Matrix Orbital GLC24064 - Manual (Page 17)

2.2.4 ACK
The idea of ACK is to indicate when the data has been received correctly. ACK does not indicate data
incorrectly received. ACK simply fails to indicate when data is correctly received. Clearly, this is of limited
usefulness and even less so with Matrix Orbital modules. Matrix Orbital modules are not capable of failing
to acknowledge an incorrectly received byte in response to that byte’s transition. They are only capable
of failing to acknowledge the bytes following the byte, which were not received. To fully understand the
reasons for this one needs to understand something about how a Matrix Orbital module processes data.
Basically, the reason why a Matrix Orbital module might fail to receive a byte correctly is that it was unable
to process the byte previous before the failed byte was transmitted. Because the module cannot possibly
know that it would be unable to store the byte before the next byte was received, it cannot know to not ACK.
The reason for this situation in deference to situations one might be familiar with (i.e., memory chips, etc)
is that the Matrix Orbital module employs a micro-processor to perform these data storage functions. A
memory chip takes care of these things entirely in hardware subsystems which operate at the same speed as
the transmission themselves.
The display uses a standard Phillips 7bit address as defined by Phillips. However, Matrix Orbital specifies
I 2 C address in 8bits. The 8th bit, or least significant bit (LSB or Low Order Bit) of the 8bit address is read
/ write bit. If we take a standard Phillips 7bit address of 45hex this would be in binary 1000101 (this is
7bits). Matrix Orbital would describe the Phillips I
2 C address of 45hex as 8Ahex. The read address would
be 8Bhex.
For more information on Phillips I 2 C please visit;
http://www.ping.be/~ping0751/i2cfaq/i2cindex.htm
2.3 General Purpose Output
The general purpose output is provided to control relays or other devices via the display. This allows an
external device to be turned on or off using the controller and software commands.
The GPO is meant to be used as a pair. The positive side of the GPO is connected to a power source of
+5Vdc supplied by the module. This connection is via a 240 ohm resistor which limits the maximum current
to 20 mA. The negative side of the GPO is connected to ground.
If the device which is being driven by a GPO requires a relatively high current (such as a relay) and
has an internal resistance of its own greater than 250 ohms, then the 240 ohm resistor may be removed and
replaced with a jumper. This resistor is located directly to the left of the positive pin of the general purpose
output.
NOTE This operation requires soldering. With the resistor removed the GPO does not
have any over current or over / under voltage protection so care must be taken when using
the GPO. For instance, if the external device is a relay it must be fully clamped using a
diode and capacitor to absorb any generated back electro-motive force (EMF).
Matrix Orbital GLC24064 13