PPP connection from Mac System 7 to Debian 10

I have a Mac SE/30 that I want to connect via PPP to Debian 10 and, in turn, access the Internet -- as if it was dial-up. I can successfully send text over a null modem link and now I want to make a PPP connection over the same link. The SE/30 is running System 7.5.5 with OpenTransport 1.1.2 and the _Config PPP_ control panel. On Debian 10 this happens:

mini31 # pppd nodetach debug /dev/ttyUSB0 9600
using channel 10
Using interface ppp0
Connect: ppp0  /dev/ttyUSB0
rcvd [LCP ConfReq id=0x42 ]
sent [LCP ConfReq id=0x1     ]
sent [LCP ConfAck id=0x42 ]
sent [LCP ConfReq id=0x1     ]
rcvd [LCP ConfReq id=0x43 ]
sent [LCP ConfAck id=0x43 ]
rcvd [LCP ConfReq id=0x44 ]
sent [LCP ConfAck id=0x44 ]
sent [LCP ConfReq id=0x1     ]
sent [LCP ConfReq id=0x1     ]
rcvd [LCP ConfReq id=0x45 ]
sent [LCP ConfAck id=0x45 ]
sent [LCP ConfReq id=0x1     ]
rcvd [LCP ConfReq id=0x46 ]
sent [LCP ConfAck id=0x46 ]
rcvd [LCP ConfReq id=0x47 ]
sent [LCP ConfAck id=0x47 ]
sent [LCP ConfReq id=0x1     ]
sent [LCP ConfReq id=0x1     ]
rcvd [LCP ConfReq id=0x48 ]
sent [LCP ConfAck id=0x48 ]
rcvd [LCP ConfReq id=0x49 ]
sent [LCP ConfAck id=0x49 ]
sent [LCP ConfReq id=0x1     ]
sent [LCP ConfReq id=0x1     ]
rcvd [LCP ConfReq id=0x4a ]
sent [LCP ConfAck id=0x4a ]
rcvd [LCP ConfReq id=0x4b ]
sent [LCP ConfAck id=0x4b ]
sent [LCP ConfReq id=0x1     ]
LCP: timeout sending Config-Requests
Connection terminated.

My guess is that Debian is expecting the Mac to send LCP ConfAck but it is not doing. However, I don't understand what this means. Any ideas on how to get it working? **Edit** The Mac appears to be sending repeated configure requests for the magic number parameter even though Debian is sending accepts for them. Similarly, the Mac is not replying to Debian's configure requests. I wonder whether the Mac isn't receiving data; could be a problem with the physical connection between USB on Debian and Mac's RS422?