COM20051I Datasheet, PDF (16/74 Pages) SMSC Corporation – Integrated Microcontroller and ARCNET (ANSI 878.1) Interface

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 (ITT)
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 ITT (Invitation To Transmit: ASCII code 04H)
!"Two (repeated) DID (Destination ID) characters
ALERT
BURST
ITT DID DID
Free Buffer Enquiries (FBE)
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 FBE - Free Buffer Enquiry: ASCII code 85H)
!"Two (repeated) DID (Destination ID) characters
ALERT
BURST
FBE DID DID
Data Packets (PAC)
A Data Packet consists of the actual data being sent to another node. It is sent by the following sequence:
!"An ALERT BURST
!"An PAC (Data Packet--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 00Hex 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)
!"Two CRC (Cyclic Redundancy Check) characters. The CRC polynomial used is: X16 + X15 + X2 + 1.
ALERT
PAC SID DID DID COUNT data
BURST
data CRC CRC
Acknowledgements (ACK)
An Acknowledgement is used to acknowledge reception of a packet or as an affirmative response to FREE BUFFER
ENQUIRIES and is sent by the following sequence:
!" An ALERT BURST
!" An ACK (ACKnowledgement--ASCII code 86H) character
ALERT
BURST
ACK
SMSC DS – COM20051I
Page 16
Rev. 03/27/2000