication

In addition to being able to communicate via I

C the VK204-25-USB communicates natively through

the RS-232 protocol at at a default baud rate of 19,200 bps and is capable of standard baud rates from 9600 to
115,200 bps. Furthermore the VK204-25-USB is also capable of reproducing any non-standard baud rate in
between using values entered into our baud rate generation algorithm and set through command 164 (0xA4).
The display module communicates at standard voltage levels of -30V to +30V or at TTL levels of 0 to +5V
by setting the Protocol Select Jumpers to TTL.

Matrix Orbital

VK204-25-USB