Author: Robert Hyatt
Date: 10:06:04 11/30/98
Go up one level in this thread
On November 30, 1998 at 12:12:29, James T. Walker wrote:
>Thanks Bob for trying to help. I don't have an external modem so I'm lost as to
>how to test the ports. All indications are that the computers are working and
>happy. They even ask if I want to end the connection when I exit the "Direct
>cabel connection" program. But they also time out while waiting for each other
>to connect! They both give messages indicating that I should check to see if
>the other one is trying to connect. I have checked the null modem cable(It is
>DB9F on both ends) for continuity. The pins are all straight through except for
>2&3 which cross. I think this is normal for RS232. So I guess I'm stuck with
>trying to find a way to check the ports. One computer has only 1 serial port
>set up as com1. The other has two ports (com1 & com2). The IRQ & base
>addresses are normal (com1-IRQ4,com2-IRQ3). The windows 98 device manager says
>there is no conflicts and everything is working normal.
>Thanks again,
>Jim Walker
we may be getting to the problem now. I am not sure that "straight through" is
correct for a null-modem cable. Yes you certainly have to cross 2 and 3 to
match transmit data (TD) on one end with receive data (RD) on the other. But
there are other issues... your computer should supply "DTR" (data terminal
ready) and it *requires* "DCD" (data carrier detected). As a general rule, for
DB 25's, here is what I used to do:
side a side b
1 1
2 3
3 2
4/5 jumpered to each other
4-5 jumpered to each other
6/8/20 jumpered
6/8/20 jumpered
it is certainly possible to avoid the "jumpering" but you have to do something
with 4/5, which is request to send, clear to send. They need to be crossed for
flow control if you need it (you probably do). 6/8/20 are data terminal ready,
data set ready and data carrier detect, and jumpering them lets your computer
supply DTR as it should, which will convince it that the other end is alive and
well since it will see DCD.
When I get to the office I'll ohm out a null-modem cable (db9 to db9) that I
know works and let you know how the pins should be connected end-to-end...
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