I want to use the line jack of a dial-up modem

Question

Typically, an ISA dial-up modem card has two RJ11 jacks: one labeled Telco and the other labeled Line or Phone, intended to be connected to a telephone. Normally, when issuing commands to the modem to dial out to an ISP, the dial tone is heard only from the Telco jack while the Line jack's dial tone is blocked.

However, I am seeking a method to dial out to the ISP using the Line jack. Is there a way to achieve this?

The reason I want to do this is because of my Amstrad PPC640 computer. The location of the Telco is not very convenient for me to use. Also, it's not a jack but a cable.

Answer 1

As far as I understand, you're asking to use the Phone jack as Line Out, that is the one being connected with your (land)line. Right?

That would be an extremely unusual feature, as no modem I know supports this. All I have ever seen are based on a two way switch routing the signals coming from the Telco socket to either the internal modems or to the Phone socket. There is usually no connection between modem and phone socket. In fact the switch is often a fixed mechanical one (relay), operating as basic change-over switch:

The whole thing is a bit like asking your stereo to output sound via the microphone input - if that analogy helps. Of course, one can always power up one's trusty soldering iron :))

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(And yes, I know there are soundcards that can reconfigure all ports, but that's a completely different technology)

Answer 2

It isn't quite clear what you're asking for, but it would seem to be more of a misunderstanding of what the jacks are for.

When you're using a modem on a telephone line, the modems on each end are communicating data by the way of modulated tones that fit within the audible frequency range that standard "land lines" could handle reliably. These protocols were very sensitive about the timing and shape of the sound wave that was used, so if someone picked up a phone handset and started talking during a connection, it would confuse the modems communicating and introduce errors in the data transmission.

While earlier, slower standards weren't very sensitive, 1200 baud and later standards were far more complicated and introduced tricks like Quadrature Amplitude Modulation which was effectively changing the shape of a sound wave mid-sine as means of representing more data. While this made for speed improvements, it also meant that instead of just a voice saying "Hello?", the communication would now even get thrown off by the "click" introduced by a phone being picked up even if the person was being silent.

To avoid these disturbances in your modem communications, people often had dedicated phone lines just for the computer. However, it was also desirable to use the phone line for voice too, so one would typically also connect a phone handset to the line as well. This is where the two jacks come in. The intent is that the "telco" jack would go to the wall (and thus to the telephone company) and the "phone" jack would be where you'd plug in a phone handset.

The difference between the Telco and Phone jack is that when the modem is using the phone line, it disconnects the 'Phone' jack so that you couldn't accidentally pick up the phone and disturb the modem communication. When the modem is done and hangs up, it re-connects the phone jack so that you can use the handset again. It's that simple.

Trying to think about what you were asking for in your question, I believe it might be that you're trying to have your PC (with the ISA modem) communicate with your Amstrad by hooking the two together with a phone wire. If this is the case, I'm sorry to say it won't work without some additional hardware that would emulate phone service and provide the proper voltages and simulated switching. Modems by themselves rarely provide enough signal voltage to direct-connect to each other. So your option is to either get one of these devices (see related question here), find two land lines, or resort to a non-modem way of linking the two machines together.

Answer 3

The following is based on the Amstrad PPC 640S Service Manual. I have never had an Amstrad so this is based on that service manual.

Using the features of the Amstrad and standard features of telphone system circuits it may be possible for you to achieve your preferred configuration.

Starting point is the Amstrad is designed to sit between the incoming telephone line and your telephone handset. As other answers have correctly assumed there is a relay (R901 on Modem Chassis Schematic Diagram 2) that disconnects your telephone handset from the telephone line when the Amstrad Modem is active. This normally prevents the handset socket being used to connect the Amstrad Modem via the handset socket to a telephone line.

Wiring of the telephone system has to be adaptable using different wiring configurations. The Amstrad is designed not to disrupt existing telephone wiring. Using this feature it may be possible to achieve your desired configuration.

Amstrad was a UK designed consumer device, the catch: this might not be legal wiring in your country. Laws and regulations vary between countries and I do not know your circumstances. Your responsibility to check any Legal/Regulatory requirements before modifying any telephone cabling, leads, etc.

Historically telephone systems would use two pair cable for domestic installations. This would permit connection of two separate telephone lines and the telephone sockets are provided to accomodate multiple lines. A standard handset connected to the Amstrad socket will use pins 2 and 5 for a connection to the telephone line. To accommodate other wiring configuration pins 3 and 4 are shown directly wired to the Amstrads telephone line cable. If you were to connect a telephone line to pins 3 and 4 of the handset jack the incoming telephone line should then appear on the connector of the Amstrad telephone lead. Just this step is likely to have broken any number of Laws and/or Regulations in most countries. You may also have an electrical safety issue of the telephone line voltage on the Amstrad telephone lead.

The incoming telephone line now has to be looped back to the Amstrad Modem. This means you need the Amstrad telephone lead plugged into a standalone socket that is appropriately wired. Installing a suitable socket in a small plastic box is one solution.

Best guess from the poor quality Amstrad schematic image I'm using is the telephone cable connection are part numbers:

Amstrad: CD951
Manufacture: AMS45125

With the following information on the Schematic:

P1 GRE (i.e. Green) - Amstrad Telephone Socket Pin 4
P2 YEL (i.e. Yellow) - Telephone line to Amstrad Modem
P3 BLA (i.e. Black) - Telephone line to Amstrad Modem
P4 RED (i.e. Red) - Amstrad Telephone Socket Pin 3

Best guess:

GRE = Green
YEL = Yellow
BLA = Black
RED = Red 

Obviously this colour code does not match the colour of the wires in your photograph, nor the pin numbers of a current standard telephone service. The Schematic's Electrical Parts List, Miscellaneous includes:

Circuit Reference: 
Description: Modem Extension Cable
Part Number: 177325

There are no details for the wiring of this Modem extension cable. I can not tell from your photograph if the Amstrad telephone extension lead is 2 or 4 wire. If it is 4 wire then the best guess is the Amstrad Telephone lead is wired in exactly the same configuration as the Amstrad Handset socket. If it is only two wire then you are out of luck. The standalone/isolated Telephone style socket (e.g. RJ45) would be wired pins:

P3 to P2
P4 to P5

Use a multimeter to confirm this wiring.