Serial cables made easy

It is possible to make up any RS-232 cable required using a single type of cable with an adaptor at each end - the trick lies in the adaptors.  In total,  eight different types of adaptor are required to cover all possibilities of:

With a little bit of thought, it is also possible to know exactly which adaptors are required without having to try all the possibilities - the trick here lies in the naming and labelling of the adaptors.


As already stated, you need to have a total of eight types of adaptor available. The following tables gives all the detail you need to make up all eight types. Full details can be found the in the R&D drawings quoted.

What to buy

The first step is to buy the unwired adaptors. These come as a pre-wired RJ-45 jack with push-fit D-type pins on the ends of the wires. The kit also includes the D-type and a nice moulded bit of plastic to hold it all together.

9 pin 25 pin
Plug Socket Plug Socket
CEMAST 0832724 0832732 0832740 0832759
RS stock code 447-667 447-673 447-689 447-695

How to wire it up

Each adaptor has to be assembled before use. This tables shows you which wire to stick in which hole - it doesn't matter whether you're assembling a plug or a socket here, they both use the same pin numbers.

9 pin 25 pin
(usually DTE)
(usually DCE)
(usually DTE)
(usually DCE)
Plug Socket Plug Socket Plug Socket Plug Socket
Name 9P-1 9S-1 9P-2 9S-2 25P-1 25S-1 25P-2 25S-2
R&D drawing no.
...65A4 ...67A4 ...66A4 ...68A4 ...69A4 ...71A4 ...70A4 ...72A4
Wiring details
RJ-45 pin no. Wire colour Wire colour
(New RS adapters)
D-type pin no.
1 Blue [Black] 7 8 4 5
2 Orange [Yellow] 4 6 20 6
3 Black [Orange] 3 2 2 3
4 Red [Red] 5
7 7
5 Green [Green] 1 1
6 Yellow [Brown] 2 3 3 2
7 Brown [Slate] 6 4 6 20
8 Slate [Blue] 8 7 5 4


The clever bit

Ok, so now you have a whole bunch of adaptors, but how do you use them? The trick is to identify the piece of equipment to be connected, and then remember that every cable you ever need has a Type-A adaptor at one end and a Type-B adaptor at the other end with a piece of straight UTP (or FTP/STP) cable in between. Note that if a reversed (not crossed) cable is used then one adaptor should be swapped for the opposite type (i.e. the cable becomes A-to-A or B-to-B).

The only slight catch left now is that you need to know what is a Type-A and what is a Type-B, which is where the next table comes in. It is suggested that each adaptor in use is labelled as shown in the table with the suffix -A or -B as appropriate, thus Sun-A, VT220-B and so on.

BBC Coded equipment

Unit code Name Label Adaptor
CD4/30 DAB Multiplexer OldMux-A 25P-1
OldMux-B 25P-2
CD4/31 DAB Multiplexer Mux-A 9P-2
Mux-B 9P-1
CD4/31A DAB Multiplexer Mux-A 9P-2
Mux-B 9P-1
CO4/49 Network Adaptor NINA-A 9S-1
CO4/50 Network Adaptor NANI-A 9S-1
CO9/21 Low Sampling Frequency Adaptor LSFA-A 9P-1
CO10/21 Serial-Parallel Interface Box SPIB-A 9P-2

Commercial equipment

Equipment Label Adaptor
Acton 3508 Acton-A 9S-1
Acton-B 9S-2
Bay Switch 350 Bay350-A 9S-2
Bay350-B 9S-1
CS200 CS200-A 25P-2
CS200-B 25P-1
IBM-PC25 PC25-A 25P-1
PC25-B 25P-2
IBM-PC9 PC9-A 9S-1
PC9-B 9S-2
Itis (ML2000) Itis-A 9P-2
Itis-B 9P-1
Philips DAB452 452-A 9P-1
452-B 9P-2
Psion Series 3c and 5 Psion-A 9P-2
Psion-B 9P-1
Pow-R-Switch PwrSw-A 9P-2
PwrSw-B 9P-1
Powerware UPS UPS-A 25P-1
UPS-B 25P-2
Rapco 1804L Rapco-A 9P-1
Rapco-B 9P-2
Sun TTy Sun-A 25P-1
Sun-B 25P-2
VT220 terminal VT220-A 25S-1
VT220-B 25S-2
Xilinx X-Checker XChek-A 9P-2
XChek-B 9P-1

Some pieces of equipment have in-built RS-232 connections on RJ-45 jacks. To connect to these, you can still use the standard piece of straight cable with an adaptor on the end, and you still follow the same rules of Type-A at one end and Type-B at the other. The following table shows how the in-built RJ-45 is wired - you need the opposite type adaptor at the other end.

Equipment RJ-45 wired as
Lantronix Type-A
None known Type-B

That's all there is to it! You can now make up any cable you need between any two pieces of equipment listed above without having to worry about whether it's a plug or a socket, or whether it should be a null-modem cable or a straight cable. Enjoy!


The following examples may make things a little clearer.

  1. Sun console port to VT220 terminal
  2. Any of the following four cables will connect a Sun console port to a VT220 terminal.

    1. Sun-A (25P-1) via straight cable to VT220-B (25S-2) preferred
    2. Sun-B (25P-2) via straight cable to VT220-A (25S-1)
    3. Sun-A (25P-1) via reversed cable to VT220-A (25S-1)
    4. Sun-B (25P-2) via reversed cable to VT220-B (25S-2)

  3. Sun console port to CS200
  4. Any of the following four cables will connect a Sun console port to a CS200.

    1. Sun-A (25P-1) via straight cable to CS200-B (25P-1) preferred
    2. Sun-B (25P-2) via straight cable to CS200-A (25P-2)
    3. Sun-A (25P-1) via reversed cable to CS200-A (25P-2)
    4. Sun-B (25P-2) via reversed cable to CS200-B (25P-1)

  5. Sun console port to Lantronix LRS Remote Access Server
  6. Either of the following two cables will connect a Sun console port to a Lantronix RAS.

    1. Sun-A (25P-1) via reversed cable to Lantronix
    2. Sun-B (25P-2) via straight cable to Lantronix

    Option one is usually used (so that a terminal can be easily plugged in without changing the adaptor) however option two allows the use of a standard length of UTP/FTP/STP cable. If needed, two straight cables can be joined using a coupler (available as RS part 186-3054, not CEMASTed) to make a single reversed cable.

  7. Laplink-cable (PC9 to PC9)
  8. Any of the following three cables will connect a pair of laptop PC's together.

    1. PC9-A (9S-1) via straight cable to PC9-B (9S-2) preferred
    2. PC9-A (9S-1) via reversed cable to PC9-A (9S-1)
    3. PC9-B (9S-2) via reversed cable to PC9-B (9S-2)

  9. Psion to Lantronix LRS Remote Access Server
  10. Either of the following two cables will connect a Psion Organiser (Series 3c, Series 5 or Siena) to a Lantronix LRS-series Remote Access Server (aka terminal server)

    1. Psion-B (9P-1) via straight cable to the LRS preferred
    2. Psion-A (9P-2) via reversed cable to to the LRS

Thanks... the following people for their invaluable help in producing these adaptors:

Written by Mike Ellis. All errors are completely my own work.
Version 0.3 © BBC R&D 1997-8