COM200221 Datasheet, PDF (13/82 Pages) SMSC Corporation – 10 Mbps ARCNET (ANSI 878.1) Controller with 2Kx8 On-Chip RAM

10 Mbps ARCNET (ANSI 878.1) Controller with 2Kx8 On-Chip RAM
Datasheet
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 6 to 15.3 mS.
4.4
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 5.7 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 (see Table 6.5) to a logic "0".
4.5
Extended Timeout Function
There are three timeouts associated with the COM20022I 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 COM20022I to start sending a
message in response to a received message) which is approximately 3.2 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 7.75 μS translates to a distance of about 1
mile. The flow chart in Figure 3.1Figure 2.1 uses a value of 18.7 μS (7.75 + 7.75 + 3.2) to determine if any
node will respond.
4.5.2 Idle Time
The Idle Time is associated with a NETWORK RECONFIGURATION. Figure 3.1Figure 2.1 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 20.5 μS. This 20.5 μS is equal to the Response Time of 18.7 μS plus the time it takes the
COM20022I to start retransmitting another message (usually another INVITATION TO TRANSMIT).
4.5.3 Reconfiguration Time
If any node does not receive the token within the Reconfiguration Time, the node will initiate a NETWORK
RECONFIGURATION. The ET2 and ET1 bits of the Configuration Register allow the network to operate
over longer distances than the 1 mile stated earlier. The logic levels on these bits control the maximum
distances over which the COM20022I can operate by controlling the three timeout values described above.
For proper network operation, all COM20022I's connected to the same network must have the same
Response Time, Idle Time, and Reconfiguration Time.
SMSC COM20022I
Page 13
DATASHEET
Revision 09-27-07