Hewlett Packard adopted the simple bell protocol for its Access port remote console/session operation on Series 9X7s, 9X8s and earlier generations. The style being used is different from the DTC port or MUX port operation most people are used to. HP's implementation of the bell protocol is as follows:

DTR is used to enable a modem or disconnect an existing modem connection/session.

CD Carrier Detect from the modem enables the remote port (port is active when true). CD low of course causes the computer port to disconnect or go inactive.

RTS/CTS hardware handshake is not implemented therefore only XON/XOFF will work.

Bell Mode AP protocol 1 Use this protocol in North America (Does not support Hardware Handshake)

This is sometimes called Bell simple protocol. It raises DTR when it can accept a connection. The connection is valid when it sees DCD. It drives RTS whatever it was when Remote is enabled (usually, RTS is low) and does not look at DSR or CTS. When a disconnect is done, DCD must drop before a new connection can be made (i.e. it will not raise DTR until DCD drops and raises).

Hewlett Packard mainframes and some CBX phone systems, require CD to stay high (true) until the modem disconnects and then go high again.

Hewlett Packard mainframes and some CBX phone systems, require CD to stay high (true) until the modem disconnects and then go high again. This is where all the trouble starts and boy do I mean trouble!

Many modems do not support this mode of CD operation. A typical Hayes compatible modem, like say a ZOOM; only has 2 modes for CD operation

AT&C0 Force Carrier Detect High

AT&C1 Let Carrier Detect follow carrier signal (normal operation)

More advanced modems like the MultiTech MT2834ZDxb provide additional levels of CD control:

AT&C2 Let Carrier Detect drop on disconnect then go high again (What HP likes)

HP needs one more feature in a modem. Whenever the computer lowers DTR; the modem must drop CD then raise it again. DTR will not raise until CD raises high again. That means HP needs a special modem to do the things they need to do. HP calls it a Remote Support Modem. This feature is often called the remote console mode. Almost no modem supports this feature. We will call this feature "DTR cycles CD"

Here comes the other problem. DIAL-OUT won't work unless CD is high.

It is the state of the CD signal that enables the computer to dial out. Two (2) things must occur for dial out to succeed.

1) CD must be true. This tells the computer the remote port has a valid hardware connection (not to be confused with a real session already on the port).

2) showdev 21 must show the LDEV as AVAILABLE.

Who uses the Remote Console Port for dial out? That's right VISION/PREDICTIVE!

There is a large inventory of cheat sheets/cook books on the Internet that give you different ways to configure a modem. If the cook book is for remote support modems: Beware, unless you have the real thing from HP the modem will frequently lockup.

Next time you get a chance. Look at the Status Line when the console is in control mode(control-b). Notice how the 'inactive' or 'active' status follows the state of the CD line. "ACTIVE" means CD is high; "INACTIVE" means CD is low. Unless you need dial out; do not use CD high mode. Use the normal CD follows carrier mode.

Without the "DTR cycles CD" function built into the Ideal or HP Remote Support modem; sessions not properly terminated by the HP computer will still be running.

NEVER EVER USE the LR Lock Remote function unless you have a HP Remote Support Modem or an Ideal Modem. If you lock the remote using LR command followed by unlock remote command; the modem will be locked up. It will be necessary to cycle the power on the modem.

Try this exercise:

With CD follows carrier AT&C1 (normal)

Lock the remote access port with the LR command. UR unlock the remote. Dial in to the modem. The modem will not answer. Cycle the power on the modem. Dial in again and everything is fine.

With CD forced always on AT&C0

Lock the remote access port with the LR command. UR unlock the remote. Dial in to the modem. The modem will not answer. Cycle the power on the modem. Dial in again and everything is fine.

With CD high and drops upon disconnect then goes high again AT&C2

Lock the remote access port with the LR command. UR unlock the remote. Dial in to the modem. The modem will not answer. Cycle the power on the modem. Dial in again and everything is fine.

CD Resets the modem when Carrier Detect drops AT&C4

Lock the remote access port with the LR command. UR unlock the remote. Dial in to the modem. The modem will not answer. Cycle the power on the modem. Dial in again and everything is fine.

The "Phantom Session"

If you have a modem connected to the port with CD high and you enable Remote Console: the status line will show Remote Console "PENDING" and not "ENABLED". This is called a Phantom Session and is a result of the CD line being high as required by HP.

To clear this:

If Using an Ideal Modem or HP Remote Support Modem

Lock the modem with LR command followed by Unlock remote UR command

-OR- Dial into the modem then disconnect; thereafter, the remote console will be "ENABLED"

-OR- Power Cycle the Modem

If Using any other modems

Power Cycle the Modem

Caveats:

* UR or LR to unlock/lock remote modem answer/dial out. (Use Only on Ideal/HP Modems)

* ER or DR to enable/disable remote console access.

