User manual

A maximum common mode voltage rating of +7V provides good noise immunity from voltages

induced on the twisted pair lines. The signal ground line connection is necessary in order to keep the

common mode voltage within that range. The circuit may operate without the ground connection but

may not be reliable.

Parameter Conditions Min. Max.

Driver Output Voltage (unloaded) 4V 6V

-4V -6V

Driver Output Voltage (loaded) LD and LDGND 2V

jumpers in -2V

Driver Output Resistance 50Ω

Driver Output Short-Circuit Current +150 mA

Driver Output Rise Time 10% unit interval

Receiver Sensitivity +200 mV

Receiver Common Mode Voltage Range +7V

Receiver Input Resistance 4KΩ

Table A-2: RS422 Specification Summary

To prevent signal reflections in the cable and to improve noise rejection in both the RS422 and

RS485 mode, the receiver end of the cable should be terminated with a resistance equal to the

characteristic impedance of the cable. (An exception to this is the case where the line is driven by an

RS422 driver that is never "tristated" or disconnected from the line. In this case, the driver provides a

low internal impedance that terminates the line at that end.)

Note

You do not have to add a terminator resistor to your cables when you use the cards. Termination

resistors for the RX+ and RX- lines are provided on the card and are placed in the circuit when you

install the jumpers. (See the Option Selection section of this manual.)

RS485 Data Transmission

The RS485 Standard allows a balanced transmission line to be shared in a party-line mode. As many

as 32 driver/receiver pairs can share a two-wire party line network. Many characteristics of the drivers

and receivers are the same as in the RS422 Standard. One difference is that the common mode

voltage limit is extended and is +12V to -7V. Since any driver can be disconnected (or tristated) from

the line, it must withstand this common mode voltage range while in the tristate condition.

The following illustration shows a typical multidrop or party line network. Note that the transmission

line is terminated on both ends of the line but not at drop points in the middle of the line.

Manual PCI-ICM-422(485)/2(4)