Cisco High-Speed - Expansion module - HDLC/ RS-232/ PPP/ RS-530/ X.21/ V.35/ RS-449/ SLIP/ RS-530A - 4 ports - for Cisco 28XX/ 28XX 2-pair/ 28XX 4-pair/ 28XX V3PN/ 29XX/ 38XX/ 38XX V3PN/ 39XX