Z85233 Datasheet, PDF (272/317 Pages) Zilog, Inc. – The Zilog SCC Serial Communication Controller

Application Note
Technical Considerations When Implementing LocalTalk Link Access Protocol
The second routine decrements the receiver buffer interframe gap, which is 200 µsecs. This difficulty is even
address by two to account for the two CRC bytes that are greater than it might appear. The 200 µsec gap starts
1 ^{read from the SCC before the special condition interrupt when the frame is received; it ends when the leading flags}
occurs. Note that the SCC does not filter these CRC bytes, and destination address of the response are sent. Start
nor does it filter the address byte. Everything received after sending the response soon enough to allow sending two
the leading flags and before the trailing flags appears in leading flags (plus a possible leading flag fragment) and
the receive buffer.
the first data character, and to allow for the 3-bit delay in
the SCC shifter. Therefore, start sending early enough to
One difficulty that arises in LLAP that was not addressed _{transmit 35 bits before the interframe gap expires, or about}
here is that the receipt of a frame very often creates an _{70 µsecs after you receive the frame.}
obligation to send a frame back to the sender within the
CONCLUSIONS
The problems of sending the sync pulses, the timing of
transmission packets, and the problems associated with
the reception of packets as defined by LLAP are handled
by the Z80181 and its peripherals. It was demonstrated
that LLAP frames can be transmitted and received by
using the straight forward polling method and by using
interrupt routines. In a much busier environment where the
processor cannot strictly be an LLAP engine, other
methods such as using DMA in a fully interrupt driven
environment must be used. It was also demonstrated that
severe CPU overhead is used in setting up the sync
pulses, timing out delays, etc., before each LLAP frame. A
modified SCC that transmits and receives special LLAP
frames helps in off loading some of this overhead, hence
freeing the CPU to do other tasks.
UM010901-0601
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