Described here is an interface which permits two Desk-Fax type 6500/6499A machines to function together. The interface is constructed from two-dozen-or-so 4000-series CMOS ICs. It would probably be simpler from a hardware perspective to program a PIC or other microcontroller for this task, but I had a stock of CMOS ICs around to be used.

Contents (this page): Technical Description Controls and Indicators Basic Operation Adjustments

Other Info: Interface schematic (contact).

1. Technical Description

Figure 1 depicts the functional organisation of the interface, showing the area of influence of controls and origin of sources for indicators.

The interface must handle three primary tasks:

a) Transmission Control: The main objective here is to detect active status of the incoming unit (ready to receive) and indicate to the outgoing unit to go active (begin scanning/transmission). Similarly, absence of activity from either unit is used as indication to terminate the transmission. Activity here means that the drum is moving along the shaft (Y motion) and is indicated by the presence of sync pulses on the communications line. The Desk-Fax does not place sync pulses on the line until linear drum movement begins, even though the drum is already rolling.

Due to an anomaly in making two Desk-Faxes work together, one aspect of the (presumed) protocol not fully implemented is that of Reception Acknowledgement. This is a result of the Incoming unit terminating ahead of the outgoing unit because the outgoing unit became active slightly after the incoming unit. This was probably handled with an asymmetry in the scan time in the original operating scenario.

b) Synchronisation: One approach to dealing with synchronisation would be to adapt the Desk-Faxes to adjust to received sync pulses in one mode. This is the approach the hams used, as described in several QST articles. I preferred not to do this as I didn't want to have to make any alterations to the Desk-Faxes.

The alternative approach is to have the interface deal with the phase difference by providing it with the ability to store one line of image data (one drum rotation). This is accomplished with a digital delay line implemented from a 4K-bit static RAM and two address counters. This is enough storage to provide for a sampling rate of 4 times the pixel rate. One counter indicates the address at which to write data from the transmitting Desk-Fax, the second indicates the address at which to read data to be sent to the receiving Desk-Fax. The two counters are incremented at the same rate.

Two approaches were tried for the determination of the phase difference:

• Using the sync pulses from each Desk-Fax to reset the appropriate counter, so that re-sync occurs for every line (every drum rotation), much like the horizontal sync in conventional raster-scan television. While in theory this would produce a stable synchronization due to the frequent update of the sync state, in practice the mechanical contacts generating the sync pulses seemed to produce enough jitter between subsequent sync pulses as to produce poor line alignment.
  

• Establishing the phase difference once at the beginning of transmission and then relying on the stability and equality of the drum rotation rate on both machines to retain that same phase difference through to the end of the image transmission. This approach was more successful. The phase difference is established by holding both counters in reset, releasing reset on the write counter with a sync pulse from the transmitter, then releasing reset on the read counter with the first subsequent sync pulse from the receiver. The read address thus lags the write address by a constant count (delay) for the remainder of the transmission.

c) Signal inversion: The image data signal inversion is accomplished within the binary storage path. As the data comes out of the storage RAM, the binary state which represents black enables the output of pulses at a 2500 Hz rate. The duty cycle of these pulses (pulse width) can be adjusted.

2. Controls and Indicators

The following tables present descriptions of the front panel controls and indicators of the interface:

3. Basic Operation

Starting with Desk-Fax units plugged into the interface and the following initial state:

• interface POWER turned ON,
• both FORCE switches OFF,
• DIRECTION switch at IDLE,
• MODE switch at SYNC.

1. Press OUTGOING on one Desk-Fax, the according XMT REQUEST lamp should light on interface.
2. Set DIRECTION switch appropriately on interface.
3. Press START on interface, the other Desk-Fax should buzz (Incoming Requested).
4. Press INCOMING on the buzzing (receiving) Desk-Fax.
5. After both units have warmed up, the interface should detect ACTIVE from the receiver. This will enable the transmitter. The interface should detect ACTIVE from the transmitter, sync will be established (no indication) and image transmission proceeds.
6. When one of the Desk-Faxes finishes, the interface detects absence of activity and disables both units.

4. Internal Adjustments

There are two variable resistors in the Desk-Fax, accessed by removing the top cover.

• RCV Level: Adjust for a 'medium' blackness. Too high a level may result in a 'smoky' or blurred image. The BLACK mode of the interface can assist with this:
  1. Set DIRECTION switch to select the appropriate Desk-Fax as receiver.
  2. Set MODE switch to BLACK.
  3. Press START on interface, the Desk-Fax should buzz (Incoming Requested).
  4. Press INCOMING on the buzzing Desk-Fax.
  5. After unit has warmed up (ACTIVE detected) perform the level adjustment.
  
  
• XMT Level: See XMTR WHITE DETECT adjustment above.

There is also a trimmer for the crystal-controlled sampling clock inside the interface but it shouldn't normally need adjustment. The sampling clock should be around 100KHz, as measured at the output of the oscillator or 10KHz measured at one of the outputs of the 10-stage counter.