I'm playing with an old modem and am trying to create a cable for it to use on my QL. It's tiny and doesn't have a power input (Stowaway 2400 Vocal).
Modem.png (108.23 KiB) Viewed 347 times
I initially thought it used the power from the phone line (like telephones did) but plugged it into my WIndows laptop and got it to recognize AT commands, so it's drawing power from the serial port. It's a 25 pin connector with 8 lines connected:
So it uses all but pin 22. I do have a DB25 breakout box somewhere so I can try and figure out where it's getting power from (keep adding pins to see when it starts working). Curious if anyone has an idea where it's getting continuous voltage from.
I haven't been able to find any documentation on the modem. It was sitting in my drawer for 20+ years. It matches the QL look (actually more the ZX81 with its red lettering). I wonder if it's designed to deal with the range of voltages. On wikipedia it says that some "RS232 compatible" devices only output 5 volts, whereas it can range up to 13 volts (+/-) and I even saw 25 volts. So I can imagine the manufacturer may have accounted for that.
The standard specifies a maximum open-circuit voltage of 25 volts: signal levels of ±5 V, ±10 V, ±12 V, and ±15 V are all commonly seen depending on the voltages available to the line driver circuit. Some RS-232 driver chips have inbuilt circuitry to produce the required voltages from a 3 or 5 volt supply. RS-232 drivers and receivers must be able to withstand indefinite short circuits to the ground or to any voltage level up to ±25 volts. The slew rate, or how fast the signal changes between levels, is also controlled.
