2.3.4 Transmit CRC Generator
This logic block appends the FCS field (a 32-bit CRC) onto transmitted frames. Some
packets may already contain the FCS field, in which case this logic block may be used
as a CRC checker instead of a CRC generator. When used as a checker, a bit is set in
the interrupt event register when the transmitted CRC field is incorrect. This causes an
interrupt if this event is not masked in the interrupt mask register. This logic block
computes the CRC on data bytes it reads from the FRAME_DATA bus that come from the
send frame block. It also receives control signals from the transmit FSM.
2.3.5 Transmit Finite State Machine
The transmit FSM implements the transmit process as described in the FDDI MAC
standard. Specifically, the transmit FSM repeats packets from other stations on the ring,
determines when it can capture the token, how long it can hold the token, what type of
token to issue, what kind of frame to send (but does not actually send it), and participates
in the ring recovery procedures, etc.
This logic block contains all the MAC protocol timers (TRT, THT, and TVX timers), the
associated register fields used to load them (T_Max, TRT time remaining register,
T_BID_RC/T_INFO, and TVX_VaIue), and the LATE_CT. The MAC standard specifies an
additional register, T_Opr, which can contain redundant information and therefore is not
The TVX timer is used to ensure that a good frame (i.e., correct CRC and valid data
length) or a nonrestricted token is seen by this station on every regular basis. It can be
used to detect events such as a babbling station, an infinitely circulating restricted token,
a lost token, etc.
The TRT timer is used to determine the time taken for each rotation of the token. The
transmit FSM uses this information to decide whether the token has been lost and
whether this station can then transmit when it receives the token. If the token takes too
long to get to this station, it implies that the ring is busy and this station may have to defer
its lower priority transmission. This timer also times the claim and beacon recovery
procedures to determine if they will complete.
The THT timer controls the length of time that a station can transmit when it has captured
the token. This timer is loaded with the value of the TRT timer when a token is captured.
A station can then transmit asynchronous frames until this timer expires. This procedure
ensures that the sum of this station's transmissions and that of the previous station's
transmissions during this rotation of the token are (approximately) less than the mutually
agreed upon token rotation time.
MC68838 USER'S MANUAL