P300H P300 Series Modem Installation and Operating Handbook Page 177
X.21 DTE OPERATION
This only has a
clock
to
the modem
from the terrestrial equipment. Configure a cable with a 15 pin `D`
male at one end (X.21 DTE) and either a 25 pin or 37 pin `D` male (not both) at the other end as follows:
Min
Req
X.21 DTE
(15 pin D male)
Signal
Description
RS422
Name
Circuit
Number
25 pin
37 pin
Electrical
Levels
Name
A
B
A
B
A
B
DATA CIRCUIT TOWARDS MODEM (TX)
S
6
13
Clock In
TT
113
24
11
17
35
RS422
R
4
11
Data In
SD
103
2
14
4
RS422
22
12
4
19
7
25
I
5
Sig
`
Valid In
RS
105
RS422
DATA CIRCUIT FROM MODEM (RX)
T
2
9
Data Out
104
RD
3
16
6
24
RS422
C
109
3
10
Sig
`
Valid Out
RR
8
10
13
31
RS422
COMMON LINES
102
19, 20, 21
Sig gnd
8
Signal Ground
SG
7
Ground
Prot gnd
Shield / Prot
`
ground
101
1
1
PG
1
Screen
Clocking: IMPORTANT
The interface will not work as X.21 unless you set the clocking correctly! The
Tx Clocking
menu should
be set take the single clock from the terrestrial by selecting
̨ Ý´±½µ ײò
The Rx Clock menu should also
be set to return Rx data in time with the single Tx Clock by also selecting
̨ Ý´±½µ ײ
. NO OTHER
COMBINATIONS WILL WORK.
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).
By selecting
ݸ¿²¹»
,
Ì»®®óײ¬º½
,
ݱ²¬®±´ Ô·²»-
it is possible to configure the modem to raise an alarm if
either the `Sig` Valid In` line (I) switches to the inactive state. As there is no X.21 equivalent of the
RS422 `Device (DTE) Ready In` line (TR), set this to
×¹²±®»
. It is also possible to configure the Tx
carrier to be switched on and off by the `Sig` Valid In` line (I) by selecting
ݸ¿²¹»
,
̨
,
Ó±¼«´¿¬±®
,
Ý¿®®·»®
,
ÎÌÍóݱ²¬®±´´»¼
.
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 In line.
