You will need a modem that handles the AMTOR protocol.
There are several of them, new and 2nd hand.
2nd hand modem are sold in most countries, as many have
been sold and most are now lying idle
on the shelf in many HAM shacks. And the owners usually
part with them at a reasonable price.
These are the most popular all-mode datacontrollers used
over the years:
They all connect as any other TNC, to the HF tranceivers
PTT & AF In/Out and to the computers serialport.
KAM PLUS by Kantronics
PK232MBX by AEA (Now TIMEWAVE)
MFJ-1278 by MFJ Enterprises
AMT-1 by ICS Electronics LTDDownload AMT-1 Manual (8,1 MB)
AMT-2 by ICS Electronics LTD
Download AMT-2 Manual. Courtesy of Cor A6/A92DU(1,3 MB)
AMT-3 by ICS Electronics LTD
Download AMT-3 Manual. Courtesy of McMurdo LTD(19 MB)
For information on the German SCS PTC modems - have a look here.