Starting to put together the BSC approach based on Mattis Lind's SyncDongle

THE SYNCDONGLE

This PCB converts synchronous serial messages in IBM's Binary Synchronous Communications protocol, routing the messages over a normal async serial line. It allows connection to the IBM 3174 adapter in BSC mode, but doesn't require the other side to have to deal with all complications of sync serial mode and the BSC protocol. 

 ORDERED PCB AND COMPONENTS TO BUILD A SYNCDONGLE

KiCad design for PCB

I opened the KiCad design files, produced the Gerber and drill files, then sent them off to JLCPCB.com to build five of them for $2 after which I splurged on express shipment. Mouser.com had the various components to mount on the board except for the STM32 Blue Pill board which I bought from Amazon.com

Mattis' picture of bare PCB and assembled board

I began reading Mattis' code that runs in the Blue Pill, called BSCbridge, when I realized that he worked out a very clever way to use the processor to handle sync serial without the start bits that are normally required to achieve character framing. He uses an SPI link and interrupts to receive an undifferentiated stream of clocked bits, which he can frame based on receipt of SYN characters. Very clever idea, wished it had occurred to me. 

He coded an Arduino sketch for the main routine that establishes the SPI link (and the normal serial link), then added some C++ library routines to handle framing, buffers and other protocol details. Since this board runs a 72Mhz ARM Cortex processor it definitely can keep up with reasonable speed BSC links.