In 2010, NASDAQ OMX PHLX LLC (PHLX) launched NASDAQ OMX PSX (PSX) as a platform for trading NMS stocks using a price/size execution model. Now PHLX has filed a proposed rule change to adopt a price/time model for PSX. PHLX is amending PSX rules to replace the price/size execution model with a price/display/time priority algorithm that is substantively identical to corresponding rules in effect at the NASDAQ Stock Market (NASDAQ) and NASDAQ OMX BX, Inc. (BX).

PHLX also has proposed rules to allow member organizations that are PSX participants to register and act as market makers. The proposed rules would impose quoting obligations on PSX market makers identical to those in effect at NASDAQ and BX.

Additionally, the proposed rule change: (1) introduces midpoint peg post-only orders and price to comply post orders, (2) adjusts the operation of minimum quantity orders and post-only orders and (3) eliminates minimum life orders.

The Securities and Exchange Commission is accepting comments on the proposed rule change. Comments should refer to File Number SR-Phlx-2013-24 and must be submitted to the SEC by April 16, 2013.

The notice of filing is available here; the rule filing with the text of the proposed rule change is available here.