RS-232 and other serial ports and interfaces
EIA-530, or RS-530, is a balanced serial interface standard that generally uses a 25 pin connector. The RS530 isn't an actual interfaceis, but a generic connector specification. The connector pinning can be used to support RS422, RS423, V.36/V.37/V.10/V.11 (not V.35!) and X.21 to name the most popular ones.

TIA-530 (1987) relies on EIA(RS)-422/423 and uses a differential signaling on a DB25 - RS232 format - EIA-530 Transmit (and the other signals) use a twisted pair of wires (TD+ & TD-) instead of TD and a ground reference as in RS232 or V.24. This interface is used for HIGH SPEED synchronous protocols. Using a differential signaling allows for higher speeds over long cabling. This standard is applicable for use at data signaling rates in the range from 20,000 to a nominal upper limit of 2,000,000 bits per second. Equipment complying with this standard, however, need not operate over this entire data signaling rate range. They may be designed to operate over a narrower range as appropriate for the specific application.  All signals are EIA-422 (balanced) except LL (pin 18), RL (pin 21), and TM (pin 25) which all use EIA-423 (unbalanced). 

TIA-530-A (1992) differs slightly by changing pins 6 and 20 to EIA-423 (unbalanced), adding a Ring Indicator (RI) on pin 22 using EIA-423, and grounding pin 23

Pin Name Dir Description Circuit Paired with
1   --- Shield    
2 TxD --> Transmitted Data BA 14
3 RxD <-- Received Data BB 16
4 RTS --> Request To Send CA 19
5 CTS <-- Clear To Send CB 13
6 DSR * <-- Data Set Ready CC 22 (not paired in TIA-530-A)
7 SGND --- Signal Ground Ground  
8 DCD <-- Data Carrier Detect CF 10
9   <-- Rtrn Receive Sig. Elmnt Timing DD 17
10   <-- Rtrn DCD CF 8
11   --> Rtrn Transmit Sig. Elmnt Timing DA 24
12   <-- Rtrn Transmit Sig. Elmnt Timing DB 15
13   <-- Rtrn CTS CB 5
14   --> Rtrn TxD BA 2
15  ST <-- Transmit Signal Element Timing DB 12
16   <-- Rtrn RxD BB 3
17  RT <-- Receive Signal Element Timing DD 9
18 LL --> Local Loopback LL Unbal, not paired
19   --> Rtrn RTS CA 4
20 DTR * --> Data Terminal Ready CD 23 (not paired in TIA-530-A)
21 RL --> Remote Loopback RL Unbal, not paired
22  ** <-- Rtrn DSR CC 6 (not paired in TIA-530-A)
23  *** --> Rtrn DTR CD 20 (not paired in TIA-530-A)
24  TT --> Transmit Signal Element timing DA 11
25  TM <-- Test Mode TM  Unbal, not paired
25 pin D-SUB male connector layout
25 pin D-SUB male connector
at the DTE (Computer)
25 pin D-SUB female connector layout
25 pin D-SUB female connector
at the DCE (Modem)
According to 6 reports in our database (4 positive and 1 negative) this pinout should be correct.

Is this pinout
RS-530 (EIA530) interface visual pinout:
RS-530 (EIA530) interface diagram
click to enlarge
Source(s) of this and additional information: RS530 Page at Connectivity Knowledge Platform (Made IT), arcelect, TIA-530-A (1992)
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