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BB-H348C-RM_1982
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swskit-v21/documentation/modem.mem
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! d i g i t a l ! I N T E R O F F I C E M E M O R A N D U M
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To: DECnet-20 Users
Date: 28 August 1979
Dept: NCSS
Loc: MR1-2/H22
Ext: HOTLINE (5911)
Subj: Modem strapping for Decnet lines
When configuring a communications network one must choose
common carrier switched or leased, or private lines, modems or null
modems, and communications interfaces. The configuration must be
hardware and software compatible, fast enough, reliable enough, and
within your budget. DECnet-20 V3.0 supports only synchronous full
duplex (4 wire) point to point communication links with no dial up
or dial out capabilities. Listed below are a number of techniques
for connecting two nodes to support DECnet-20. Remember that only
the modems and communications interfaces listed in the current SPD
are supported. All configurations require network profiles and
customer support plans, and any configuration using non standard
facilities should include PL90 estimates for support.
1) Directly connected DMC11s. Two DMC11-AL's with line units
can be connected directly with coaxial or triaxial cable over short
distances. The cables connecting the 56KB line units can be up to
18000 feet. The cables connecting the 1MB line units can be up to
6000 feet. Remember that full duplex needs two cables to connect
two DMCs.
2) Two communication interfaces can be connected by a
synchronous null modem or modem eliminator over short distances.
The distance from the communications interface to the null modem
must be no more than 50 feet.
3) Two synchronous modems, each connected to one communication
interface can be directly connected by a 4 wire private line. This
can be as little as 4 wires each a few feet long. The synchronous
modems connected by local wires are often called limited distance
modems. The ICC commlink-II is a LDM.
4) Two synchronous modems, each connected to one communication
interface can be connected by a 4 wire line leased from a common
carrier such as the Bell system.
5) Two synchronous modems with dial backup capability, each
connected to one communications interface can be connected by two
DDS calls in the Bell system. This is not supported.
To: DECnet-20 Users Page 2
Subj: Modem strapping for Decnet lines
Following are the options needed for the modems used in
DECnet-20 connections.
NULL MODEM or MODEM ELIMINATOR (any speed)
This device connects two communications interfaces over a short
distance. The only option normally offered is choice of line speed.
If the speed is not switch selectably choose a speed within the
limits of the communications interfaces. Remember that a null modem
for synchronous communications must provide a clocking signal. The
Digital H312-A cannot be used; it has no internal clock.
COAXIAL or TRIAXIAL CABLE (56 KB or 1000KB depending upon the
device)
When connecting two DMC11-AL/DMC11-MA or DMC11-AL/DMC11-MD
communications interfaces, only cables and connnecting wires are
required. They can be connected only over short distances. These
devices contain their own clock sources. No other communications
interfaces can be connected without modems or modem eliminators.
BELL SYSTEM 201C (2400 bits per second EIA only)
Bell 201C and 208A modems are similar. The following options
are specific to the 201C. See also the modem strapping options for
the 208A.
No automatic calling unit. The Bell 801/Digital DN11
configuration for making outgoing calls is not supported.
Permanent auto answer. This doesn't matter for leased
line connections but is important in dial backup situations to
prevent the need for operator intervention at the called site.
Four wire line interface. Full duplex communication
requires a 4 wire interface.
Zero millisecond RTS/CTS delay. Full duplex communication
with constant carrier does not do line turn around or in any
other way wait for the carrier to settle.
BELL 208A (4800 bits per second EIA only)
The following modem strapping options are common to the Bell
208A and the Bell 201C.
AA connected to AB. This option connects protective
ground (AA) to signal ground (AB).
Clock source internal. Clocking for data transfer is
provided by the modem.
To: DECnet-20 Users Page 3
Subj: Modem strapping for Decnet lines
Continuous carrier. Full duplex operation requires
continuous carrier.
New synch without. This option is only used in multidrop
configurations.
Alternate voice either way. This is the customers choice,
it makes no difference to the communications interface.
