CommunicationsAudioInterface for Remote Operations


Packet Radio has its place in Raynet, e.g. at 'data-intensive' emergencies, and is becoming an increasingly viable mode as more Raynet members equip themselves with the requisite TNC. CAIRO-8, being designed for rigs to host any active module, is well suited as the TNC interface. The pin-6, Audio-Line output is an ideal 'Rx-AFSK' fixed-level signal against which the TNC input sensitivity may be pre-set, once and for all, and the pin-8, Squelch output may be applied to a modified 'RF-DCD' input to engage the TNC's 'superior' performance, via this hard-wired control. In particular, this will allow vocal and Packet traffic to share the same Raynet channel, with the vocal traffic having priority, being a dual-mode operation: "Clandestine" Packet. (RP .2)

RP .1 CAIRO-8 and TNCs

For full compatibility with CAIRO-8 (and basic CAIRO) signalling, the TNC output interface should be modified to present isolated 'Tx-AFSK' (i.e. mike) and 'metallic-contact' PTT. This will then ensure that the TNC is compatible with any CAIRO-adapted transceiver, including the series-input hand-helds which may well be used as the basis of a 'portable' Packet outstation.

The simple conversion to CAIRO-8 may be achieved by installing a small PCB, inside the TNC, with the full *OPTO* arrangement of Fig. 15, but with a 1K resistor instead of the 8K2, together with a 5V SIL relay, supplied by the *REG* and operated by the TNC's PTT-transistor.

As part of the conversion, the TNC's original rig-signals connector (often a DIN-5 socket) would be replaced with an Audio-8 chassis plug so that a single Loose-Tail is used for all signals, including the 12V d.c.-supply, which is now taken directly from the CAIRO-8 host transceiver. This makes the TNC's previous 12V power connector a redundant provision which may be removed. In its place, the hole would be 'filled' with a suitable grommet through which passes a short length (6", 15cms) of CAIRO line (4-core, individually-screened cable) terminated with a DIN-7 free socket, wired to the normal CAIRO pin conventions. Inside the TNC, the mike lines of this CAIRO "Terminating Tail" pass to the appropriate pins of the Audio-8 chassis plug, in parallel with the wires from the *OPTO* isolated output, on the adaptor PCB. The speaker line (A8:2), which is 'unused' in the TNC, is coupled directly through to DIN socket pins 1 & 6, and the PTT line, of the Terminating-Tail, passes to the interface PCB to control the host rig, via the TNC interface relay.

Thus, all CAIRO-8 signals now enter the TNC, via the Loose-Tail, and signals equivalent to those of basic CAIRO (without the PALS) pass onwards to the Terminating-Tail DIN-7 socket. This maintains the bus-signals convention, for the CAIRO signals, to support onward connections to remote operator accessories which may be operated 'in parallel' with the TNC when required, Fig. 17. (For simplicity, a TNC converted to the CAIRO-8 interface may be known as a "C8-TNC".)

RP .2 "Clandestine" Packet

For Raynet purposes, it is feasible to operate Nets where voice messages and data traffic co-exist on a single channel. Consider a pair of Raynet stations, one at a temporary field hospital, where casualties receive preliminary treatment, and the other at the district main Hospital, where the casualties are taken subsequently. Two 'layers' of traffic would exist in this scenario.

Verbal exchanges would cater for the general activities overall, e.g. requests for medical supplies, whilst 'data' (i.e. Packet) might be used to transact the details of individual casualties; names, addresses, medical conditions, etc., as supplied by the medical staff of the User-Service.

CAIRO-8 host rigs, working with C8-TNCs, will support this dual mode if a modified 'RF-DCD' protocol is enabled. In this protocol, the C8-TNC will be inhibited against sending its packets unless the channel is quiet and the host transceiver is not being keyed by the voice operator for a verbal transmission. This gives rise to the notion of "Clandestine" Packet since the packet data will only be sent when the channel is not being used for voice traffic.

It follows that, because voice exchanges are the primary mode (e.g. F3E) and the Packet exchanges, though operationally significant, are the secondary mode (F2D), this dual-mode "Clandestine" operation may be conducted on simplex voice Nets or else via Talk-Through Relays. For Raynet at emergencies, this offers an extremely powerful method of 'two-layer' operation.

Fig. 17

"Clandestine" Packet Station

for dual-mode working on a single (Raynet) channel

RP .3 DCD Logic

For correct operation of "Clandestine" Packet, it is necessary to control the conditions under which the TNC is permitted to launch packets, more precisely than in 'normal' Packet Radio, so that the bursts may only occur at the moments of turnaround between consecutive voice 'overs'. Were they to occur at other times, they would degrade the voice service without benefiting the operation overall. In fact, it is the retry bursts, which are subject to random-delays, which must be inhibited because an acknowledgement packet can not be received until the next turnaround.

