P300H P300 Series Modem Installation and Operating Handbook Page 176
P1440 IN X.21 MODE
The P300 Series along with its standard P1440 RS422/V.35/RS232 interface can provide both X.21
DCE and X.21 DTE operation. For X.21 operation the card must be set to RS422 mode, and the Tx & Rx
Clocking modes must be set correctly. With the use of the appropriate adaptor lead (to convert to the
X.21 standard 15 pin D connector), a full X.21 DCE or DTE interface is available.
X.21 DCE OPERATION
This only has 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 either a 25 pin or 37 pin `D` male (not both) at the other end as
follows:
25 pin
Min
Req
X.21 DCE
(15 pin D female)
Signal
Description
RS422
Name
Circuit
Number
37 pin
Electrical
Levels
Name
A
B
A
A
B
B
DATA CIRCUIT TOWARDS MODEM (TX)
2
Data In
103
T
9
SD
2
14
4
22
RS422
C
3
10
Sig
`
Valid In
RS
105
4
19
7
25
RS422
DATA CIRCUIT FROM MODEM (RX)
S
6
13
Clock Out
RT
115
17
9
8
26
RS422
R
4
11
Data Out
RD
104
3
16
6
24
RS422
I
5
12
Sig
`
Valid Out
RR
109
8
10
13
31
RS422
COMMON LINES
Sig gnd
8
19, 20, 21
Signal Ground
SG
102
7
Ground
Prot gnd
1
Shield / prot
`
ground
Screen
PG
101
1
1
Clocking: IMPORTANT
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 (C) 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 (C) by selecting
ݸ¿²¹»
,
̨
,
Ó±¼«´¿¬±®
,
Ý¿®®·»®
,
ÎÌÍóݱ²¬®±´´»¼
.
The interface will not work as X.21 unless you set the clocking correctly! The
Rx Clock Menu
is used to
set the source of the single clock from the modem as
Í¿¬»´´·¬»
,
ײ¬»®²¿´
, or
ͬ¿¬·±² Ý´±½µ
. Do not select
̨ Ý´µ ײô
as this does not exist in X.21. The
Tx Clocking
should simply be set to
Ψ
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.
