User Manual

“Halo” System: Communication Protocol 970-00002-000
EXI Wireless Systems, Confidential Page 27 of 33 July 27, 1999
Figure 10 shows network traffic in a case of two devices on the bus with reliable communication.
Maximum number of nodes to be polled is 9 per second in ideal condition (all node responses are
received). “Host Request” message packet and “RIM” response packet are separated from each other 100 µs
only, and not as shown in the Figure 10.
7.3. Communication parameters
UARTS on both sides (Master and Slave) are configured for asynchronous full duplex mode. Byte format is
1 start bit, 8 data bits and 1 stop bit. Both UARTs transmit and receive LSb first. Standard communication
speed is 57.6 kbaud what refers to 17.36 µs per bit.
7.4. Message format
Messages are initiated by the Host computer and the RIM responds to most of them but not all. Messages in
both directions have the same format and the difference among them is in data they carry (different type and
length of data), but the message structure is the same.
Table 10 illustrates format of these messages.
Table 10 485 Message Format
Bytes Field Description
1 SOH Start of Header (01 Hex)
1 Length Length of message from (including) a message type to (including)
checksum.
1 Message Type Upper 2 bits: message number
Figure 10
HOST
REQUEST
1
RIM 1
RESPONSE
HOST
ACKNOWLEDGMENT
HOST
REQUEST
2
RIM 2
RESPONSE
HOST
ACKNOWLEDGMENT
HOST
REQUEST
1
55 ms 55 ms 55 ms 55 ms
FULL CYCLE: 2x110 ms = 220 ms