User's Manual

PMAC User Manual
Talking to PMAC 25
TALKING TO PMAC
Basic Aspects of Communicating with PMAC
This section covers basic aspects of communicating with PMAC from a host computer. At this level,
there is a program for the host computer that processes these communications. The PMAC Executive
Program (Accessory 9D) is the most common of these programs.
If there will be a host computer in the final application, communications routines must be written for the
host computer as part of the front-end software for the application. That is a more advanced topic, and it
is covered in the Writing a Host Communications Program section of this manual. This section
concentrates on the actual communications.
At a basic level, PMAC can communicate to a host dumb terminal, either over the serial (RS-422) or the
parallel (bus) interface. The communications mostly consists of lines of ASCII characters sent back and
forth. Of course, most of the time the host will be a computer with considerably more intelligence, but at
root it will talk to the card as if it were a terminal. The PMAC Executive PC program has a terminal
emulator mode to do this directly.
Communications Ports
Each version of PMAC can communicate either over its serial port or its parallel (bus) port. The main
difference between the different hardware versions of PMAC is the type of bus interface: PC, STD, or
VME.
Note:
It is important not to command PMAC simultaneously from both ports; the
characters can be intermixed and the commands garbled.
Active Response Port
Either the serial port or the bus port is the active response port, where PMAC will send its responses to
the commands. PMAC powers up/resets with the serial port as the active response port. However, any
command received over the bus port makes the bus port the active response port (this happens
immediately in most bus-host applications, so is transparent to the user). Further responses are returned
to the bus port.
A subsequent command from the serial port does not automatically make the serial port the active
response port again, so it is possible that PMAC will respond to a command over the serial port by
sending data to the bus port. This will probably confuse both host computers. To make the serial port the
active response port again, you must send a <CTRL-Z character to PMAC.
Caution:
If using a bus-based system with an auxiliary computer over the serial port to do
some diagnostic work, such as data gathering or tuning with the PMAC Executive
Program, it is important to stop bus communications.
Serial Interface
The hardware configuration for the PMAC serial interface port is slightly different on different versions
of PMAC.