Q-Lite Satellite Modem Installation and Operating Handbook
9-9
X.21 DCE Operation
This has only a clock
from
the modem to the terrestrial equipment. Configure a cable with a
15 pin `D` female at one end (X.21 DCE) and a 25 pin `D` male at the other end as follows:
Min
Req
X.21 DCE
(15 pin D female)
Signal
Description
RS422
Name
Circuit
Number
25 pin
Electrical
Levels
Name
A
B
A
B
DATA CIRCUIT TOWARDS MODEM (TX)
T
2
9
Data In
SD
103
2
14
RS422
C
3
10
Sig
’
Valid In
RS
105
4
19
RS422
DATA CIRCUIT FROM MODEM (RX)
S
6
13
Clock Out
RT
115
17
9
RS422
R
4
11
Data Out
RD
104
3
16
RS422
I
5
12
Sig
’
Valid Out
RR
109
8
10
RS422
COMMON LINES
Sig gnd
8
Signal Ground
SG
102
7
Ground
Prot gnd
1
Shield/ Prot
’
ground
PG
101
1
Screen
Clocking
For the interface to function as X.21, the clocking must be set as follows. The
Rx Clock
Menu
is used to set the source of the single clock from the modem as
Satellite
,
Internal
, or
Station Clock
.
Tx Clk In
does not exist in X.21 and cannot be used. The Tx clock should be
set to
Rx
as the Tx data is clocked by the same clock as the Rx data.
The maximum data rate is 10Mbps (over good quality twisted pair screened cable), but this
rate is reduced as the connecting cables extend beyond 10m. As an estimate, assume the
maximum rate halves as the distance doubles (ie 5Mbps to 20m, 2.5Mbps to 40m etc).
The input clock/data phase is selected automatically, reporting a `Data Marginal` warning
whenever it has to switch phase. If the modem reports Data Marginal frequently then this is
not a modem fault but an indication that you have a clocking problem external to the
modem (ie changing clock/data phase, a problem which would otherwise have caused
untraceable data errors). Check the phase of the
Data In
with respect to the
Clock Out
line.
It is possible to configure the Tx carrier to be switched on and off by the `Sig= Valid In` line
by selecting Edit
->Tx->Carrier
and selecting
RTS enabled
.
