6100 BSC Programming Manual

6100 BSC Concepts and Context
If a station receives RVI and ignores it, treating it like
ACK 0 or ACK 1, the replying station is not allowed to repeat
the RVI. A second RVI is allowed only where the first was
garbled or not received, i.e., where the response to RVI
was ENQ, "Please repeat that reply."
RVI is a positive acknowledgement; the last message block was
received without error.
Conversational Mode. After receiving a block of text, a station
is permitted to reply with a message instead of with ACK,
NAK, WACK, or RVI. This response is called a conversational
reply and has the same format as any other message. It can have
a heading, transparent text, intermediate text blocking, and so
on.
An application makes a WRITEREAD request to send a message and
receive a conversational reply. To send the reply, the other
application also makes a WRITEREAD request. A conversational
reply is limited to one transmission.
NOTE
In many BSC implementations, a station can send a
conversational reply only to a block that ends in ETX.
6100 BSC does not enforce this restriction.
Many devices allow only limited conversational mode. In this
mode, a station that receives a conversational reply may not
transmit a message in response. Rather, it transmits an
acknowledgement-- ACK 1 or NAK. (To send ACK 1, the
application uses a READ request. BSC sends a NAK automatically
if the situation demands it.) In effect, a conversational reply
reverses the roles of the stations. The replying station becomes
the sending station, as shown in Figure 1-18. The former sending
station can regain the line with a subsequent conversational
reply. Either station can issue an EOT sequence to end the
exchange of messages.
6100 BSC extends conversational mode in two ways. First, a
station that receives a conversational reply may send a message
in response; each station is allowed one text reply, accomplished
with a WRITEREAD request, after which the first station to make a
text reply must send an acknowledgement. (A READ request is used
for this purpose.) Thereafter, data transfer can continue, or
either station can send an EOT to end the exchange of messages.
A station may not make two consecutive conversational replies.
1-36