STANDARD SERIAL COMMUNICATION INTERFACE and ...

Standard Serial Interface Protocol Description
Where:
1. ACK = ASCII acknowledgment character, 06 (Hex)
2. Packet # = Packet number of the previously transmitted host message package, 0-255
possibilities.
3. Packet Checksum = Equals the least significant 8 bits of the summation of all elements contained
in message packet fields 1 and 2.
4. Carriage return = ASCII Carriage return, or 0D in hexadecimal.
5. Line feed = ASCII line Feed, or 0A in hexadecimal.
The negative response format would appear as follows:
Where:
Field Description NAK Packet # Packet
Checksum
Carriage
Return
Line
Feed
Packet Field # 1 2 3 4 5
Byte #/ Field 1 1 1 1 1
Table 4.2.2 Sign Negative Response Message Packet Format
1. NAK = ASCII acknowledgment character, 15 (Hex)
2. Packet # = Packet number of the previously transmitted host message package, 0-255
possibilities.
3. Packet Checksum = Equals the least significant 8 bits of the summation of all elements contained
in message packet fields 1 and 2.
4. Carriage return = ASCII carriage return, or 0D in hexadecimal.
5. Line feed = ASCII line Feed, or 0A in hexadecimal.
4.3 Host and Sign Module Communication Action Description
A master sign/display module will respond to a host command if and only if the address within the host
transmitted packet matches the hardware address setting on the respective sign module. A positive
response will be sent if the host command packet is valid and the checksum received matches the
checksum calculated by the master module controller. A negative response will be transmitted to the host
if a checksum mismatch occurs due to a communication median problem, or if the host command was
invalid.
If a negative response or no response is received within 3 seconds after a host command is issued, the
host should re-transmit the message packet to the sign or controller module. If after three consecutive
unsuccessful host transmissions, the host computer should assume the respective sign module is "offline",
and re-try communication at a later time.
4.4 Broadcast Messages
Broadcast messages are host transmitted messages where the host wants all signs or displays
connected or in the proximity of the host (if the communication median is a wireless link) to implement the
requested command within the message. Broadcast messages are typically used for less sophisticated
type display systems where unique address capability is not required or for transmitting safety warning
type messages, such as “Fire in Building, Please Evacuate”.
Broadcast messages are the only type of messages where the signs do NOT transmit a response packet
back to the host computer. If the host packet received at the sign is not corrupted (sign calculated
Standard Serial Communication Interface Description, © 1998-2002 Kent Displays Inc., doc. # 25016 Revision T, Page 7 of 52