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Here are a few notes about the modems collected over the years.

CDM-550 Modems

There are also some CIM-25 adapters on some of the modems that we were playing with. These dongles have a DB-9 interface that plugs into the remote serial port of the modem. With a RJ-45 jack on the back, they are powered by the modem and accessible on the network. They present a web interface for communications and configuration of the modem. They are fixed at 19200 baud and require the modem to be in remote control mode and have the data serial port connected to something!. They support SNMP, but seem to get a little confused if they are hammered with data requests too quickly. While they password protect their access, they do not clear or timeout half-opened TCP connections, so they sometimes get hung up by probes. Best to use them behind some sort of NAT or firewall that can restrict access to them. The default user accounts for web access are "admin" (full access), "opcenter" (read/write), and "monitor" (read-only), all with default passwords of "1234".

When setting up a Comtech modem, it is particularly important to make sure that the remote control port is set to 19200 baud and that it agrees with the setting on the Digi terminal server. To set the baud rate of this port, one needs to configure the remote control baud rate, then set it back to local mode. The value should default to 19200, but it is worth a double check.

In order for the Comtech CDM-550 modems to work well with Cisco serial ports through a CAB-530MT (or CAB-SS-530MT) cable, a DB-25 to DB-25 adapter from Comtech needs to be used. This adapter, among other things, drops pin 23 so that the modem and router do not argue about who provides the clock for the link. Comtech has been happy to provide the adapters (part number CA/WR0056 as per Comtech in 2005...or maybe CA/WR9718 as per Comtech in 2011) for free if you talk to their service department (Richard Rivera is the most helpful there). The CDM-570 modems do not require this adapter so long as the switch by the port is in the default 1:N mode.

Sometimes all of the settings look good on a modem, but it still does not lock. The frequency, data rate, and FEC are all correct, the signal shows up on the spectrum analyzer, and the cabling is good, but the demod still does not lock. This may be a spectral inversion issue if both sides are not matching normal or inverted (flip it and see if that fixes it). The problem may also be in the signal gain that is getting to the modem. If there is too much loss in the cabling, the modem may not see enough signal. Look at the modem's Rx signal gain to see how much it sees. Less than about -60 dB, and it will turn off the demodulator. It should be up in the -40 level to get a good solid lock.


A good data connection really needs to be made on the DB-25 interface in order for the remote serial interface to work. This either involves no data cable being plugged in at all or having a Comtech supplied magic adapter installed if the CDM-550 is connected to a Cisco. It is something strange about the way they pulled pins for that remote control serial interface from the modem's data interface. If the CDM-550s are plugged into a Cisco modem without a Comtech magic adapter, their remote control port will not work.

Upgrading firmware

In the uncommon case one needs to upgrade firmware, it is a pretty simple process provided:

  • You are using a windows computer with a serial port
  • You are able to power cycle the modem easily and promptly
  • You use a straight-through serial cable with a female 9-pin gender bender
  • You unplug the data connector (25 pin) when doing the update or it may not work. Some pins are shared between those interfaces, so it wont work with a RS-530 connection (232 may be fine though...)

The quirks may also apply to CDM-570 modems, but I havent tried upgrading them yet.

Firmware files are available from the Comtech support website. They support both the current CDM-570 modems and discontinued/legacy products. Note that we use CDM-550s and not CDM-550R modems.

+5V PSU Undervoltage alarm

The older CDM-550 modems sometimes started kicking out an error about "+5V PSU Undervoltage" and turning their alarm light red. This was an indication that the +5V pin on the modem motherboard was not really at 5V. According to Comtech, the connector using that 5V was not particularly robust in the older models of modems, and would sometimes oxidize a bit in moist environments and see less than its lower threshold of happiness on that pin (something liek +3.5V, I think). The error wasnt super critical, but they did send us the new-and-improved jumper cables to make the connection more solid inside the modem. The Comtech-suggested solution was to open the modem (voiding an already outdated warranty), locate the cable, pull it on and off a few times to scrub the pin a bit, then replace the cable with a new one (if available).

After replacing the cables in modems identified to have the problem, we havent see this issue since.

CDM-570 Modems

These are the newer versions of the CDM-550 modems. Most of what is written applies to the CDM-570s as well, but the CDM-570s are equipped with an Ethernet port for web and SNMP control, along with a switch that avoids needing the Cisco adapter (that 1:N switch). Otherwise, the perform in very much the same way. There are sometimes some strange web login quirks. See Why does my Comtech CDM-570 modem not let me login?

Factory Reset

To factory reset one of these (and the CDM-570L as well), do the following:

  1. Get back to the main screen of the modem front panel. This is the one that shows firmware version and model number. Pressing the "Back" button a bunch of times ought to do this.
  2. Press the up and down pointing arrows at the same time for just a second. This should bring up some options for resetting the modme.
  3. Select the NVRAM option with the blinking cursor. Hit Enter.
  4. At the confirmation screen, press enter to reinitialize all NVRAM.

At this point, the modem should be in a default factory configuration. You will need to reconfigure it to have all of the right settings for this modem to operate in HiSeasNet.

CDM-570L Modems

These modems are used on the Ku-band ships that have L-band equipment instead of the 70MHz equipment. That means the 1m 4006 and 1.5m 6006 antennas and BUCs, and NOT the 1.2m 4996 antennas and converters. The CDM-570L modems are essentially identical in features to the CDM-570s except they work in L-band and not 70MHz on the RF side. As a result, they do present the entire band at the modem instead of requiring a converter to single out a transponder into the 70MHz IF range. This makes it easier to see other transponders (and see other carriers) on SatMex5. It also makes for some strange IF frequencies if you are used to 70MHz.

There have been times when the Tx and Rx data rates were locked to 1544 kbps. The trick is to check the interface type. Set it correctly to the RS-422 (aka EIA-530) value and then the data rates should be editable again. The only time we saw this happen, we were unable to determine how the interface got changed in the first place.