Workstation 5 Field Service Guide
2-27
Workstation 5 System Board Technical Descriptions
Point Of Sale Interfaces
IDN Transmit
The DE input enables transmitter. To transmit data,
TRANSMIT_EN1
goes
high, enabling the transmitter in U24. The transmitter in U25 is disabled by
ANDing
422_MODE_1#
at U20-12 with
TRANSMIT_EN1
at U20-13,
producing a Low at U20-11, disabling the transmitter in U25. Transmit data
from the UART,
TXD3
is routed through level shifter U2 to produce
F_TXD3
, and this is fed the
D
input of U24 and U25.
IDN Receive
The
RE#
input of each transceiver enables the receiver.
RS422_MODE_1
is set high at the
RE#
input of U24 to disable the receiver. At the same
time,
422_MODE_1
is inverted by U11A, enabling the receiver in U25.
IDN receive data is gated onto
RXD_422#
at U20-10, ANDed with
422_232_SEL_1
at U20-9, to produce
RXD_422
at U20-8.
RXD_422
is ORed with
RXD_232
(inactive) to produce
F_RXD3
at U4-11
and fed to level shifter U2. U2 is required to interface the VCC5 based IDN
interface with the +3.3V based UART in Companion Device U41.
RS422 Termination
The transmit and receive pair is connected to the RS422 termination
circuit. In general the RS422 Termination circuit performs three functions:
a) provide the proper impedance matching to the communications cable, b)
protect the 75176 transceivers from differential-mode transient voltages
induced into the IDN cable, and 3) suppress RF emissions originating from
the IDN circuitry.
