To provide for the metering of the flow of individual data streams, a
Function Control Sequence (FCS) is added to each transmission block.
The pes is a sequence of bits, some of which represent a particular transmission stream. The receiver of several data streams can
temporarily stop the transmission of a particular stream by setting the
corresponding FCS bit off in the next transmission to the sender of that
stream. The stream can subsequently be resumed by setting the bit on. However, since only single data streams are supported, RSCS does not
support this metering capability. If bit one of the FCS (wait-a-bit) is
on, or if bits 4, 9, or 15 (print, console, punch stream identifiers)
are off, transmission will be suspended. Thus, the bit pattern of X'SSC1' represents the m1n1mum acceptable FCS configuration for
transmission to be continued.
Finally, for error detection and correction purposes, a Block Control
Byte (BCB) is added as the first character of each block transmitted.
The BCB, in addition to control inforaation, contains a hexadecimal
block sequence count. This count is maintained and verified by both the
sending and receiving systems to exercise a positive control over lost
or duplicated transmission blocks.
In addition to the normal binary synchronous text control characters (STX, ETB, etc.), MULTI-LEAVING uses two of the ESC control characters, ACKO and NAK. ACKO is used as a "filler" by all systems to maintain
coamunications when data is not available for transmission. NAK is used
as the only negative response and indicates that the previous
transmission was not successfully received. MULTI-LEAVING Control Specification
This section describes the bit-by-bit definitions of the various MULTI-LEAVING control fields and includes notes concerning their Appendix B: MULTI-LEAVING 351

RECORD CONTROL BYTE (RCB; OIIITTTT a 7 To identify each record type within a transmission block OIIITTTT --or--
o 1110000 III
--or--
o
IIITTTT
TTTT 00000000 1 000 001 010 011 100 101 111
1 0001 0010 0011 0100 0101 0110 0111 1000-1100 1101-1111 End of transmission block
Non-EaT RCB III is control information:
Reserved
Request to initiate a function
transmission (prototype RCB for
function in SRCE )
Permission to initiate a function
Transmission (RCB for function
contained in SRCB )
Reserved
Reserved
Available for location modification
General control record (Type
indicated in SRCB) Non-EaT RCB
III is used to identify streams
of multiple identical functions
(such as multiple print streams
to a multiple printer terminal).
TTTT is the record type identifier. Operator message display request Operator command
Normal input record Print record Punch record
Data set record
Terminal message routing request
Reserved
Available to user
352 IBM VM/310 System Programmer's Guide