Digi C/X Remote Troubleshooting Guide

• Analog modems, DSUs, ISDN TAs, FRADs (COM devices) all operate the same way.
• In remote operation, Digi sync lines are passive. Synchronous clocking is done by the COM device. COM devices will inject TxC and RxC into both the host adapter and concentrator thereby causing data to flow. So long as valid clock signals (TxC & RxC) are present, Digi hardware will send and receive data.
• If possible, place the COM devices in a back-to-back configuration. This will ensure that possible phone line problems are immediately isolated.
• Digi Synchronous protocol is a proprietary HDLC.
• 99% of all remote concentrator installation problems can be traced to one or more of the following. The other 1% of problems can be traced to phone line problems and (very rarely) an incompatibility between Digi sync ports and installed COM devices.
1. Sync cables built incorrectly
2. Improperly configured COM device sync clocks
3. One or both COM devices latched in a hardware flow control state. Be sure to force all COM device input signals high where applicable. Force at least RTS and DTR high on COM devices.

1. Verify that the software for the C/X (EPC/X) is installed correctly for the Digi host adapters synchronous line in question.
• The number of physically installed concentrators must match the number of concentrators configured in software. A concentrator which displays "P2" is a common symptom when the software has more concentrators installed than are physically present. This rule applies only to remote concentrators. For local concentrators, a mismatch is acceptable.
• Verify that the sync line (host to modem) is configured for the electrical characteristic used by the COM device - RS232 or RS422. Digi software configuration RS422 will also work with V.35 COM devices.
• Verify that the host adapters sync line speed is set to a value that is equal to or less than the speed of the COM device. A direct match in sync line speed is not required. The COM device will dictate the actual speed of the Digi host adapter synchronous port. The COM device will supply both transmit and receive clock (TxC & RxC). Digi clocking goes into a passive state when configured for remote connections.


2. Make sure concentrator node numbers are set correctly.
• Remote concentrator installations with a single concentrator - set node to 1n
• Remote concentrator installations with more than one concentrator - set node numbers contiguously starting at 1n, 2n, etc. Node numbers can not be skipped. E.g. 1n, 3n, 4n


3. Verify cabling is correct.
• The Digi concentrator manual covers both RS232 and RS422 cable options.
• Make the sure the cable matches the diagram pin for pin.
• Make sure to include needed loopback signals as required. For RS232 COM devices, loopback pins 4 to 5 and 6 to 20 to ensure that the COM device doesn't get stuck in a hardware flow control state.
• Often, a remote concentrator(s) will stay at P1 even though the COM devices have connected and DCD is asserted. It is possible that the one or both of the COM devices are stuck in a flow control state.


4. Check the lights (LED's) on the COM devices. It is VITAL to monitor the lights on the COM devices. The condition of the COM device lights can tell a great deal about the progress of data flowing from the host card, through the COM devices, to the remote concentrator, back through the COM devices and finally back to the host card.



1. Checking the Local COM Device for proper operation


• Check the Local COM device first. SD (TxD) should be flickering indicating that the host card is sending data. If SD (TxD) not flickering, the host card is not sending data. The remote concentrator is probably sitting at P1. Proceed no further until the Local COM device is sending data.
1. The cable (host to Local COM device) may be pinned improperly.
2. The Local COM device may be stuck in a flow control state. Check loopback signals on the Local COM device cable. Check COM device settings that may be related to hardware flow control.
3. Check the Local COM device to ensure it is connected to the Remote COM device. DCD (modems) should be asserted.
4. Check for proper clock configuration on the Local COM device.



If the Local COM device is sending data but the concentrator does not boot to AC, proceed to the next section



2. Checking the Remote COM Device for proper operation
• Check the Remote COM device. The RD (RxD) light should be flickering. This indicates that data is passing from the Digi host card and through both Local and Remote COM devices. The concentrator should go to at least P2.
• If the Remote COM device's RD (RxD) light in not flickering, go back the Local COM device. The problem is likely there. See Local COM device section.
• If the Remote COM device's RD (RxD) light is flickering and the concentrator is still at P1.
1. Check the Remote COM device to concentrator cable. P1 indicates that no packets are arriving at the concentrator.
2. Check Remote COM device clock configuration.
3. Force input pins high. COM device may be stuck in a hardware flow control state.
• If the Remote COM device's RD (RxD) light is flickering and the concentrator display sticks at P2, data is getting to the concentrator but not getting back to the host card.
• Check the Remote COM device's SD (TxD) light. It should be flickering indicating data is passing from the concentrator to the COM device.
1. If the remote COM device's SD (TxD) light is not flickering, check the Remote COM device to concentrator cable.
2. Check the Remote COM device itself. It may be stuck in a hardware flow control state.
3. Check Remote COM device clock configuration.
• Check the Local (back at the computer side) COM device. RD (RxD) light should be flickering indicating that data is passing from the Remote COM device to the Local COM device.

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Modem Tips
• Important Hayes compatible "at" commands. Other modems may have different register settings. Check your documentation for equivalent commands.

&X0 - Modem generates transmit clock (TxC) and supplies it on pin 15.
&M1 - Places the modem in Sync mode - on line. Async mode - off line.
&D2 - DTR normal
&R1 - CTS always true
&S0 - DSR always on
&C1 - DCD pin 8 reflects actual condition of the carrier signal.


• The following "at" command will force a Hayes compatible modem into synchronous mode with TxC generated as an output on pin 15. (Note: receive clock - RxC - Pin 17 - is always an output signal from a modem)

AT &X0&M1&D2&R1&S0&C1&W



• Modem Auto-dialing.
• Modems can be programmed to auto-dial on the transition of DTR. When the modems DTR (input pin) transitions from off to on, the modem will go off hook and dial the pre-defined phone number stored in NVRAM. Check your modems documentation for correct "at" command.
• Auto-dialing is done from the remote side of the link When the remote concentrator is powered on, pin 13 (+5 volts) will transition from off to on.
• Add an additional wire will need to be added to the COM device to concentrator cable.
• Add additional wire pin 13 (+5 volts concentrator) to pin 20 (DTR on the modem)

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DSU Tips
• Clock sources
• Back to back mode: Set one DSU to internal clocking. Set the other to external or repeater clock timing
• In-circuit mode: Clock source is the phone circuit. Set DSU clocking to external.
• Test DSUs in a back to back configuration first.
• Force all applicable input signals on the DSU high. See Digi cable diagram and DSU manufacturers cable diagrams.

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Frame Relay Device (FRAD) Tips
• Know your DLCIs
• Set HDLC mode as opposed to SDLC, NRZ, Flag Fill, clock from Frame circuit.
• Force input signals high

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ISDN Sending Unit (ISU) Tips
• Verify correct SPIDs and LDNs.
• Verify correct switch type (i.e. National, 5ESS, DMS100)
• Clock from circuit
• Force all input signals high