Serial Modem Wiring

[Volume]

I proudly own a little older computer (Pentium II, 233) and additionally now an old serial ISDN modem. (Not this modern USB stuff)
Now the modem wire ends in a 9 pin D-Sub while my computer has a 25 pin D-Sub COM 2 port (I use COM 1 instead of the PS/2 for the mouse because this highly sophisticated piece of Micro$oft operating system does run more stable with the wheelmouse on the COM 1 and the IRQ for the PS/2 disabled, otherwise I always have a blue screen when scrolling through the start menue with the wheel :mad: )

Which pins must be connected from 9 to 25 to make the modem run ?
Transmit Data, Recieve Data, and Signal Ground as well as Protective Ground on one connector (computer side I think), but what about hardware handshake lines ? Or don´t such things exist any more ?
And which pin no of the 9 pin to be connected with which one of the 25 pin ???? I think the wire crosing is already done in the 9 pin cable, so I suppose there is no further crossing needed.

I used to work on computer hardware in the times when floppies had 8" diameter and were hard sectored and 64 kB main memory and 10 MB hard disks (5 1/4", 5 disks, 10 surfaces, two slots high) were state of the art :D
At that time only 25 pin connectors existed and RTS/CTS handshake was normally used.

Any hints are wellcome

Volume

[RomeoTangoFoxtrotMike]

Which pins must be connected from 9 to 25 to make the modem run ? Transmit Data, Recieve Data, and Signal Ground as well as Protective Ground on one connector (computer side I think), but what about hardware handshake lines ? Or don´t such things exist any more ?
And which pin no of the 9 pin to be connected with which one of the 25 pin ???? I think the wire crosing is already done in the 9 pin cable, so I suppose there is no further crossing needed.

You should be able to get away if Tx, Rx and Gnd if you can configure your modem driver to use software flow control. The issue of wire-crossing is a bit misleading. The IEEE specs transposed the Tx and Rx pins on the 9-way connector :rolleyes:, so connecting pins 2-to-2 and 3-to-3 will give the effect of a "cross-over" cable. Pin 5 is signal ground on the 9-way, IIRC.

HTH.

[Ronbmy]

see...........

http://www.anotherurl.com/library/rs232.htm


Just reread your original post.

You had modern stuff didn't you? 8 inch floppies, oh what fun.

I remember the depressing IBM mainframe we had - card punch & reader, 10MB (removable) hard drive the size of a cow pie.

Waiting for the reader to process 8000 lines of Cobol. Go for dinner, have a kip and still not finished on return. Then you find the typo and have to go through it again.

Thank God for Fujitsu Cobol and a pc.

[Mac the Knife]

You should be able to find a little dongle adaptor that converts 9-pin to 25 pin D-subs. It's a completely passive connection converter that consts thruppence ha'penny and there should be dozens of them lurking around your local electronics junkshop. You'll just have to look to make sure it has the appropriate male/female sockets and if necessary find a so-called gender changer socket. Easier than soldering it yourself and figuring out the TX/RX/DTS/RTS connections. The same shop may well have a 9-to-25 pin cable which is less bulky and clumsy than the adapter.

[Avtrician]

OK here goes

25 Pin Plug 9 Pin

2 Transmit Data 3 TX
3 Receive Data 2 RX
4 Reqest to send 7 RTS
5 Clear to send 8 CTS
6 Data set ready 6 DSR
7 Signal Ground 5 SG
8 Carrier Detect 1 CD
20 Data Terminal ready 4 DTR
22 Ring Indicator 9 RI

Wire up the plug male or female as required as described and you are away being careful as its easy to connect to the wrong pin.

Or go to your local electronics/ computer shop and buy ready made.

Good luck:)

[Ronbmy]

To quote Mac the Knife...

You should be able to find a little dongle adaptor that converts 9-pin to 25 pin D-subs.

Care should be excercised when using a gender bender with some motherboards. If the 25 pin connector is soldered directly to the m/b then use of a gender bender plus the normal cable can put too much strain on the m/b connection. If the solder reflow system used in manufacture was not perfect then the chance of a dry joint - and intermittent working - increases. The number of new m/bs returned for reworking is enormous.

Best use a 25 to 9 cable. This is the cheapest good option

Better still...roll your own.

[Volume]

Thanks so far,

but because the modem 9 pin cable is too short anyway, I have to use a 9 to 25 pin wire. I´ve already found the COMPLETE 9 to 25 connection, the big question is : how many lines are NEEDED ?
The modem connector is a 6 line telephone wire plug, so definitively 3 of the 9 pins on the other end are unused.
I still have a 25 to 25 cable, once used to connect my 286 with an Ampex terminal (Yes, with really good operating systems it was possible to have multi user operations on a 286 in the mid 80s) which has just 5 pins connected, the question is are they enough ? Or is there a 6th important line ?

[Mac the Knife]

"..The modem connector is a 6 line telephone wire plug, so definitively 3 of the 9 pins on the other end are unused..."

Sorry Vols, I'm now completely confused about your question

"...really good operating systems..."

Do you mean Digital Research's 'Concurrent CP/M' for the 8086/8088 or MP/M II running on a Z80 (eg. TRS-80)? Both sorta work if you're happy with a glass Teletype. Or do you mean DR's 'Concurrent DOS' (needed a 386)? Perhaps you mean Minix on a 8088.

The 286 (deliberately) didn't have the LOADALL instruction for switching into real mode, so the small vs. large memory model problem remained and multi-tasking multi-user systems never developed further for this CPU.

IMS's REAL/32 is the descendent of DR's Multiuser DOS and still being marketed for special purposes (like POS terminals) thought it needs at least a 386.

[RomeoTangoFoxtrotMike]

Volume,

If both your modem and comms software can be set to use software flow control, then you only need Transmit, Receive and Signal Ground, i.e. 2, 3 & 5/7.