Subject: Heath HK-21 hwd flow ctrl

 A short time ago I had the need to have full hardware flow control on
a Heathkit HK-21 pocket tnc. Normally hardware flow control is handled
via pins 4 & 5 (RTS/CTS) on a DB25 connection. That is when the TNC
pulls the CTS line low the computer/terminal stops sending data.
And when the computer/terminal pulls the RTS line low the TNC stops
transmitting data.  The problem is that on the Heathkit HK-21 TNC pin 4
(RTS) is not used by the TNC. However Heath did provide a means to
obtain full hardware flow control. On the HK-21 TNC, pin 20 functions
exactly the same as RTS. That is when pin 20 is pulled low the TNC stops
sending data and buffers it until pin 20 goes high again. Therefore it is
a simple matter of rewiring the cable connector at the TNC end.

    Computer (DTE)                    HK-21 (DCE)
        DB25                              DB25
  Shield/GNC       Pin 1    <---->   Pin 1      Shield/GND
  Trans. data      Pin 2    ----->   Pin 2      Trans. data
  Rec. data        Pin 3    <-----   Pin 3      Rec. data
  Req. to send     Pin 4    ----->   Pin 20     Data term. ready
  Clear to send    Pin 5    <-----   Pin 5      Clear to send
  Data set ready   Pin 6    <-----   Pin 6      Data set ready
  Common           Pin 7    <---->   Pin 7      Common
  Carrier detect   Pin 8    <-----   Pin 8      Carrier detect
  Data term ready  Pin 20   ----->   NC

Note: On the HK-21 TNC pin 6 (DSR) and pin 8 (CD) are tied high.
      On other TNC's pin 8 (carrier detect) goes high when a connection
      is established with another TNC.
73's AL


Y