art@acc.arpa (08/25/87)
>This is maybe a dumb question, but what is the intended use of pin 6, >the one named DSR or Data Set Ready in the RS232 standard? I've seen >that some devices (especially modems) uses it, but most ignores it (such >as computers and terminals). Is it really needed for modem-control >signaling or can I simply connect it to pin 20 (DTR) at the other end? >How should it be connected in a null-modem cable when I want to connect >two DTE devices with modem control? Data Set Ready basically indicates that the modem is in an operational state (Data Set is an old Telco term for a modem). DSR often indicates that the modem is in data mode rather than voice mode. DSR can be considered the counterpart to Data Terminal Ready (DTR), which indicates that the customer equiptment is ready to operate. For asynch RS-232 cables, I recommend the following: SD 2 -------------\ /----------------- 2 SD x RD 3 -------------/ \----------------- 3 RD RTS 4 ---+ +--- 4 RTS | | CTS 5 ---+ +--- 5 CTS DSR 6 ---+ +--- 6 DSR | | SG 7 ---|-------------------------|--- 7 SG | | DCD 8 ---o---------\ /-------------o--- 8 DCD x DTR 20 -------------/ \----------------- 20 DTR Art Berggreen art@acc.arpa ------
tim@ora.UUCP (Tim O'Reilly) (08/26/87)
In article <8998@brl-adm.ARPA>, art@acc.arpa writes: > > For asynch RS-232 cables, I recommend the following: > > > SD 2 -------------\ /----------------- 2 SD > x > RD 3 -------------/ \----------------- 3 RD > > RTS 4 ---+ +--- 4 RTS > | | > CTS 5 ---+ +--- 5 CTS > > DSR 6 ---+ +--- 6 DSR > | | > SG 7 ---|-------------------------|--- 7 SG > | | > DCD 8 ---o---------\ /-------------o--- 8 DCD > x > DTR 20 -------------/ \----------------- 20 DTR > > > Art Berggreen > art@acc.arpa > > ------ Note that the cable shown here is a null modem cable for connecting tw DTE (Data Terminal Equipment) interfaces. DTE-DCE (i.e. terminal to modem) connections would not cross lines 2 and 3, and 8 and 20. There are a variety of ways to do cable workarounds for DSR/DCD/DTR; you'll probably get people recommending other versions of this cable that also work. Most hinge on the fact that carrier detect is a more crucial indication of connection than DSR, and so most UNIX systems monitor DCD instead of DCR. However, I do want to point out one drawback of the cable shown here, which has nothing to do with DSR/DCD/DTR: A cable with CTS and RTS jumpered together will work fine for normal terminal operation (people don't type all that fast) but if you try to upload text from a PC using a cable like this, the receiving system will tend to lose characters. While CTS/RTS were originally designed to turn around a half-duplex modem connection, nowadays, they are often used for hardware flow control. If there is any chance your cable will have to carry data from the terminal to the host at computer speeds rather than at human speeds, you should be sure to include RTS and CTS. For the null-modem cable shown above, the two lines should be crossed. For a straight through cable, they should (obviously) be straight through. For UUCP, creating a cable like this is not absolutely necessary, since UUCP will retry packets to make sure everything gets through. However, you are increasing system overhead and decreasing performance for the sake of two wires. -- Tim O'Reilly (617) 527-4210 O'Reilly & Associates, Inc., Publishers of Nutshell Handbooks 981 Chestnut Street, Newton, MA 02164 UUCP: seismo!uunet!ora!tim ARPA: tim@ora.uu.net