--- chapter.sgml.orig Wed Sep 7 20:57:22 2005 +++ chapter.sgml.3 Wed Sep 7 20:44:59 2005 @@ -78,7 +78,7 @@ RS-232 - RS-232C cables + RS-232-C cables EIA standard for hardware serial communications @@ -117,17 +117,22 @@ null-modem cable - A null-modem cable passes some signals, such as signal - ground, straight through, but switches other signals. For - example, the send data pin on one end goes to the - receive data pin on the other end. + A null-modem cable passes some signals, such as System + Ground, straight through, but switches other signals. For + example, the Transmitted Data pin on one end goes to the + Deceived Data pin on the other end. - If you like making your own cables, you can construct - a null-modem cable for use with - terminals. This table shows the RS-232C signal names and the pin - numbers on a DB-25 connector. + You can also construct your own null-modem cable for use with + terminals (e.g., for quality purposes). This table shows the RS-232-C + signal names and the pin numbers on a DB-25 connector. Note that the + standard also calls for a straight-through pin 1 to pin 1 + protective ground line, but it is often omitted. + Some terminals can get by using only pins 2, 3 and 7, while others + require different configurations than the examples shown below. - + + DB-25 to DB-25 Null-Modem Cable + @@ -189,11 +194,11 @@ - DCD + DTR + 20 + connects to 8 - - 6 - DSR + DCD @@ -203,25 +208,220 @@ 20 DTR + + + DCD + 8 + connects to + 20 + DTR + - +
+ Here are two other schemes more common nowadays. + + + DB-9 to DB-9 Null-Modem Cable + + + + + Signal + Pin # + + Pin # + Signal + + + + + + RD + 2 + connects to + 3 + TD + + + + TD + 3 + connects to + 2 + RD + + + + DTR + 4 + connects to + 6 + DSR + + + + DTR + 4 + connects to + 1 + DCD + + + + SG + 5 + connects to + 5 + SG + + + + DSR + 6 + connects to + 4 + DTR + + + + DCD + 1 + connects to + 4 + DTR + + + + RTS + 7 + connects to + 8 + CTS + + + + CTS + 8 + connects to + 7 + RTS + + + +
+ + + DB-9 to DB-25 Null-Modem Cable + + + + + Signal + Pin # + + Pin # + Signal + + + + + + RD + 2 + connects to + 2 + TD + + + + TD + 3 + connects to + 3 + RD + + + + DTR + 4 + connects to + 6 + DSR + + + + DTR + 4 + connects to + 8 + DCD + + + + SG + 5 + connects to + 7 + SG + + + + DSR + 6 + connects to + 20 + DTR + + + + DCD + 1 + connects to + 20 + DTR + + + + RTS + 7 + connects to + 5 + CTS + + + + CTS + 8 + connects to + 4 + RTS + + + +
+ - Connect Data Set Ready (DSR) and - Data Carrier Detect (DCD) internally in the - connector hood, and then to Data Terminal - Ready (DTR) in the remote hood. + Usually, when one pin at one end connects to a pair of pins + at the other end, it is implemented with one short wire between + the pair of pins in their connector and a long wire to the other + single pin. + + The above designs seems to be the most popular. In another + variation (explained in the book RS-232 Made + Easy) SG connects to SG, TD connects to RD, RTS and + CTS connect to DCD, DTR connects to DSR, and vice-versa. - Standard RS-232C Cables - RS-232C cables + Standard RS-232-C Cables + RS-232-C cables - A standard serial cable passes all the RS-232C signals - straight-through. That is, the send data pin on one - end of the cable goes to the send data pin on the + A standard serial cable passes all the RS-232-C signals + straight-through. That is, the Transmitted Data pin on one + end of the cable goes to the Transmitted Data pin on the other end. This is the type of cable to use to connect a modem to your FreeBSD system, and is also appropriate for some terminals. @@ -243,8 +443,8 @@ construct a cable, you need to make sure it will fit the ports on your terminal and on the FreeBSD system. - Most terminals will have DB25 ports. Personal computers, - including PCs running FreeBSD, will have DB25 or DB9 ports. If you + Most terminals will have DB-25 ports. Personal computers, + including PCs running FreeBSD, will have DB-25 or DB-9 ports. If you have a multiport serial card for your PC, you may have RJ-12 or RJ-45 ports. @@ -972,43 +1172,64 @@ modem If you are using an external modem, then you will of - course need the proper cable. A standard RS-232C serial + course need the proper cable. A standard RS-232-C serial cable should suffice as long as all of the normal signals are wired: - - - Transmitted Data (TD) - - - - Received Data (RD) - - - - Request to Send (RTS) - - - - Clear to Send (CTS) - - - - Data Set Ready (DSR) - - - - Data Terminal Ready (DTR) - - - - Carrier Detect (CD) - - - - Signal Ground (SG) - - + + Signal Names + + + + + Acronyms + Names + + + + + + RD + Received Data + + + + TD + Transmitted Data + + + + DTR + Data Terminal Ready + + + + DSR + Data Set Ready + + + + DCD + Data Carrier Detect (RS-232-C's Received Line + Signal Detector) + + + + SG + System Ground + + + + RTS + Request to Send + + + + CTS + Clear to Send + + +
FreeBSD needs the RTS and CTS signals for flow-control at speeds above @@ -1033,7 +1254,7 @@ Serial Interface Considerations FreeBSD supports NS8250-, NS16450-, NS16550-, and NS16550A-based - EIA RS-232C (CCITT V.24) communications interfaces. The 8250 and + EIA RS-232-C (CCITT V.24) communications interfaces. The 8250 and 16450 devices have single-character buffers. The 16550 device provides a 16-character buffer, which allows for better system performance. (Bugs in plain 16550's prevent the use of the