Multiplex Interface

Multiplex Interface

       IBM Systems 9370 use an asynchronous point-to-point
interface (blue cable) to connect to an external bus control unit
(BCU) which adapts to an I/O bus (SPD bus).  This interface requires
35 signal lines, one line less than the selectable cable can support.

      For parallel processing, it is important to connect as many IBM
9370 processing units (PUs) as possible to an external bus (VME bus).
      The general concept of the proposed design is to:
      1.   change the existing interface to a multiplex of
three-state drivers, which allows the connection of multiple PUs (4)
to one external BCU by the existing cable;
      2.   control this driver multiplex with the spare line of the
blue cable;
      3.   allow the full blue cable protocol, so that each connected
PU regards the common BCU as dedicated to itself,
      4.   allow an additional BMX (blue bus multiplex card) status
interface to the external bus which is enabled when no PU is
communicating (few signals (6) are used by the external BCU to
control the multiplex), and
      5.   establish a driver control sequence, so that no
three-state driver can drive against another one and be destroyed.

      BLUE BUS MULTIPLEX CARD: The BMX card is shown in Fig. 1.  It
contains a special control logic BMX_CTRL, status drivers BMX_STAT,
and four sets of blue cable drivers BMX - DR 1, 2, 4, 5.   For
a multiplex, three-state drivers are used which are also capable of
driving the signal lines of the blue cable.

      Only one control line, N_PU_SELECT, is not multiplexed in the
blue cable.  The signal on this multi-function line controls the
different operating phases of the BMX.  If inactive, this signal
enables the multiplexed BMX status and control lines via BMX_CTRL.
In this state, any PU may request the blue cable via four control
lines BBn_PU_REQ; n = 1, 2, 4, 5.  The BCU may also select a PU
by placing the binary address N_PU_A0,1 on the remaining two control
lines and by activating the N_PU_SELECT line.  The PU address is
stored in the PU_ADDR_REG in the BMX_CTRL by the transition of
N_PU_SELECT to active.  This address is decoded by the
PU_ADDR_DECODER after the latter has been enabled by the delayed
N_PU_SELECT.  The decoded address enables one set of the blue cable
drivers N_EN_BBn.  As long as N_PU_SELECT is held active, the BMX
status and control lines are disabled and the selected PU may
communicate with the BCU.  At the end of the PU operation, the BCU
deactivates N_PU_SELECT, which disables the selected blue cable
drivers and enables the BMX status and control l...