20.017 What kind of cable should I use to connect a modem to my IIc?
Description
This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
20.017 What kind of cable should I use to connect a modem to my IIc?
Here is the pinout of the IIc serial port looking at the back of the computer:
DIN-5F (female)
socket with
Apple's numbering
5 1
4 2
3
The functions are:
1 Handshake Out (nominally DTR)
2 Data Out (TxD)
3 Ground
4 Data In (RxD)
5 Handshake In (nominally DSR)
To connect a IIc to a typical modem use the following pinout for a non-hardware
handshaking cable:
IIc Modem
DIN-5M Dsub-25M DIN-5 Dsub-25M
plug male connector
1 Handshake Out 20 DTR
2 Data Out 2 TxD 1 5 ,--------/ /---------.
3 Ground 7 Gnd 2 4 \ 1 2 ... 12 13 /
4 Data In 3 RxD 3 \ 14 ... 25 /
5 Handshake In 6 DSR* `------/ /-------'
*You might want to use pin 8, DCD in some cases.
To connect a IIc to a modem with a 9-pin connector you can use the pinout below
for a non-hardware handshaking cable:
IIc Modem
DIN-5M Dsub-9M DIN-5 Dsub-9M
plug male connector
1 Handshake Out 4 DTR
2 Data Out 3 TxD 1 5 ,--------/ /---------.
3 Ground 5 Gnd 2 4 \ 1 2 ... 4 5 /
4 Data In 2 RxD 3 \ 6 ... 9 /
5 Handshake In 6 DSR* `------/ /-------'
*You might want to use pin 1, DCD in some cases.
The IIc cannot do hardware handshaking** very well, but this is as close as you
can get:
IIc Modem
DIN-5M Dsub-25M DIN-5 Dsub-25M
plug male connector
1 Handshake Out 4 RTS
2 Data Out 2 TxD 1 5 ,--------/ /---------.
3 Ground 7 Gnd 2 4 \ 1 2 ... 12 13 /
4 Data In 3 RxD 3 \ 14 ... 25 /
5 Handshake In 5 CTS `------/ /-------'
** Note that you need comm software which supports hardware handshaking
on the IIc to do this properly. I expect ProTerm does, but ZLink and
Talk Is Cheap almost certainly don't.
The IIc's handshaking lines have annoying side effects, which cause problems
with hardware handshaking:
1. The "Handshake Out" signal is implemented to mean "I want to send data" (the
official and original meaning of RTS). If you turn off the output handshake
line, the IIc will stop sending data. For a hardware handshaking modem, RTS is
supposed to mean "You are allowed to send me data" (from the computer's point
of view).
If the computer tells the modem to stop transmitting, the computer will also
be unable to transmit. This will reduce the rate at which data can be
transferred bidirectionally, but doesn't cause any other problems.
2. The "Handshake In" signal is implemented to mean "There is receive data
present" (the official meaning of DCD). If the incoming handshake line is
disabled, the IIc will stop receiving data (ignore any data on RxD). For a
hardware handshaking modem, CTS is supposed to mean "You are allowed to send me
data" (from the modem's point of view).
If the modem tells the computer to stop transmitting, the computer will also
be unable to receive, and will discard any data sent by the modem while CTS
is not active. This can cause screen corruption and loss of data blocks or
acknowledgements during a file transfer, which will require retransmission. It
is only likely to be a problem while a lot of data is being sent, so is more
likely to cause problems during a file upload than a download. If the comms
software is quick enough, it can drop RTS imm
ediately when CTS is lowered, which will prevent the modem from sending any
more data.
By: SuperTimer
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