User Manual

“Halo” System: Communication Protocol 970-00002-000
EXI Wireless Systems, Confidential Page 26 of 33 July 27, 1999
7. EIA-485 Communication
Information between Controller and the Host computer are exchanged across twisted pair cable driven by
EIA-485 transceivers. The Controller can not communicate directly to the Host but rather through the RIM
which is a network adapter for the Controller. On the Host side, the PC has an EIA-485 network card to
make this link compatible.
7.1. General
The EIA-485 is a hardware specification which determines physical layer only, and doesn’t specify
communication protocol used but imposes some limitations to that protocol in regards to maximum
communication speed, maximum number of nodes allowed on the bus and how these nodes could be
interconnected.
The maximum communication speed and the distance across these nodes are communicating, are closely
linked together and by increasing one parameter, the other must be decreased.
The maximum number of nodes is determined by electrical properties of transceivers and the maximum
number today is 256. These nodes must be linked following a bus topology and the bus must be terminated
at each end to avoid reflections which could significantly degrade network performance.
7.2. Network performance
The 485 network used by the Halo system can operate on three different speeds: 9600, 19200 and 57600
baud. This is selectable by a jumper in the RIM on the one side, and through the Halo program that is
running on the Host PC on the other.
Maximum of 99 nodes is allowed on the bus and the Host is a master. The Host is polling slave nodes in a
round robin fashion. As a consequence of this type of communication, higher number of nodes means that
the system response will be slower.
The system response will slow down even more if the Host software is configured for the non-existing
nodes or if some nodes on the bus are powered down or unable to communicate. For every node that
doesn’t respond to the Host’s request (for any reason), the system response is prolonged approximately 600
ms (the Host waits that long for the response).
If the node responds on the Host’s request, the next request (for the next node) is sent after 55 ms (55 ms is
internal PC tick and messages can not be sent more often). This indicates that expected polling rate is 110
ms per device in a case of reliable communication.
If the Host upon receiving the message sent by the RIM sends “Ack” for that message, but that “Ack” is not
received by the RIM, next time when the Host requests the message from the RIM, the RIM will send the
same message. This could cause additional delay for the message to be displayed on the computer screen.