Low Cost ARCNET (ANSI 878.1) Controller with 2K x 8 On-Board RAM
activity, the COM20019I starts sending INVITATION TO TRANSMIT with the Destination ID (DID) equal to
the currently stored NID. Within a given network, only one COM20019I will timeout (the one with the
highest ID number). After sending the INVITATION TO TRANSMIT, the COM20019I waits for activity on
the line. If there is no activity for 597.6 S, the COM20019I increments the NID value and transmits
another INVITATION TO TRANSMIT using the NID equal to the DID. If activity appears before the
597.6 S timeout expires, the COM20019I releases control of the line.
During NETWORK
RECONFIGURATION, INVITATIONS TO TRANSMIT are sent to all NIDs (1-255).
Each COM20019I on the network will finally have saved a NID value equal to the ID of the COM20019I
that it released control to. At this point, control is passed directly from one node to the next with no wasted
INVITATIONS TO TRANSMIT being sent to ID's not on the network, until the next NETWORK
RECONFIGURATION occurs. When a node is powered off, the previous node attempts to pass the token
to it by issuing an INVITATION TO TRANSMIT. Since this node does not respond, the previous node
times out and transmits another INVITATION TO TRANSMIT to an incremented ID and eventually a
response will be received.
The NETWORK RECONFIGURATION time depends on the number of nodes in the network, the
propagation delay between nodes, and the highest ID number on the network, but is typically within the
range of 192 to 488 mS.
4.4
4.5
BROADCAST MESSAGES
Broadcasting gives a particular node the ability to transmit a data packet to all nodes on the network
simultaneously. ID zero is reserved for this feature and no node on the network can be assigned ID zero.
To broadcast a message, the transmitting node's processor simply loads the RAM buffer with the data
packet and sets the DID equal to zero. Figure 4 illustrates the position of each byte in the packet with the
DID residing at address 0X01 or 1 Hex of the current page selected in the "Enable Transmit from Page
fnn" command. Each individual node has the ability to ignore broadcast messages by setting the most
significant bit of the "Enable Receive to Page fnn" command to a logic "0".
EXTENDED TIMEOUT FUNCTION
There are three timeouts associated with the COM20019I operation. The values of these timeouts are
controlled by bits 3 and 4 of the Configuration Register and bit 5 of the Setup 1 Register.
4.5.1 Response Time
The Response Time determines the maximum propagation delay allowed between any two nodes, and
should be chosen to be larger than the round trip propagation delay between the two furthest nodes on
the network plus the maximum turn around time (the time it takes a particular COM20019I to start sending
a message in response to a received message) which is approximately 101.6 μS. The round trip
propagation delay is a function of the transmission media and network topology. For a typical system
using RG62 coax in a baseband system, a one way cable propagation delay of 248 μS translates to a
distance of about 32 miles. The flow chart in Figure 3.1 uses a value of 597.6 μS (248 + 248 + 101.6) to
determine if any node will respond.
4.5.2 Idle Time
The Idle Time is associated with a NETWORK RECONFIGURATION. Figure 3.1 illustrates that during a
NETWORK RECONFIGURATION one node will continually transmit INVITATIONS TO TRANSMIT until it
encounters an active node. All other nodes on the network must distinguish between this operation and an
entirely idle line. During NETWORK RECONFIGURATION, activity will appear on the line every 656 μS.
Rev. 03-07-06
Page 12
SMSC COM20019I
DATASHEET
SMSC [ SMSC CORPORATION ]
