Some users of the LI-7000 CO₂/H₂O Analyzer use modems to transfer data over large distances (e.g. the Short Range Modem SRM-3A, which can transfer signals up to a distance of approximately 2.5 miles). When using modems of this type with the LI-7000, data does not travel through the modem to the computer. This is due to two different conditions that are occurring simultaneously.
LI-7000s with the newest digital board (s/n IRG4-0471 and higher, or those that have been upgraded) have a special setting that affects the RS-232 output. The LI-7000 enters a power-saving mode after about 30 seconds of inactivity; this turns off power to the pin that tells the LI-7000 that the computer is connected and ready to transmit/receive data. The power-saving mode is not an issue during normal operation with a computer, as the software sends a signal to the LI-7000 that “wakes” the instrument so that data transfer can take place.
When a modem such as the SRM-3A is used with the LI-7000, however, a problem occurs, because these types of modems do not have a built in power supply; they use the voltage passing through the transmission wires to power them. Since these modems have no power supply, they cannot act as a power source, and have no way to send a voltage signal to the LI-7000 to “wake” it and start data transfer. Thus, until a connection is established, the user is unable to send data between the LI-7000 and the computer.
The solution to this problem is to apply a small voltage to one of the unused pins on the RS-232 connector on the LI-7000. Pins 1, 6, or 8 can be used; we recommend using the Data Set Ready pin 6 (see below).
This voltage should be a DC voltage of 2.7 to 25 volts. The DC power supply should be a continuous source, as the LI-7000 enters power-saving mode after 30 seconds of inactivity. A small 9-volt battery, or two AA batteries will work fine. Wire the battery directly to the RS-232 connector on the back of the LI-7000. The battery will keep the LI-7000 in an “awake” state so that data transfer can occur normally, even when the modem is being used.