by Jim La Porta, N1CC

1. Why use Modems?
2. Finding the Right Solution
3. Getting the Right Modems
4. External Modems or Internal Modems?
5. Setting up the Modems and CT to work together
6. Modem Settings

1. Why use Modems?

If your operating positions for a multi-multi are separated by more than fifty feet, yet within the size limitation that particular contests permit (i.e. ARRL Field Day within a 500 foot diameter circle), you might need to "extend" your PC-to-PC connections. There are other reasons you might consider using an alternative to shielded EIA-compliant cables. All of these factors may come into play:

1. Distance between individual PC's.
2. Proximity to RF near-field (Interference to the PC-to-PC link as well as EMI from the PC's linked together getting into HF receivers at the site).
3. EMI from one or more PC's affecting another PC (Part 15 only requires that Class B devices not radiate beyond 100 feet, when in the same room multiple PC's can and do cause mutual interference).

2. Finding the Right Solution

Some solutions have used expensive Fiber-Optic links (Typically more than $200 per PC-to-PC Link) or Extended Distance Cables with double shielding (Around $1.50 a foot with expensive shielded hoods on the EIA connectors). Few solutions have considered the amount of mutual interference between computers, which may be aggravated in a high RF-Field.

You can bulletproof your multi-multi or remote your operating position using readily available dial modems. To use dial modems, however, you usually need to provide a "dial-up circuit". Such a circuit typically requires that there be "loop current" and "ringing current" - which, for most users means a Telco-provided dial circuit for each computer link.

A three-computer network would require PC #1 one-RJ11C Telco circuit, PC #2 two-RJ11C Telco circuit, and PC #3 one-RJ11C Telco circuit; for a total of four Central Office (CO) circuits. In the Albany NY area this would cost $17 a month for each, plus an installation fee per circuit of $175. This means the first year's cost for the circuits would be ($68 X12) + ($175 X 4) = $1,516. For most of us this is too much to pay. There are two ways to solve this "cost" limitation:

1. Build a false dial telephone network in your station. This is accomplished by creating the necessary conditions for dial-modems. That is, "Loop Current" and "Ringing":
   1. For Loop Current you need to provide DC Voltage of +12V up to +48V on each circuit. With proper bypassing you can do this with one 12V automotive battery, use Diodes and .01 mmfd capacitors to isolate each of the faux dial circuits from each other so they do not have a common AC path. Note: The typical load of a modem on this false loop will be less than 100 mA.
   2. For "Ringing" a modem needs between 90 and 145 Volts AC at 20 to 30 Hz, switched on for 1 second .... off for 2 seconds, on for one second... at which time the dial-modem should answer by going off-hook and sending answer back tone. Note: One modem should be configured for "Originate Only" and the second modem for "Answer Only". Once answerback begins don't switch the ringing voltage on again. You can use a rotary switch to select the circuit you wish to ring.

      CAUTION: Fuse the output of the "Ringing" circuit so that an operator coming in contact with the equipment cannot be badly shocked!

      It should cost less than $50 to put together a simple network using the false dial network described above.

2. Use "Dial" modems that can work on "Leased Lines". Modems that have this capability do not need Loop Current or Ringing to make their connection.

3. Getting the Right Modems

The modem speed isn't as important as you might expect. CT supports internetworking at speeds up to 19,200 bps. Even in a major multi-multi you may not need that speed. Modems have long been available using Microcom Networking Protocol (MNP) Level 4 and Level 5, in newer modems this is supplemented with ITUV.42 and ITU V.42bis protocols.

MNP-4 and ITU V.42 provide for error correction so that this part of the support suite ensures that any damaged data is corrected before it is delivered to the next PC in the chain. There is a small amount of delay, typically under 30 milliseconds added by the error correction during normal operation, and if a "CT-Packet" is damaged, it is retransmitted up to seven times until the far-end modem accepts the data (we'll discuss the effect of that delay at various speeds).

MNP-5 and ITU V.42bis provide for data compression of up to four (4) and eight (8) times the modems actual analog-side speed. The table below provides the reasonable expectation for error-free compressed data with different modem modulation technologies:

*Terminal speed shown lists MNP-5 first and ITU V.42bis second. Note that MNP-5 has a maximum speed of 38400 bps, and that 115200 is shown as the maximum speed for V.34. New V.34 modems will often go faster, however, are of little use in this configuration.

From this table, you can readily see that a V.22bis modem will provide you with 9600 bps speed. These units are current market-priced at as little as $20 for new products, and a plethora of models available for Amateur Consumption at flea markets and at second hand dealers. Some models that may work on 4-Wire or 2-Wire leased lines include: Codex 2233, Codex 2234, Concord Data Systems 224, GDC224, Hayes Optima 2400, Microcom AX/2400C, SupraFaxPlus 2400, and others. For the cost of a "false dial network" virtually "any" V.22bis modem will work.

The next speed to consider is 9600, or V.32 (4800 is a fall-back speed). A large number of devices that work on leased lines may be found in the used marketplace. In particular the Codex 2260, 2264, 3261 (early) and UDS 3225 modems all work on 2W or 4W leased lines. For example, one Ham bought two 2264's at a hamfest this spring for $50 total.

