5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
Idle Time
The Idle Time is associated with a NETWORK RECONFIGURATION. Figure 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 41 μS. This 41 μS is equal to the Response
Time of 37.4 μS plus the time it takes the COM20020I to start retransmitting another message (usually another
INVITATION TO TRANSMIT).
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 2 miles stated earlier. The logic levels on these bits control the maximum distances over which the
COM20020I can operate by controlling the three timeout values described above. For proper network operation, all
COM20020I's connected to the same network must have the same Response Time, Idle Time, and Reconfiguration
Time.
5.6 Line Protocol
The ARCNET line protocol is considered isochronous because each byte is preceded by a start interval and ended with
a stop interval. Unlike asynchronous protocols, there is a constant amount of time separating each data byte. On a 5
Mbps network, each byte takes exactly 11 clock intervals of 200ns each. As a result, one byte is transmitted every 2.2
S and the time to transmit a message can be precisely determined. The line idles in a spacing (logic "0") condition. A
logic "0" is defined as no line activity and a logic "1" is defined as a negative pulse of 100nS duration. A transmission
starts with an ALERT BURST consisting of 6 unit intervals of mark (logic "1"). Eight bit data characters are then sent,
with each character preceded by 2 unit intervals of mark and one unit interval of space. Five types of transmission can
be performed as described below:
Invitations To Transmit
An Invitation To Transmit is used to pass the token from one node to another and is sent by the following sequence:
An ALERT BURST
An EOT (End Of Transmission: ASCII code 04H)
Two (repeated) DID (Destination ID) characters
ALERT
BURST
EOT
DID
DID
Free Buffer Enquiries
A Free Buffer Enquiry is used to ask another node if it is able to accept a packet of data. It is sent by the following
sequence:
An ALERT BURST
An ENQ (ENQuiry: ASCII code 85H)
Two (repeated) DID (Destination ID) characters
ALERT
BURST
ENQ
DID
DID
Data Packets
A Data Packet consists of the actual data being sent to another node. It is sent by the following sequence:
An ALERT BURST
An SOH (Start Of Header--ASCII code 01H)
An SID (Source ID) character
Two (repeated) DID (Destination ID) characters
A single COUNT character which is the 2's complement of the number of data bytes to follow if a short packet is
sent, or 00H followed by a COUNT character if a long packet is sent.
N data bytes where COUNT = 256-N (or 512-N for a long packet)
SMSC COM20020I 3.3V
Page 13
Revision 12-06-06
DATASHEET
SMSC [ SMSC CORPORATION ]
