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P300H P300 Series Modem Installation and Operating Handbook Page 177
Eurocom D/1 interface `D` operation
The Eurocom `D` interface uses transformer coupled differential clock and data signals. The data signal
is AMI coded, and the amplitude of both clock and data signals is ±0.5V (ie 1.0 V peak to peak). The
differential lines are of 130
impedance, and the centre tap of the line coupling transformers may either
be grounded, or left floating.
The Eurocom `D` interface will operate from <16kbps to >2048kbps, although the Eurocom specification
call for only 256, 512, and 1024kbps. In addition to the clock and data lines in and out (four lines), a fifth
line is implemented over and above the Eurocom specification which carries the modems internal clock out
when the Tx clock is set to internal.
Min RS422 Circuit
Req Name Number
Signal Description centre tap of
25 pin Link to ground
transformer
AB
DATA CIRCUIT TOWARDS MODEM (TX)
Clock In TT 113 19 6 LK 1 *
Data In SD 103 18 5 LK 2 *
Int’ Tx Clock Out ST 114 21 8 LK 7 *
DATA CIRCUIT FROM MODEM (RX)
Clock Out RT 115 4 17 LK 4 *
Data Out RD 104 16 3 LK 5 *
COMMON LINES
Ground to Activate Eurocom `D` 14 n/a
Do Not Connect 12, 20 n/a
Signal Ground SG 102 2, 7, 13, 15, 22, 25 n/a
Shield/Protective ground PG 101 1 n/a
* Links
All links settings are:
Centre tap grounded: 1-2 (link `up` with the PCB legend the right way up)
Centre tap floating 2-3 (link `down` with the PCB legend the right way up)
The typical configuration would normally be to ground the outputs, and leave the inputs floating ie:
Links 4, 5, and 7 in pos 1-2 (up), and links 1 and 2 in pos 2-3 (down)
This provides protection by ensuring the signal levels remain close to ground potential (as might happen
if both inputs and outputs were floating), but avoid any ground currents between distant locations (as might
happen if both inputs and outputs were grounded).
The modem will detect and display the `Data Marginal` warning if the Tx input clock and data phase is
incorrect, and will automatically flip the clock phase to correct this. If repeated `Data Marginal` alarms occur
then it is likely the Tx clock and data are not from the same clock course and are `slipping past` each other.