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each connector on the daisy-chain. The allowable common mode differences in the RS485 specification
is 7 volts, and the PGC4 complies with this. Connector pin numbers are defined in Appendix C. Each
used pin should be connected to all similarly-numbered pins at every PGC4 and host computer
connector on the interface. The wiring should be daisy-chained from the host computer to each
instrument in turn; no branches or spurs should be allowed. The most remote PGC4 should have a
termination connected, as described in section 2:3, above.
2:5 Host computer line drivers and receivers.
It is advisable to check certain conditions in the host computer line drivers and receivers, even if these
are guaranteed to comply with the relevant standards.
If facilities exist for terminating lines at the receiver this should be done. For compliance with RS485 the
line driver should also be terminated.
Consideration should be given to the receiver output state when the all the PGC4s are silent and their
lines are in the "TRI-STATE" condition.
For RS422/RS485 the commonly used DS75176, SN75176 and similar transceivers do not guarantee
that the output state of a receiver is defined when the inputs are terminated, but undriven, because of
the ±200 millivolt offset of the receiver. The claim on the data sheet that "The receiver incorporates a
fail-safe feature which guarantees a high output state when the inputs are left open" refers to a condition
without termination. Connection of a termination, external cables or a driver with its outputs in the "TRI-
STATE" condition will give unpredictable results and result in transmission errors. Normally, a 1 kilohm
pullup resistor connected between the RXD+ line and the receiver +5 volt supply and a 1 kilohm
pulldown from RXD- to the 0 volt supply line will ensure a high output state under properly terminated
conditions.
For RS232 receivers an open-circuit input normally ensures a high output state. If this is not the case
then adding a 10 kilohm pulldown resistor between the received data line and the receiver negative
supply voltage will correct this.
Because the number of PGC4 instruments driving an interface may be between 1 and 16 it is not
possible to fit these additional components on the drivers in the PGC4.
2:6 Demonstration software.
Two demonstration programs are available for IBM PC XT, AT or compatible computers. The computer
requires DOS 3.0 or higher, a colour display and a free serial port. The baud rate (2400, 4800, 9600 or
19200 baud) and serial port (COM1: or COM2:) are selectable.
PGC4DEMO.EXE:
This program is used to control a single PGC4 instrument (although other instruments may be
connected to the party line). The PC displays the pressures of all active gauges and the status of relays,
and can assume control of the instrument in order to switch gauges, change setpoints, display
messages, etc. The instrument type, ROM version/date and internal settings can be displayed.
PGCMULTI.EXE:
This program logs the status of up to 16 instruments on a party line. Pressures, relay status and errors
are displayed, and the internal settings can be shown. To physically identify an instrument, it can be
instructed to display its ID. The program does not control individual gauges but has the ability to start or
stop all gauges in all instruments.
