User's Manual

PMAC User Manual
Talking to PMAC 31
Multiple-Card Applications
If there are several cards communicating with the host, there must be a way for the host to distinguish
between the different cards. The host computer must be able to talk to each of the cards individually, and
sometimes to talk to the cards collectively. Therefore, the host must have a means of addressing the
cards.
This section covers the basic concepts of communications issues dealing with multiple cards. For more
detailed information, refer to the Writing a Host Communications and Synchronizing PMACs to Other
PMACs sections of this manual.
Bus Communications
When the interface is a bus-type interface (e.g., PC-bus, STD-bus or VME-bus), the distinction between
cards is taken care of by hardware addressing. This means that the different cards respond to different
bus address locations as selected by the bus address lines. The setup of the hardware addressing of
multiple cards is done just as it is for single cards (above); one simply cannot put two different cards at
the same hardware address on the bus.
Simultaneous Commands
A little trickier task is to initiate simultaneous action on all boards, because commands must be issued
sequentially to the different boards. However, characters can be sent to all of the boards in sequence so
quickly that this delay will not be the major limitation in keeping action simultaneous between the boards.
The commands should be sent so that everything except the carriage return character is sent to all of the
boards; then the <CR> command is sent to each board in rapid succession. (If checking for the write-
ready bits, make sure they are true on all boards before sending the character to any board.) On a typical
bus system, send these characters one microsecond apart. This is much faster than the approximately one
millisecond software scan time of the command interpreters on PMAC, so the commands are effectively
issued simultaneously.
Serial Communications
However, if serial communication is being used (RS-232 or RS-422), the daisychaining of PMAC does
not permit separate hardware addressing, so there must be a software addressing scheme.
PMAC cards equipped with PROM version 1.13 and higher are capable of daisychained communication
using the RS-422 port. PMAC Lites and PMAC STDs cannot use daisychained communication with the
RS-232 port; the RS-422 port is required (Option 9L for PMAC Lite). Up to 16 PMAC cards can be
connected and synchronized using serial port communications. To do this however, a few hardware and
software set up procedures must be followed.
Connections
When using serial communications to multiple PMACs from a single host serial port, the connection is
made through a single multi-drop daisy chained cable. At one end is the connector for the host computer
(usually a DB-25 connector). At the other end of the cable is one connector (drop) for each PMAC on the
chain. Each strand of the cable is brought out on the same pin of each connector.
Multi-Drop Cable
The PMAC STD has both a RS-232 port and a RS-422 port. If daisy chaining is desired, the RS-422 port
must be used. Delta Tau does not provide this cable. The Acc-3D cable provides serial connection to the
RS-422 port of a single PMAC PC or PMAC VME, and each Acc-3E ordered with it provides an extra
drop for an additional PMAC PC or PMAC VME. If connected to an RS-232 port of a host computer, it
is strongly recommended that an Acc-26 or similar converter be used, especially in multi-drop
applications.