DESCRIPTION AND OPERATION
I-E96-217A
2 - 7
The message code (i.e., command code) determines the func-
tion the receiving module is to perform. After receiving and
interpreting a message, a module responds with a reply mes-
sage. This message contains a data field with either an
acknowledge or the requested data. The SPM stores the
received data in RAM until the computer is ready for it.
Before performing an operation, a receiving module performs
consistency checks to determine if a message is valid. The type
of message and number of bytes must conform to standard
transaction formats.
POWER
Standard INFI 90 power (+5 VDC, ±15 VDC) drives the SPM cir-
cuitry. It connects through the top 12-pin card edge connector
(P1) shown in Figure
. The +5 VDC supply is for the logic
circuitry, the ±15 VDC for the RS-232C interface. In addition to
the power, the module bus and Power Fail Interrupt (PFI) con-
nect through P1.
MACHINE FAULT TIMER
A Machine Fault Timer (MFT) generated on the SPM is a secu-
rity feature common to all microprocessor based INFI 90 mod-
ules. The timer is reset periodically by the microprocessor
(approximately once every 2.2 seconds). If it is not reset within
this time, the timer expires causing the module to shut down
(
time out
). It is reset on power up or by the manual reset switch
accessed through the module faceplate.
If a
time out
occurs, the front panel module status LED lights
red and the SPM will no longer communicate with the INFI 90
system or computer.
A MFT status signal is also used to develop the Clear To Send
(CTS) output to verify for the computer that the module is
capable of receiving data transmission.
ON-LINE TESTS
When the SPM is first installed or after a reset, it conducts
on-line self tests to verify that its logic circuitry is operating
properly.
RAM Test
A RAM test consists of writing data to RAM, and then compar-
ing the data in RAM to the data that was written. If this test
fails, the microprocessor stops resetting the MFT causing a
time out
to shut the module down.
ROM Test
A ROM test computes a checksum of ROM memory contents. It
then compares this checksum to the known checksum. If they
