In a TDMA system, all thermostats and the Host controller must be synchronized to ensure proper timing for control of the Host controller’s
receive lines . This is accomplished by synchronizing on the receipt of the Carriage Return <CR> character . Upon receipt of the <CR> character,
each connected node resets its communication timer, slot counter and frame counter, regardless of message syntax or content . The node may not
begin sending unsolicited messages until a carriage return is received and synchronization has occurred . In addition, the Host controller transmits
a <CR> every 12 hours to ensure accurate synchronization . Also, upon completion the last slot (maximum address), the Communication Frame
resets, setting the slot counter to 1, and the cycle repeats . The following figure depicts slot timing with and without receipt of a carriage return .
Slot 1 Slot 2 Slot 3 Slot 4 Slot 5
•••
Slot 63 Slot 64 Slot 1 Slot 2 Slot 3 Slot 4 Slot 1 Slot 2 Slot 3 Slot 4
•••
t = 0
Time
Slot counter is reset
after max address
slot is completed
Slot counter is reset
after Carriage Return
is received
NOTE:
The number of slots shown is based on the max number of thermostats and thus a max address of 64 . Applications with a lower max
node address restarts the slot counter after the max address slot has completed .
HOST MESSAgE TyPES
Host controller message transmissions are not governed by slot timing since the Host controller is the only device on its respective transmit line .
The Host controller does however need to meet specific timing to prevent missed commands and collisions . Command timing is determined by
the type of response as follows:
When a response to an Explicit Address Command is desired,
slot width + sub-slot width
time must elapse after issuing an Explicit Address
Command before issuing a second command to the same node .
When a response to a Global Address Command is desired,
slot width * max node address
time must elapse after issuing a Global Address
Command before issuing a second command when a response is desired .
RESPONSE ExPECTED
When no response to either an Explicit Address Command or Global Address Command is desired,
slot width + sub-slot width
time must
elapse after issuing either command before issuing a second command to the same node .
NO RESPONSE ExPECTED
NODE MESSAgE TyPES
There are three node message types . Each message type has a specific time for the node to transmit a message . The three message types are
as follows:
1 . Explicit Address Command Response
2 . Global Address Command Response
3 . Unsolicited Message
As shown in the figure below, during the n
th
slot the n
th
node responds to a global or explicit address command during the first sub-slot and/or
sends an Unsolicited Message during the second sub-slot .
Slot
n
Address
n
global or Explicit
Response Sub-slot
Address
n
Unsolicited
Message Sub-slot
Expansion Sub-slot 1
Expansion Sub-slot 2
3