Switched network backup either way. Switched network
backup allows the customer to place two phone calls through the
dds network to provide a communications path if his lease line
is failing. If uninterrupted service is important choose this
option. With the cooperation of your local Bell salesman this
option can also be used to provide 4 wire service without
purchasing a leased line. This would provide a low volume
decnet link with much cheaper common carrier rates than would
be possible with a leased line.
The following options are specific to the Bell 208A only.
CC indication of analog loopback on. In analog loop test
mode this option enables the data set ready lead (DSR) which
the DN20 requires before transmission can begin.
Continuous request to send in continuous carrier mode.
Switched request to send is only used in certain mutidrop
configurations.
One second holdover at receiver on line dropout provided.
This prevents loss of the connection when carrier drops for
very short times. Since DDCMP performs its own error checking,
any errors encountered during short carrier losses will be
corrected through retransmission.
ICC COMLINK II (up to 19200 bits per second EIA only)
No information currently available.
OTHER
Note that local DMC's, and EIA KDPs (up to 19.2KB) have been
mentioned. DECnet-20 can also be purchased with a remote DMC (DMC11
AR/FA) which uses the CCITT V.35 electrical standard. This is for
remote operations above 19.2KB. We currently have no information on
modems or null modems for the V.35 interface. (perhaps the GTE
L500A-5 modem is used?) The Bell 303 current loop modem can be used,
but this requires a CCITT V.35 to Bell 303 conversion unit. Good
luck.
Note also that Digital makes an EIA DMC. It is not however,
sold with DECnet-20. The KDP interface is superior to the DMC at
speeds up to and including 19.2KB and EIA doesn't work well above
19.2.
To: DECnet-20 Users Page 4
Subj: Modem strapping for Decnet lines
As a special bonus here is a copy of the Pin definitions for
the 25 Pin connectors you will find on your modems. These will be
helpful when you are using a breakout box to try to diagnose a
communications problem.
To: DECnet-20 Users Page 5
Subj: Modem strapping for Decnet lines
+--------------------------------------------------------------------------+
| | | | | |
| PIN | NAME | TO TO | FUNCTION | CIRCUIT |
| | | DTE DCE| | (CCITT) (EIA) |
|--------+-------+--------+-----------------------------+----------+-------|
| 1 | FD | | FRAME GROUND | 101 | (AA) |
| 2 | TD | > | TRANSMITTED DATA | 103 | (BA) |
| 3 | RD | < | RECEIVED DATA | 104 | (BB) |
| 4 | RTS | > | REQUEST TO SEND | 105 | (CA) |
| 5 | CTS | < | CLEAR TO SEND | 106 | (CB) |
| 6 | DSR | < | DATA SET READY | 107 | (CC) |
| 7 | SG | | SIGNAL GROUND | 102 | (AB) |
| 8 | DCD | < | DATA CARRIER DETECT | 109 | (CF) |
| 9 | | < | POSITIVE DC TEST VOLTAGE | | |
| 10 | | < | NEGITIVE DC TEST VOLTAGE | | |
| 11 | | | UNASSIGNED | | |
| 12 | SDCD | < | SEC. DATA CARRIER DETECT | 122 | (SCF) |
| 13 | SCTS | < | SEC. CLEAR TO SEND | 121 | (SCB) |
| 14 | STD | > | SEC. TRANSMITTED DATA | 118 | (SBA) |
| 15 | TC | < | TRANSMITTER CLOCK | 114 | (DB) |
| 16 | SRD | < | SEC. RECEIVED DATA | 119 | (SBB) |
| 17 | RC | < | RECEIVER CLOCK | 115 | (DD) |
| 18 | | > | RECEIVER DIBIT CLOCK | | |
| 19 | SRTS | > | SEC. REQUEST TO SEND | 120 | (SCA) |
| 20 | DTR | > | DATA TERMINAL READY | 108.2 | (CD) |
| 21 | SQ | < | SIGNAL QUALITY DETECT | 110 | (CG) |
| 22 | RI | < | RING INDICATOR | 125 | (CE) |
| 23 | | > | DATA RATE SELECT | 111/112|(CH/CI)|
| 24 | (TC) | > | EXTERNAL TRANSMITTER CLOCK | 113 | (DA) |
| 25 | | > | BUSY | | |
+--------------------------------------------------------------------------+