HARDWARE INTERFACE 2-1
2.0 HARDWARE INTERFACE
2.1 RS-485 4-Wire Operation
RS-485 and RS-422 interfaces use differential
transceivers for increased noise immunity, since any
noise induced in one wire will usually be induced in
the other wire and thus will be canceled out at the
differential receiver.
RS-485 4-Wire operation allows 32 drivers and 32
receivers to be party-lined together at distances up to
4000 feet. This allows a host computer to talk to 31
drives. The host computer has its transmitter and
receiver enabled at all times. The drives always have
their receivers enabled but only one drive transmitter
on the party-line can be enabled at any one time.
The host computer transmits a query to a specific
drive (queries have an address field that identifies the
destination drive). Even though all drives on the
party-line or network receive the query and decode it,
only one drive will prepare and transmit a response.
The destination drive enables its transmitter during its
response and disables it immediately after
transmission is complete. This sequencing of the drive
transmitters is built into the PHOENIX drive software.
The host software requires no special hand-shaking
since the transmitter and receiver are enabled at all
times.
There are a number of electrical supply houses that
offer RS-485 interface cards for IBM-compatible PCs.
US Drives has decided to only support 4-Wire RS-485
and RS-422 operation because standard MS-DOS serial
device drivers may be used without modification. The
proper hookup for 4-Wire RS-485 Multi-Drop between
a host PC and a number of PHOENIX drives is shown
in Figure 2-1.
2.2 RS-422 4-Wire Operation
RS-422 4-Wire operation is similar to RS-485 4-Wire
operation except that the wiring is “point-to-point”
with no other drives party-lined. This mode works at
full duplex and is illustrated in Figure 2-2.
2.3 RS-232 Operation
The PHOENIX drive permits a direct RS-232 interface.
It is felt, however, that the differential transmission
scheme offered by the RS-422/485 standard is much
more suitable for an industrial environment. Direct
connection to a PC using the RS-232 scheme is not
recommended for drives operating on the factory
floor.
Those users that still wish to use a RS-232 interface
have the following alternatives.
2.3.1 RS-232C-to-RS-422 Interface
Adapter
Users who wish to use the PC’s RS-232C serial port
can install a RS-232C-to-RS-422 converter that is
readily available from a number of electrical suppliers.
It need not be isolated, as the PHOENIX user serial
port is already electrically isolated from the rest of the
drive. These converters typically look much like a “25
pin gender changer” plug with the conversion circuits
built into the plug. Power is normally supplied to the
converter by an AC-DC adapter that plugs into an
110vac duplex outlet.
2.3.2 Direct RS-232 Wiring
The following “scheme” is not recommended for
permanent use. By connecting the +RXD and +TXD
pins to the PC RS-232 ground, a quasi-single ended
RS-232 interface can be accomplished.
Normal RS-232C signals bounce between +10 volts
and –10 volts (the positive rail voltage can be between
the values of +3 volts and +25 volts – likewise for the
negative rail). Thus, +10 volts is interpreted as a logic
“0” while -10 volts is interpreted as a logic “1”. Most
RS-232C receivers will detect zero volts as a logic “1”
due to hysteresis effects which allows the wiring
scheme shown in Figure 2-3 to work (usually!).
This direct connection generates RS-232C levels of
approximately +3.7 volts to -3.7 volts. Note that the
isolated RS-422/485 common on the PHOENIX control
board (TB6-5) must not be grounded or it will short-
circuit the +TXD line if the PC is grounded. This direct
RS-232 wiring scheme, if it must be done, is most
appropriate when using a laptop PC that is floating
from earth ground.
