6100 BSC Programming Manual

Writing Applications that Use 6100 BSC
If the receiving station is temporarily unable to receive data,
it can make a CONTROL request with the WACK option. (See the
discussion of WACK in Section 1). The request sends a WACK
reply and awaits a response, normally ENQ. The application
can make repeated CONTROL WACK requests, then finally make
a READ request when it is ready to receive data.
If the receiving station has an urgent message for the other
station, it can make a READ request with the RVI option.
(See the discussion of RVI in Section 1). The request sends
an RVI reply to the last accepted block and awaits the next
transmission, normally a block or an EOT sequence. If the
next transmission is not an EOT--if the other station keeps
sending data--the receiving station makes READ requests
until the transmitting station relinquishes the line. When
finally an EOT arrives, the receiving station bids for the
line, as shown in Figure 3-6.
-----------------------------------------------------------------
| |
| Local Station Remote Station |
| |
| | READ |
| | request |
| / msg ---------> / |
| WRITE request | \ |
| (completes | <---------- RVI | READ |
| with status | | request |
| 165--RVI) \ | (RVI option) |
| / | |
| WRITE | msg ---------> | |
| request | / |
| | <---------- ACK n \ |
| \ | READ request |
| / | (completes with |
| READ | EOT ---------> | status 163--EOT) |
| request | / |
| | <---------- ENQ \ |
| | | WRITE |
| | ACK 0 -------> | request |
| | | (bid) |
| \ <---------- msg | |
| |
| The underscore shows the information applications |
| supply and receive. In each case, the information |
| resides in the text field of the request/response |
| buffer (see Figure 3-9). Items not underscored--for |
| example, most control sequences--are supplied and |
| received by the protocol, not by the application. |
| |
-----------------------------------------------------------------
Figure 3-6. Reverse Interrupt and Line Turnaround
3-14