Network Control Module 300 Series Technical Bulletin 66
Short Haul Modems (Line Drivers)
A short haul modem has electronic drivers for two dedicated twisted
pair Electronic Industries Alliance (EIA) interface devices (for
example, NCM to Remote Terminal) where no telephone line exists.
Some of the ways in which these modems are used result in noise
being picked up by the NCM. For example, if the remote terminal is a
portable PC that is removed after use, then the open terminal can be a
source of noise to the NCM. Alternatively, if the modem is used in a
printer application, the unused receive terminals on the NCM end may
pick up noise.
Short haul modems may not be reliable for mission critical
applications. Do not use them in applications that cannot tolerate any
downtime, such as hospital, pharmaceutical, or other critical industrial
applications.
Short haul modem pairs must be identical to communicate. If you
replace one modem, make sure your replacement is identical to the
remaining modem or replace them both.
Precautions When Using Short Haul Modems (Line Drivers)
Any short haul modem used in Metasys System operation must have
DC electrical isolation between the two output pairs and the RS-232
port built within the modem. This is not the same thing as noise or
transient suppression. While transient suppression is acceptable, only
models of modems having the electrical isolation are allowed.
Conventional trunk noise reduction wiring goes a long way toward
preventing this problem. Shielded twisted pair wires with proper noise
grounding techniques cure most problems before they can occur.
For example, earth ground the shield on the NCM side in applications
where a remote terminal is used. Consider using a soft ground with a
560 pf capacitor between the remote end of the shield and earth.
In cases where the trunk is set up for multi-drop use, such as a portable
remote terminal, attach an end-of-line resistor at the farthest
termination from the master device. A 220-ohm 1/4-watt carbon film
resistor should be sufficient. One resistor should be attached to each
twisted pair, receive and transmit circuits. These resistors, and the
internal impedance of the master device, prevent a high-impedance
antenna effect from bringing unwanted noise into the system.
In extremely noisy conditions, connect a Johnson Controls line
conditioning device/noise limiting capacitor (ACC-22-0) between the
two receive wires at the NCM side of the modem. If the modem is
used as an out-going signal only, such as in a printer application, the
unused receive terminals on the NCM end should be shorted together
to prevent noise pickup.
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