The necessary control is achieved by 'inserting' supplementary logic into the existing 'DCD' path, between the TNC's MODEM IC (e.g. 7910, TCM3105, XR2211) and the USART IC (e.g. 8530(A), Z80SIO/0 or 63B03X), by bringing two wires from the PCB to the logic, usually from the relevant pins of the hardware 'DCD' jumper and removing the jumper itself, or, in the absence of a jumper, by cutting the TNC's PCB track at a convenient point somewhere between these two ICs.

The logic incorporates the condition of the CAIRO-8 Squelch and the terminating-tail's PTT to inhibit the TNC whenever the host rig is receiving or when it is being used for voice transmission. The logic also incorporates the original modem-output, or "soft" DCD, so that the TNC behaves normally in all other (i.e. non-"Clandestine") applications. Lastly, an option is included in this logic to allow it to be programmed for correct USART control, depending on whether the original 'RF-DCD' coupling was an active-high or active-low signal.

The logic uses a 74151, 1-of-8 (8-to-1) Multiplexer, Fig. 18, and the control is simply hard-wired, either to logic '0' (0V) for active-low, or logic '1' (+5V) for active-high signalling, when the PCB is installed in the TNC. At the time of writing (1991), most TNCs appear to require the active-low (L) signalling option; the two exceptions being the KAM All-Mode and BSX-1 TNCs.

(Please consult the TNC handbook very thoroughly in this matter and program this control only after careful examination of the original 'DCD' coupling, e.g. with a logic probe.)

(Readers who are conversant with logic design techniques may recognise that the hard-wired control, for the Active High/Low selection, is a "Map-Entered Variable" on the MUX which is being used as a 3-variable, two-level NAND "DCF" implementation of the 'DCD' function.)


RP .4 PTT Logic

To accompany the DCD logic, the PTT uses a 5V SIL relay, e.g. [345-555] on the TNCs PTT transistor and a diode pull-down onto the terminating-tail's PTT line. Thus the host rig is keyed, by metallic contact closure from this relay, either when the TNC sends packets or when the remote operator uses a PTT device to engage a voice transmission. The diode arrangement ensures that the PTT signal input to the logic, only relates to the terminating-tail PTT-line activity. Finally, for maximum flexibility, it is recommended that the interface PCB derives its 12V supply directly from the Audio chassis plug (pin-7), not via the TNC's On/Off switch. This will ensure that the user-PTT operates correctly, even when the TNC itself is turned OFF, so that the voice operator continues to have full 'access' to the host transceiver, via the TNC's terminating-tail connection.

This "Terminating Tail" is kept short simply for TNC handling convenience. However, its signals are the usual CAIRO set and may be extended to a remote operations desk in the normal way, e.g. an Orange-Reel Line, Fig. 17, for a configuration to be terminated with operator accessories.

Fig. 18

DCD 'Insertion' Logic (*MUX*)

(Vcc = +5V at pin-16 is not shown, for clarity)

RP .5 Acceptability

Operational experience with "Clandestine" Packet is limited at the present time (1991). However, the Author and his team at Aston University have attempted several 'dummy' operations from which the following preliminary observations have been made.

(In due course, they would welcome corroborative or contradictory evidence from other sources to help further clarify, or perhaps condemn, the belief that this dual mode may be very useful for some Raynet purposes.)

Firstly, it would appear that the sound of the Packet bursts is not excessively distracting to the voice operators, contrary to initial expectation. Indeed, there is an element of "reassurance" on hearing the normally very short bursts at the beginning of each listening 'over'. One soon learns to wait for these, just like waiting for the "K" in conventional (GB3..) Repeater operations.

However, it is useful to note that if Packet traffic is the predominating mode, operator(s) may find the listening ambience of an extension speaker or single-ear accessory to be preferable to that of binaural headphones, though the latter can always be reduced to monophone equivalence, with the mute facility switch on the Single or Dual Operator Outlet Box.

Secondly, the signals pathway between any two stations on a typical Net, is usually 'solid' (i.e. voice-category "Good") and particularly so when a Talk-Through Relay is being used. Also there is no Packet traffic on the Net other than that which is related to the specific Operation. Thus, the occurrence of retry exchanges ("trashing") between TNCs is almost totally eliminated. These two factors lead to a higher than 'normal' throughput of data traffic except that the overall rate at which the TNCs 'handshake' is now determined by the voice operators, in this dual mode.

