Telog Battery Assisted Data Download Cable
It has recently been made known that some notebook PCs do not provide all the control signals on the RS232 (Com) port. Specifically the RTS signal, found on Pin 7, is missing. Telog Linecorders use the voltage usually found on this pin to provide power to the optically isolated data port. If this voltage is not available downloading from the Linecorder is not always possble.
The circuit above shows how a simple battery supply can provide the necassary power to Pin 5 on the Linecorder. What is also used are two diodes that should the voltage be available on Pin 7 on the PC port then no unecassary power is drained from the battery.
The diodes are 1N4148. The battery is any voltage from 9 to 12 volts. Nothing is prohibiting the use of a miniture 12volt battery as used in most key-ring remote controls and will facilitate the construction of a relatively small adapter.
R1 and R2 provide an "alternative ground" arrangement for notebooks that do not have Pin 5 (signal ground) connected. R1 and R2 however limit any ground loop currents that may exist should both Pin 5 and the case be connected within the PC. R1 and R2 are any value from 10 to 22 ohms.
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