Corporation

SIGNAL PROCESSING EXCELLENCE

82

supplying

±2V minimum into a 100Ω load.

Short circuit protection for the RS-422 operat-

ing mode is the same as in the RS-232 mode. The

driver outputs can be shorted to ground for an

infinite duration, with a maximum current of

±100mA.

The RS-422 receivers accept differential signals

at a 1Mbps rate, and translate them to a non-

inverted TTL output. The receivers are speci-

fied with a

±15V differential input voltage,

which means that to operate normally, the dif-

ference betweeen the voltages at the inputs

cannot exceed

±15V. The common mode volt-

age is specified as

±7V. This identifies the

midpoint of the range about which the differen-

tial input must lie so that the receiver can detect

a change of state. Within this

±7V range, the

receivers will recognize a change in state with a

±200mV differential threshold voltage. Since

the RS-232 and RS-422 inputs are shared, all

receiver inputs are protected to

±30V to guard

against inadvertently applying an RS-232 signal

to an input that is configured for RS-422.

Figure 2 shows a typical circuit for the SP304.

In this case the SP304 is shown configured for

one (1) duplex RS-422 and two (2) duplex

RS-232 communication paths.

CONFIGURING THE SP304

Figures 3, 4 and 5 show the various combina-

tions of simultaneous RS-232 and RS-422

operation that can be achieved with the SP304.

Each of these configurations are software

selectable by logic level on the SELECT A and

SELECT B control lines. Configuration can be

changed “on-the-fly”.