How do you find out if a V.32 modem works on a leased line? Usually there is a third audio position on the modem's back panel (1 for Telephone Set, one for Dial line and the third for leased line). Since most computer users consider 9600 obsolete, you might find a number of these units laying around at a good price.

The V.32bis or 14400 bps modems sell today for $49 new. These are rapidly being phased out by V.34 modems. One note of caution only originally high-end V.32bis modems work on leased lines so if your goal is the leased-line solution there are only a few to choose from. Some examples are Motorola 3261 and UDS 3229.

At 28.8 Kbps or V.34 you are dealing with the current market, low-end prices of $150 to high-end prices of $1500. In this group very few modems offer leased line support. Some that do are the Motorola Premier 336 ($495 List) and 3261FAST ($995 List). The idea of using low-end V.34 modems is perfectly acceptable, however, any V.34 is an overkill in regards to CT-Networking.

4. External Modems or Internal Modems?

Almost without exception Internal modems do not work on leased lines. If you are going to use a Telco-provided dial circuit or the "false" network described above you can use the Internal modem. I have used an internal 14.4 modem using the Rockwell chipset for several years with no problems to communicate with CT between sites.

5. Setting up the Modems and CT to work together

There isn't a lot required to set up CT to work properly with modems. In the CT-provided Terminate and Stay Resident (TSR) tools of COMTSR1, COMTSR2, COMTSR3 and COMTSR4 you have a choice of flow control and Data Character Size:

1. Flow control is optionally set for None, "inband" signaling through the use of "XON" and "XOFF" and for "hard-wire" use of RTS/CTS or DTR/DSR control signals. Here are the trade-offs:

   None

   The modem cannot signal the Computer to stop sending and hold data until the modem can pass data again. This is generally undesirable, however, some V.22bis modems will work best with this option, as the I/O buffer is typically 512 bytes ... if you are only dealing with retransmissions infrequently this will be large enough.

    

   RTS/CTS

   The modem looks for RTS (Pin 4 of a DB25) from the Computer, it then sends CTS (Pin 5 of a DB25) to the Computer. Note: Only use a straight-through cable with a Modem Link. This is probably the best setting for all modems. You need to test this option however, as some modems, such as certain Zoom and US Robotics models don't support control signals. Especially with higher speed modems, this option should be defaulted until testing may confirm a better choice.

    

   DTR/DSR

   The modem looks for DTR (Pin 20 of a DB25) from the Computer, it then sends DSR (Pin 6of a DB25) to the Computer. Note: Only use a special CT cable with a Modem Link. This is probably the second best setting for all modems. Test this option however, as with some modems, such as certain Zoom and US Robotics models support of control signals may be erratic. A secondary problem is that some Serial Boards in the Computer might not recognize the COMTSR commands and would not work. Note: You need a special cable to use a Modem with this option. Pin 6 on the modem needs to connect to Pin 5 on the Computer, so that this option becomes DTR/CTS. Especially with higher speed modems, this option should be defaulted until testing may confirm a better choice.

    

   XON/XOFF

   This inband method depends on how the computer interprets the COMTSR. The K1EA COMTSR should work with fully-compliant serial boards, however, most boards in computers today are not fully-compliant. Manufacturers have found that its cost-effective to short-cut on components that are not commonly used. This remains an option to consider.

2. Data Character Sizes of N81, N82, E71. DO NOT USE N82 with modem links as MNP-4 and ITU V.42 can only work with 10-bit codes (1 Start bit, 8 Data bits and 1 Stop bit). I recommend that you use "N81" (No Parity, 8 Data, 1 Stop) to insure the best reliability.

The most important thing to remember is to write down the settings you have made for the COMTSR and keep track of changes you might make to "fine-tue" this requirement.

6. Modem Settings

V.22bis, V.32, V.32bis and V.34 modems all use the "AT" command set you need to use a terminal program to set the modems up to support auto-connection or auto-dialing for CT prior to operation. These settings are non-volatile and will remain in the modem's firmware.

Some modems can be set up to automatically dial (when using the Telco dial network). Carefully review the Commands section of the Modem manual if available.

When using the "false" dial network you simply configure one modem as "Originate Only" and the second modem as "Answer Only"; with this configuration as soon as you power on the modems they are ready to communicate - when the "Ring" is actuated on the Answer Only modem handshaking takes place and you are ready to go.

For Lease Line applications one modem is again set to be "Originate Only" and the second modem as "Answer Only"; with this configuration as soon as you power on the modems the Answer Only modem starts to send Answerback and handshaking takes place and the call is established.

Note: In all cases unless you disable "Operating Status" you will get garbage in your network from the modems. Be sure to choose ATQ1 to disable status reporting to the Computer.

Here are some typical settings you may be able to make with "AT" commands:

You generate a "Command String" to tell the modem what you want it to remember, and then use the AT&W1 command to save the changes you have made as the default option for the modem. A "Command String" might look like this: AT&C2&D5&L2ATQ1AT*AA0*CA0*EC0*FL3 (Or, in modemspeak DCD Present when connected only, DTR seen as command to connect, 4-Wire Leased Line, reports to Computer disabled, Originate Only, Originate Only, auto negotiation, RTS/CTS Flow control).