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P300H P300 Series Modem Installation and Operating Handbook Page 173
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:
Min Signal RS422 Circuit Electrical
Req Description Name Number Levels
X.21 DCE
(15 pin D female)
25 pin 37 pin
NameAB ABAB
DATA CIRCUIT TOWARDS MODEM (TX)
T29Data In SD 103 2 14 4 22 RS422
C 3 10 Sig’ Valid In RS 105 4 19 7 25 RS422
DATA CIRCUIT FROM MODEM (RX)
S613Clock Out RT 115 17 9 8 26 RS422
R411Data 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 Signal Ground SG 102 7 19, 20, 21 Ground
Prot gnd 1 Shield / Prot’ PG 101 1 1 Screen
ground
Clocking: IMPORTANT
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 either Satellite, Internal, or Station Clock. Do not
select Tx Clk In as this does not exist in X.21. The Tx Clocking should simply 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.
By selecting Change, Terr-Intfc, Control Lines 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 Ignore. It is also possible to configure the Tx carrier to be
switched on and off by the `Sig’ Valid In` line ( C ) by selecting Change, Tx, Modulator, Carrier, RTS-
Controlled.