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Week4902 -3-
Operational Protocol
The device operates as a stand alone LED display. The integral driver is fed information in the
correct bit format from the on board microcontroller. The controller handles the RS485
communication protocol with the host system. The host system must prepare the segment data that
it will send to the display in order to illuminate the relevant segments that make up a character.
Since the device can be daisy chained each unit has a number of setting jumpers that set the address
of the unit on the RS485 bus. The device will only capture the segment data that is destined for that
unit. This is done by monitoring the address of the unit sent from the host controller. The host must
send 5 bytes in order to update all of the 41/2 digits within a particular display. The format of the
data is as follows:
D7 D6 D5 D4 D3 D2 D1 D0
n/u lead 1 sign Add 4 Add 3 Add 2 Add1 Add 0 Address/half digit
dp GFEDCBADigit 4
dp GFEDCBADigit 3
dp GFEDCBADigit 2
dp GFEDCBADigit 1
Digit format is as follows:
Half Digit Digit 4 Digit 3 Digit 2 Digit 1
If D6 is 0 then the lead 1 is off, if D6 is 1 then the lead 1 is on.
If D5 is 0 then the -sign is shown, if D5 is 1 then the + sign is shown
The system is designed to work with a number of devices on the bus at the same time. This would
cause a flickering effect if each were updated when all LED’s were in the same field of view. To
avoid this problem the devices do not update until the host sends the universal UPDATE ALL
command. This is done by sending address 0Dh. When this address is seen by the displays the data
stored in the microcontroller from the bus will be transferred to the display driver chip. This means
that the address 0Dh cannot be used as an identifier (jumper address must not be set to 0Dh)
To set the address the jumpers need to be linked. The presence of a jumper set the relevant address
matching bit to a logic 1. If all jumpers are removed then the address is 0.
Jumper address is as follows: Note the units are set to addresses 10h to 1Ah after test.
A0
A1
A2
A3
A4
Note that the +/-segments are inverted from each other. If one is off then the other is on and vice
versa.
