I'm not hugely well equipped to give a proper AH answer to the question, but I can point you in the direction of a good source: one of my old professors at Edinburgh, Jeremy Crang, was one of the editors of the first comprehensive military history of Scotland, published a couple of years ago — the creatively-titled A Military History of Scotland. It's a colossal tome, and an impressive collection of scholarship.
I've only read parts of it — because it's 900 pages long, covers 2,000 years, and Scottish history was never my specialist area — but it's pretty much the canonical work in the (admittedly not-especially-crowded) field of Scottish military history.
There was not a standing army, however all men owed service to the crown.
There were two forms of military service in Scotland. "Free Service" and "Scottish Service". Free service was performed by all free men (i.e. the barons, thanes, knights, serjents and all other freeholders) and would be horsed and have a range of good arms and armour, the numbers depending on the terms of their fee charter. Scottish Service was (primarily) demanded from the earldoms north of the Forth and consisted of all able bodied men and who would fight on foot (and who may or may not have some form of body armour (a jack or other padded leather).
In general the period of (unpaid) service was for 40 days.
Four times per year a 'wapinschaw' would also occur (issued by the crown) which would require either or both forms of service to assemble for an inspection of arms and equipment.
There was also (largely post Robert I) ship service required of the large fleets maintained by the lords in the Isles and Argyll.
By around 1500 military thinking was changing and there was a new vogue to emulate the Swiss in their use of pikes and so forth. In preparation for the Flodden campaign in 1513, James IV was importing vast numbers of pikes as well as artillery from the low countries and French knights and men at arms to train the raised soldiers. This was one of the first occurrences in Scotland of large scale national provision of arms and equipment to the soldiers raised.
Post the disaster at Flodden, Military service continued largely as before but with fluctuating engagement of the crown in providing arms and equipment.
Sources:
Brewer JS Letters and Papers, Foreign and Domestic of the Reign of Henry VIII
N. MacDougall James IV
GWS Barrow Robert Bruce & The Community of the Realm & Kingdom of the Scots.
JE Morris, Welsh Wars of Edward I
M Prestwich, Edward I