AIX Printer Tips: Twisted Pair Wiring for Async Communications: CAT 5 cabling for Printer Attachment


About This Document
Application to 128-Port Adapter

About This Document

Many people use Category 5 (CAT5) cables for attaching printers and asynchronous devices to AIX. When doing this, you should take care in matching the signals that are paired and the environment where these cables will be used. This document discusses some of the considerations that should be addressed when using this cabling for printing.

This document applies to all levels of AIX.


The following is a quote from UNIX SYSTEM ADMINISTRATION HANDBOOK by Evi Nemeth, Garth Snyder and Scott Seebass (p. 147):

"The proper way to connect shielded twisted pair wire is to use one pair for send (pin 2) and another pair for receive (pin 3). The second wire of each pair should go to signal ground (pin 7). The shields should be connected to frame ground (pin 1) at one end and not connected at all at the other end.

"Some installations cheat and use only one shielded twisted pair per terminal. If a terminal connected this way were struck by lightning, it would be fried. You pays your money and you takes your chances.

"The presence of chassis ground (pin 1) that is grounded at one end and unconnected (electrically floats) at the other end is required for full safety of the installation. In addition, the use of the second wire in the twisted pair for signal ground insures that if there is a power spike, which can easily happen if terminal wires are run near high-voltage lines, spikes on the data pins (2 or 3) will be compensated for by equivalent spikes on signal ground and the hardware will only see the difference, the true signal."

If these rules are not followed, you will find particular problems with long wiring distances when using Category 5 (network) wiring for attaching async devices.  This wiring is not recommended because it does not have the shielding that is defined in the RS/232 standards, but it is frequently used because of previously installed cable in the walls.  When printing, if the transmit and receive pins are not wrapped with signal ground, there can be flow control problems, especially when using the XON/XOFF protocol.

Application to 128-Port Adapter

Recommendation: 4-pairs Using Pinouts for 4-pair 596B Standard

RAN             Twisted Pair             Terminal
RJ-45                                    DB-25
Pin  Sig        Pin  Pair  Pin           Sig  Pin
4    TxD -----> 1     1     1  --------> RxD  3
6    SigGNd --  2     1     2  ------    SGnd 7
5    RxD <----- 3     2     3  <-------- TxD  2
6    SigGnd --- 6     2     6  --------- SGnd 7
8    DTR -----> 4     3     4  --------> DSR, DCD 6 & 8
1    DSR* <---- 5     3     5  <-------- DTR  20
2    RTS -----> 7     4     7  --------> CTS  5
7    CTS <----- 8     4     8  <-------- RTS  4   (Need for printer)

* You need to set Alternate RJ-45 pinouts so DTR from Terminal
or printer gets sent to DCD on RAN.

Twisted Pair Wiring for Async Communications: AIX Printer Tips: twstpair.html ITEM: FAX
Dated: 99/02/15~00:00 Category: zap
This HTML file was generated 99/06/24~12:42:12
Comments or suggestions?
Contact us