If the data traffic is particularly intensive, it becomes essential for the vocal exchanges to be conducted with the crisp precision of proper (formal) voice procedures so that there are plenty of turnaround occasions for the TNCs to exchange their packets. For this reason, it is recommended that a "Clandestine" station should employ at least two operators - one for the voice and one for the Packet traffic. Although they may well sit side-by-side, and have knowledge of each other's business, they each 'work' their counterpart(s), at the other station(s), almost completely independently of each other. The preliminary evaluation of this dual mode indicates that if a single operator was to attempt both tasks, confusion would set in to the detriment of both 'layers'. Therefore, it is very useful to note that if the voice operator is the licensed amateur, s/he is supervising the transmissions of that station, in a very real sense, so the Packet typist-operator need not be a licensed amateur but might well be a member of the User-Service staff instead.

RP .6 Relayed "Clandestine"

When considering dual-mode Nets for an operational requirement, it must always be remembered that "Clandestine" packets 'steal' themselves into what would otherwise be the brief turnaround silences between 'overs' in the voice Net. Thus, if a Talk-Through Relay is used for "Clandestine" Nets, it should have a (nearly) instant shut-down characteristic after an input carrier has dropped. Without this, there are no 'silences' for the packets to 'take their turn'.

It is also desirable, for the voice net too, that the Relay has a fast open-up characteristic, on subsequent re-access, so that the early milliseconds of a packet burst are coupled to the output, to be 'heard' in full by the 'listening' TNC. This implies that a Talk-Through Relay should be of the carrier-access kind. However, when a Talk-Through Relay is slow to re-engage, the software parameter - AXDelay - in the TNCs involved in these Nets, may be increased from the default setting (0) to, say, 20 (200 mS) to compensate. Alternatively, if a voice-access (VOX-type) Relay is used, it may be necessary to increase this parameter even further to, say, 50 (500 mS) to extended the 'pre-packet' tones sufficiently to engage that Relay, ahead of the 'real' packet train. These factors should be determined, by experiment, at the start of service in each configuration.

It is useful to note here, that both the CAIRO and CAIRO-8 TTUs, described later in this Manual, implement carrier-access Talk-Through and appear to perform correctly with TNCs having AXDelay set to 20. Conversely, the typical GB3.. Repeater, with its long hang-time, will not be a satisfactory form of Relay for "Clandestine" Packet operation.


RP .7 Portable Packet

Meanwhile, for 'portable' Packet operation, without a CAIRO-8 host rig, an alternative 'split-tail' may be constructed for use into a C8-TNC, in place of the full (CAIRO-8) loose-tail. This split-tail has a single Audio-8 free-socket from which emerges two separate (~ 1m) cables. One of these is a 'normal' CAIRO Line (of 4-core, individually-screened cable) to a DIN-5 plug. The DIN pin-1 (speaker/blue) core of the CAIRO Line is coupled to the Audio-8 socket pins 1 & 2 (audio-line and speaker), exactly as it would be wired in basic CAIRO (to DIN socket pins 1 & 6). The other tail is a suitable 12V power supply line, terminated in an appropriate 12V connector. Because the C8-TNC is pre-set to work to the standard 1Vpk input, the set-up procedure for portable operation, via the split-tail, is very simple:

1) Connect the split-tail 12V-line to a supply and power-up the C8-TNC (and program the actual and 'strategic' (i.e. alias) callsigns, etc., for this outstation)

2) Connect the split-tail CAIRO-line to a CAIRO-adapted Rig, e.g. a hand-held.

3) Turn the Rig's Volume and Squelch controls to zero, on a silent channel.

4) Advance the Volume control until the DCD-light (LED) illuminates (FM noise), and then advance it a "touch" more beyond that.

5) Advance the Squelch control until the DCD-light (LED) extinguishes, and then advance it a "touch" more beyond that.

6) Change ("QSY") to a Packet (or Raynet) channel and operate.

RP .8 Rationale

TNCs which have been modified to the CAIRO-8 interface, retain ALL their original operational features but may now be used with CAIRO-8 converted or CAIRO-adapted transceivers, with all the immediate benefits which accrue from this unification for various Raynet operations. However, the benefits also extend into the 'shack' where operating there is greatly simplified because the C8-TNC Terminating Tail gives direct access to the rig, for "phone QSOs" and the like, without the need to unplug the TNC first. This avoids the sometimes tiresome task of having to re-program the TNC, in respect of the clock/calendar for example, after such voice sessions. Nevertheless, it is advisable to disable the C8-TNC software, in respect of its Beacon and CWID modes, etc., before changing to a non-Packet channel !

RP .9 DCD-State Boards

These optional augmenting logic boards will operate correctly with TNCs converted to CAIRO-8, though they must be disabled whenever the full "Clandestine" Packet mode is required. This can be achieved by switching the state-board's RxD signal to ground.






Supplementary Material


Remote Operations

CAIRO Basics

CAIRO-8 Basics

Raynet Packet

CAIRO Installations

CAIRO-8 Relays

CAIRO Relays

Relay Installations

Summary Configuration

Data Card

Handy Hints