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"A?55387 300 19880422 52480 0 0 ?BPLUS.DOC"
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D. B Plus Control Sequences.
The B Plus Protocol defines several Control Sequences in
addition to packets. Control Sequences are used to manage
the progress of the B Plus Session.
1. Enquire.
The Enquire Control Sequence consists of the single
character:
<ENQ>
0x10
It is sent in one of two circumstances:
o The Responder is in terminal mode and the Initiator
wants to synchronize its <Sequence> number, and to
tell the Responder to set up for normal B Protocol.
When the <ENQ> is received, the Responder must:
> Prepare for 512 byte data blocks
> Prepare for Standard Checksum
> Initialize its Sequence to 0x30
> Transmit the string:
<DLE> + + <DLE> 0x30
> Return to terminal mode. Note: The terminal
program definately should NOT enter B Protocol
mode when the <ENQ> is received.
o The entity sending the packet has received a
Negative Acknowledge, or timed out while waiting for
an Acknowledge. The response to an <ENQ> during a B
Plus session is a Positive Acknowledge.
2. Positive Acknowledge.
When a correctly checked and sequenced packet is received,
a Positive Acknowledge is transmitted. This consists of:
<DLE> Sequence
where Sequence is the Sequence number of the last validly
received packet.
16
3. Wait.
The Wait Control Sequence informs the receiving entity that
the sending entity requires additional time to process what
it has received. Wait is sent as:
<DLE> ;
4. Negative Acknowledge.
When the receiving entity receives a packet with an
unexpected <Sequence>, improper checksum or times out, it
sends a Negative Acknowledge to the other entity. This
consists of the single byte:
<NAK> 0x15
When the sending entity receives a NAK, it initiates a
Resynchronization Process by sending two <ENQ> characters
and waiting until it receives two identical Positive
Acknowlegements.
5. Panic Abort.
The B Plus Protocol recognizes a series of four <DLE>
characters as a panic abort. The need for such a facility
can arise if the Responder computer becomes locked up during
a transfer, and it is necessary to reboot. Since <DLE> is
really control-P, sending it is a very simple procedure.
In addition, the Initiator recognizes the seqeunce:
<DLE> +
while waiting for a Positive Acknowledge as being incorrect,
and will abort the transfer. This can happen as above, but
before the operator can press control-P the Initiator sends
an <ENQ> during a down-load to find out what is happening.
The Responder, now in terminal emulation mode, responds
with:
<DLE> + + <DLE> 0