* For Vision / Predictive support to function correctly, the AP should be set to Session Mode.

* If you want to prevent both local and remote console access (Control-B Access), put the front panel keyswitch into the locked position.

* If the keyswitch is in the locked position, error messages sent to the console will not be displayed.

* Always set the Bit Rate (Baud Rate) the same for both Remote Console and Session Modes.

* LDEV 20; Remote Console Port initially is not set to support modems.

* LDEV 21; User Port (Sessions) initially is not set to support modems.

* Remote Console Access Port does not support speed sense.

* Session Mode on LDEV 21 does support speed sense.

* CPU Status LEDS: Third LED from the left is lit yellow if the remote console is enabled.

* Access Port does not support hardware handshaking with HP recommended BELL Mode.

* CAUTION: Callers connecting at Bit Rates slower than Access Port can experience Data Loss when listing files. Use Control-S (XOFF) to stop listing and Control-Q (XON) to resume listing or enable local XON/XOFF Flow Control on the modem (remember Block Mode doesn't like that).

General rules for configuring remote modems:

* DTR normal. Never use DTR always on as this defeats your system security via UR/LR commands

* CTS should follow RTS.

* DSR must follow OH, not DCD.

* For Bell mode, the modem should disregard RTS.

* Do Not use local XON/XOFF flow control. Block Mode applications don't like that.

* Set both local and remote modems to either: the same compression mode, OR to NO data compression.

* If the modem sends up-modem dialog (disable response codes), it is possible for the user to be logged off immediately when a password is enabled for the Remote Console Access.

* Always use 40233A Cable. Avoid using 92219Q; it is not always acceptable. Be careful; these cables are non-symmetrical. That means the end marked Computer is connected to Access Port 1 (LDEV 21) and other end (marked modem) is connected to the modem.

* UpLoads to the HP3000 – since we are not using flow control on the remote modem, avoid connecting at carrier speeds faster than the Access Ports Bit Rate. You may overrun your upload and loose bytes.

* DownLoads from the HP3000 – always be connected at a carrier rate equal to the Access Port Bit Rate or faster. You may overrun your DownLoad and loose bytes. The moral to the story is: connect at the same rate as the Access Port Bit Rate whenever possible.

* Always use Error Correct whenever possible. (Preferrably V.42)

* Always use Speed Buffering (also known as autobauding). This mode ensure that the modem talks to the Access Port at the fixed 'Bit Rate' setup by the 'CA' command. The modem can then negotiate a carrier or (phone) line speed that does not match.

* If you are not doing Binary File transfers (UpLoads/DownLoads); it is safe to use XON/XOFF flow control on the remote modem. This will allow you reliably connect at any speed! Typical Hayes Protocol to Use:

Restore Factory default first using: AT&F

E0 Command characters not echoed – prevents accidental ASCII character storms which locks up the modem.

Q1 Disable responses to computer during dial-in. (That prevents password lockout problems)

&D2 Modem Hangs up when Computer drops DTR. Modem can not answer or dial out when DTR is locked out by the Access Ports 'LR' Lock Remote Command.

&S1 DSR follows 'OFF Hook' and is asserted

&K0 Disables local flow control – Access Port and Programs will handle flow control via XON/XOFF

&Q5 Negotiates error-correcting connection – best for reliable connections

&C1 DCD follows phone line carrier signal.

S0=1 Auto Answer on first Ring

"Always remember to save setup in Non-Volatile Memory using AT&Y0&W0" Smart modems learn the Baud Rate, Parity and Character format to power up with from the 'AT' part of this command and are only saved in Non-Volatile memory with the &W0 command.

Final Warning

Under the right circumstances; either LDEV 20 (remote console port) or LDEV 21 (session port) can and do lockup internally.

For example: showdev 21 states that the device is available; but when you connect no session can be started.(this assumes remote console is disabled and UR unlock remote is in).

Troubleshooting this example:

1) Power cycle modem

2) Check bit rates of modem and User Port are same.

3) Reset LDEV 21 with system diagnostic Consolan

4) Disconnect modem and modem cable from remote port. Connect an ordinary terminal directly to port and try to log-in.

The above steps apply to both remote console and session modes. If you can't initiate a session of either mode with an ordinary terminal; you will never be able to get a modem to work either!

If none of these steps fix your problem: there is only one solution. You must shutdown and power cycle the system! This fortunately is an infrequent occurance but it does happen.

Back to top