128-Port modem cabling considerations

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This operating system does not require DSR in modem-control applications, and because almost all of today's modems have auto-answering capability, the Ring Indicator signal is generally unnecessary.

The 10-pin RJ-45 plugs is not the predominant cabling subsystem and may be difficult to obtain in the retail market. The TTY subsystem of this operating system provides an optional feature called ALTPIN, which swaps the logical functions of DSR (Data Set Ready) with DCD (Data Carrier Detect) for a port. When ALTPIN is enabled, DCD becomes available on pin 1 of an 8-pin RJ-45 connector (equivalent to pin 2 of a 10-pin connector).

If you wish to build an 8-wire modem cable for the 128-port RAN, use the 8-pin RJ-45 plug wired as in the following table:

Table 1. 128-Port modem cabling
Item Description Modem
SYSTEM END CONNECTOR

8-pin RJ-45

 DEVICE ENDRI 22
1 DSR 6
2 RTS 4
3 (Chassis) GND SHELL
4 TxD 2
5 TxD 3
6 (Signal) GND 7
7 CTS 5
8 DTR 20
  CD 8
Note: The physical location of DSR and CD may be swapped with the ALTPIN parameter when enabled using the stty-cmxa command.

The following table shows the asynchronous signal communication between the system unit and an attached modem. Here, data is being sent from the system unit to a remote system.

Table 2. Asynchronous signal communication
DEVICE SIGNAL ON/OFF MEANING
Computer DTR + Hey, modem, are you ready to connect to another system?
Modem DSR + Yes, I am ready. Go ahead and dial.
Modem DCD + I've got the other system on the phone.
Computer RTS + OK, can I send data now?
Modem CTS + Sure, go ahead.
Computer TxD   Sending data out to modem.
Modem RxD   I've received the data.
Modem CTS – Don't send me any more data, I'm sending it out...
Modem CTS + OK, I'm ready for more data, let me have it!
Transmit data steps may be repeated until... Computer DTR – FINISHED! Go ahead and hang up.
Modem DCD – OK.
Here is the signal communication between a RS/6000 and a modem about to receive an incoming call from another system. Computer DTR + I'm ready and have "enabled" the port for dial-in.
Modem DSR + I'm ready also but I'm just waiting around for a call.
Someone calls in! Modem DCD + Somebody called in and I've got them on the line.
Modem CTS + I've got data from another box, can I send you data now?
Computer RTS + I'm ready to receive. Go ahead and send.
Modem RxD   Here it comes!
Modem continues to send data until... Computer RTS – WAIT! My buffer is full, don't send any more data.
Computer RTS + I'm OK now. Send me more data.
Modem DCD – Call has ended.
Computer DTR – OK, please hang up.