System Interface; Timing And Synchronization - Aprilaire 8800 Programmer's Manual

SySTEM INTERfACE

The System Network communication interface is a non-isolated, 4-wire, full-duplex RS-485 communication link . The 'A' terminals are transmit
(reference automation system) and the 'B' terminals are the receive pair (reference automation system) . The baud rate is 9,600 bps by default .
It can be changed using the user interface (See Setup Screens) or through a command from the Host controller . When the baud rate is changed,
the protocol timing is scaled as well .

TIMINg AND SyNCHRONIzATION

The protocol uses time division multiple access (TMDA) to allow each node to have exclusive control of the receive lines of the Host controller
for a specified duration .
The communication frame is divided into a maximum of 64 time slots . The number of time slots are based on the setting for number of
thermostats on the network . The default setting is 64 . Each slot is divided into four sub-slots . Legacy timing is based on 16-bit timer running at
1MHz (e .g . 65536 * 1/1MHz = 65 .536 ms) .
baud Rate
9,600
19,200
The four sub-slots of a slot are:
Sub-slot Name
1 Solicited Response Sub-slot
2 Unsolicited Message Sub-slot
3 Expansion Sub-slot 1
4 Expansion Sub-slot 2
The following figure depicts the communication frame and slot divisions . (NOTE: Timing shown is for 9,600 bps and is rounded for illustrative purposes . )
Slot 1 Slot 2
262 ms
Solicited Response Sub-slot
65 ms
2
Slot Width
262 .144 ms
131 .072 ms
Description
For a node to respond to a Host command
For a node to send an unsolicited message to the Host
Reserved
Reserved
Communication frame
Slot 3 Slot 4
Unsolicited Message Sub-slot
65 ms
Sub-slot Width
65 .536 ms
32 .768 ms
Slot 5
•••
Expansion Sub-slot 1
65 ms
Slot
Slot 63 Slot 64
Expansion Sub-slot 2
65 ms