Micros Systems Workstation 5 - IDN Receive; IDN Transmit; RS422 Termination

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.